CIS 185 CCNP ROUTE Ch. 6 Border Gateway Protocol Solution for ISP Connectivity – Part 2 Rick Graziani Cabrillo College Last Updated: Fall 2010

2 Materials Show configuration for not being a transit AS

3 Materials Book:  Implementing Cisco IP Routing (ROUTE) Foundation Learning Guide: Foundation learning for the ROUTE Exam  By Diane Teare  Book  ISBN-10:  ISBN-13:  eBook  ISBN-10:  ISBN-13:

4 SVLSAC LAXRVR SVL-Agg1 LAX-Agg1 L2PAIX CUDI Level 3 Transit Cogent Transit CENIC (non-transit AS) Google Commodity Peer Hurricane Elec Commodity Peer Comcast Commodity Peer Customer Prefixes Internet 2 Research Peer NLR Research Peer Bi-lateral Settlement Free Peer Note: This diagram is by no means accurate but only a very simplified representation of an AS. Cabrillo UCSC UCLA Cuesta Public IP Address AS and Public IP Address AS and Public IP Address Public IP Address Static Default Static Default EBGP Fully-meshed IBGP EBGP MEDLocPref IGP: IS-IS used for next-hop reachability EBGP Buy transit per Mb/s per month for routes not know from non-transit AS’s Non-transit. You can get to our prefixes and we can get to your prefixes. No charge. Note: Non-customer prefixes (networks connecting peers) are not included in EBGP updates to peers. IGP is not redistributed into IS-IS and IS-IS is not redistributed into BGP. Redistribute static Non-transit. You can get to our prefixes and we can get to your prefixes. No charge.

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7 Path Attributes Each route has its own set of defined attributes, which can include:  Path information  Route preference  Next-hop  Aggregation information Administrators use these values to enforce routing policy. Based on attribute values, you can configure BGP to:  filter routing information  prefer certain paths  customize its behavior Every UPDATE message has a variable-length sequence of path attributes.

8 Path Attributes Not all vendor implementations of BGP recognize the same attributes. Path attributes come in four different types:  Well-known mandatory  Well-known discretionary  Optional transitive  Optional non-transitive

9 Well-known mandatory An attribute that has to exist in the BGP UPDATE packet. It must be recognized by all BGP implementations. If a well-known attribute is missing, a notification error will be generated  This ensures that all BGP implementations agree on a standard set of attributes. Example: AS_PATH attribute Path Attributes

10 Path Attributes Well- known discretionary An attribute that is recognized by all BGP implementations But may or may not be sent in the BGP UPDATE message. Example: LOCAL_PREF

11 Path Attributes Optional transitive An attribute that may or may not be, recognized by all BGP implementations (thus, optional). Because the attribute is transitive:  BGP should accept and advertise the attribute even if it isn’t recognized. Example: COMMUNITY

12 Path Attributes Optional non-transitive An attribute that may or may not be, recognized by all BGP implementations. Whether or not the receiving BGP router recognizes the attribute, it is non- transitive:  Should NOT be passed along to other BGP peers. Example: MED

The Route Selection Decision Process (Cisco) 13 Summary of the BGP Path Selection Process BGP uses the following criteria, in the order presented, to select a path for a destination: NOTE: Not all of these are commonly used and will be examined in more detail later in this presentation and in the next presentation. 1. If the path specifies a next hop that is inaccessible, drop the update. 2. Prefer the path with the largest weight. 3. If the weights are the same, prefer the path with the largest local preference. 4. If the local preferences are the same, prefer the path that was originated by BGP running on this router. 5. If no route was originated, prefer the route that has the shortest AS_path. 6. If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than Incomplete). 7. If the origin codes are the same, prefer the path with the lowest MED attribute. 8. If the paths have the same MED, prefer the external path over the internal path. 9. If the paths are still the same, prefer the path through the closest IGP neighbor. 10. Prefer the path with the lowest IP address, as specified by the BGP router ID WLam Weight Local Preference as path med

The AS-Path Attribute AS-path attribute – Well-known mandatory attribute Whenever a route update passes through an AS, the AS number is prepended to that update  AS number is put at the beginning of the list when it is advertised to the next EBGP neighbor. Router A: advertises network in AS Router C: prepends its own AS number to it and advertises the route to Router B. Router B: From Router B’s perspective, the path to reach is:  65500, My path to is (65500, 64520) WLam Weight Local Preference as path med

AS numbers are prepended only by routers advertising routes to EBGP neighbors. Routers advertising routes to IBGP neighbors do not change the AS-path attribute 15 The AS-Path Attribute My path to is (65500, 64520) My path to is (64520)

show ip bpg – Shows the BGP Table AS Path to is via AS: 7018, 22822, 22822, 2152, twice? AS Path prepend (later) 16

The Next-Hop Attribute Next-hop attribute - A well-known mandatory attribute Indicates the next-hop IP address that is to be used to reach a destination. IGPs:  hop-by-hop routing protocol  router-by-router BGP:  hop-by-hop routing protocol (like IGPs)  AS-by-AS (not like IGPs)  By default, the next-hop is the next AS 17

EBGP: The next-hop is the IP address of the neighbor that sent the update.  Router A advertises (EBGP) to Router B, with a next hop of IBGP: The next-hop is carried into IBGP unchanged.  Router B advertises (IBGP) to Router C, with a next hop of Router B uses as the next-hop attribute to get to Router C’s next hop to reach is not The Next-Hop Attribute

Very important that Router C knows how to reach the ( or )  IGP  static route Otherwise, Router C will drop packets destined for , because it will not be able to get to the next-hop address for that network. Must either:  Advertise the next-hop network, the network between AS’s, ( ) into the AS  Change the next-hop address (Router B’s network) - later 19 The Next-Hop Attribute

Recursive Lookup IBGP router performs a recursive lookup to find out how to reach the BGP next- hop address by using its IGP entries in the routing table. Router C has a packet to send to  Finds the longest-match for ( /16) in the routing table and finds a BGP route with a next hop of  Does a recursive lookup in the routing table for a path to network  If there is an IGP route to (ex: ) in the routing table with a next hop of  Forwards the packet destined for to  Otherwise, drops the packet 20 The Next-Hop Attribute

Third-party Next-Hop: When running BGP over a multiaccess network such as Ethernet, a BGP router uses the appropriate address as the next-hop address (by changing the next-hop attribute) to avoid inserting additional hops into the path. Because the network among the three routers A, B, and C is a multiaccess network, it makes more sense for Router A to use Router C as a next hop to reach , rather than making an extra hop via Router B. 21 Third-Party Next- Hop I’m your EBGP peer but your best next-hop path to is via Router C at EBGP

However, if the common medium between routers is a nonbroadcast multiaccess (NBMA) medium, complications might occur. Routers A, B, and C are connected by Frame Relay. Router B can reach network via (PVC). When Router B sends a BGP update to Router A about , it uses as the next hop, not its own IP address ( ). A problem arises if Routers A and C do not know how to communicate directly  Routers A and C do not have a Frame Relay map entry to reach each other Solution: Router B could advertise itself as the next-hop address for routes sent to Router A like a non-multiaccess network (later) 22 Third-Party Next- Hop EBGP PVC I’m your EBGP peer but since this is a multicaccess network I know your best next-hop path to is via Router C at Thanks but I don’t have a PVC or a Frame Relay map entry to

The Origin Attribute The origin attribute is a well-known mandatory attribute  Defines the origin of the path information. The origin attribute can be one of three values: IGP  The route is interior to the originating AS.  Normally when the network command is used (coming)  Indicated with an “i” in the BGP table (coming) EGP  The route is learned via EGP  EGP is legacy and no longer supported in the Internet  Indicated with an “e” in the BGP table. Incomplete  The route’s origin is unknown or is learned via some other means.  This usually occurs when a route is redistributed into BGP.  Indicated with a “?” in the BGP table. 23

The Local Preference Attribute Local preference - A well-known discretionary attribute  Indicates to routers in the AS which path is preferred to exit the AS Path with the higher local preference is preferred. Configured on a router Exchanged only among routers within the same AS  Passed only via IBGP not via EBGP Default value on a Cisco router is 100 Local Preference takes precedence over AS_PATH  WLam: Weight, Local preference, as path, med 24 IBGP WLam Weight Local Preference as path med

AS receives updates about network from two directions:  via AS (65500, 65350)  via (65000, 65250, 65350) Router A and Router B are IBGP neighbors. Local preference:  On Router A for network is set to 200  On Router B for network is set to 150 Local preference information is exchanged within AS via IBGP All traffic in AS addressed to network is sent to Router A as an exit point from AS The Local Preference Attribute IBGP My Local Preference is higher so I am the preferred exit point.

The MED Attribute The MED attribute - An optional nontransitive attribute. Also called the metric  Displayed in the metric column in the BGP table. Indicates to external neighbors the preferred path into an autonomous system. A way for an AS to try to influence another AS as to which way it should send its traffic when there are multiple entry points. MED is sent to EBGP peers:  Those routers propagate the MED within their AS  But do not pass it on to the next AS WLam Weight Local Preference as path med

By default, a router compares the MED attribute only for paths from neighbors in the same AS. By using the MED attribute, BGP is the only protocol that can affect how routes are sent into an AS. Router B has set the MED attribute to 150 for Router C has set the MED attribute to 200 for Router A receives EBGP updates from Routers B and C  Chooses Router B as the best next hop to get to AS The MED Attribute Which path is most attractive? My MED is 150 for so send those packets this way. My MED is 200 for so send those packets this way.

By default, a router compares the MED attribute only for paths from neighbors in the same AS. If these were two different AS’s then Router A would use MED for best path selection 28 The MED Attribute My MED is 150 for so send those packets this way to AS My MED is 200 for so send those packets this way to AS Different AS’s so I won’t use the MED unless configured with “bgp always compare med” AS AS AS 65002

The Weight Attribute (Cisco Only) Weight attribute - a Cisco-defined attribute used for the path-selection process. Configured locally and not propagated to any other routers Higher weight is preferred when there are multiple routes to the same network. Weight takes precedence over Local Preference Value from 0 to Default of WLam Weight Local Preference as path med

Router A has two ways to reach  via Router B (AS 65000)  via Router C (AS 65500) Router A is configured to set the weight of updates coming from:  Router B to 200  Router C to 150 Weight for Router B is higher so Router A uses Router B as a next hop to reach The Weight Attribute (Cisco Only) Router A AS AS

The Path-Selection Decision Process with a Multihomed Connection An AS rarely implements BGP with only one EBGP connection, so generally multiple paths exist for each network in the BGP forwarding database. Using the 11-step route selection process as outlined in Diane Teare’s book… 31

The Route Selection Decision Process (Cisco) 32 Summary of the BGP Path Selection Process BGP uses the following criteria, in the order presented, to select a path for a destination: NOTE: Not all of these are commonly used and will be examined in more detail later in this presentation and in the next presentation. 1. If the path specifies a next hop that is inaccessible, drop the update. 2. Prefer the path with the largest weight. 3. If the weights are the same, prefer the path with the largest local preference. 4. If the local preferences are the same, prefer the path that was originated by BGP running on this router. 5. If no route was originated, prefer the route that has the shortest AS_path. 6. If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than Incomplete). 7. If the origin codes are the same, prefer the path with the lowest MED attribute. 8. If the paths have the same MED, prefer the external path over the internal path. 9. If the paths are still the same, prefer the path through the closest IGP neighbor. 10. Prefer the path with the lowest IP address, as specified by the BGP router ID WLam Weight Local Preference as path med

Step 1 Prefer the route with the highest weight. (Recall that the weight is Cisco-proprietary and is local to the router only.) Step 2 If multiple routes have the same weight, prefer the route with the highest local preference. (Recall that the local preference is used within an AS.) Step 3 If multiple routes have the same local preference, prefer the route that was originated by the local router. (network command) Step 4 If none of the routes were originated by the local router, prefer the route with the shortest AS-path. Step 5 If the AS-path length is the same, prefer the lowest origin code (IGP < EGP < incomplete). Step 6 If all origin codes are the same, prefer the path with the lowest MED. (Recall that the MED is exchanged between autonomous systems.) The MED comparison is done only if the neighboring AS is the same for all routes considered, unless the bgp always-compare-med router configuration command is enabled. Step 7 If the routes have the same MED, prefer external paths (EBGP) over internal paths (IBGP). Step 8 If synchronization is disabled and only internal paths remain, prefer the path through the closest IGP neighbor. This means that the router prefers the shortest internal path within the AS to reach the destination (the shortest path to the BGP next-hop). Step 9 For EBGP paths, select the oldest route, to minimize the effect of routes going up and down (flapping). Step 10 Prefer the route with the lowest neighbor BGP router ID value. Step 11 If the BGP router IDs are the same, prefer the route with the lowest neighbor IP address. 33 The Route Selection Decision Process (ROUTE)

Step 1 Prefer the route with the highest weight. (Recall that the weight is Cisco-proprietary and is local to the router only.) Step 2 If multiple routes have the same weight, prefer the route with the highest local preference. (Recall that the local preference is used within an AS.) Step 3 If multiple routes have the same local preference, prefer the route that was originated by the local router. (network command) Step 4 If none of the routes were originated by the local router, prefer the route with the shortest AS-path. Step 5 If the AS-path length is the same, prefer the lowest origin code (IGP < EGP < incomplete). Step 6 If all origin codes are the same, prefer the path with the lowest MED. (Recall that the MED is exchanged between autonomous systems.) The MED comparison is done only if the neighboring AS is the same for all routes considered, unless the bgp always-compare-med router configuration command is enabled. Step 7 If the routes have the same MED, prefer external paths (EBGP) over internal paths (IBGP). Step 8 If synchronization is disabled and only internal paths remain, prefer the path through the closest IGP neighbor. This means that the router prefers the shortest internal path within the AS to reach the destination (the shortest path to the BGP next-hop). Step 9 For EBGP paths, select the oldest route, to minimize the effect of routes going up and down (flapping). Step 10 Prefer the route with the lowest neighbor BGP router ID value. Step 11 If the BGP router IDs are the same, prefer the route with the lowest neighbor IP address. 34 The Route Selection Decision Process (ROUTE)

Multiple Path Selection (BGP Multipath) BGP chooses only a single best path for each destination. The maximum-paths paths affects only the number of routes kept in the IP routing table, not the number of paths selected as best by BGP. Defaults to one. R2 and R3 are advertising network  With maximum-paths 2: both paths appear in the IP routing table  Without maximum-paths 2: only one path to in R1’s routing table  Only one path is still selected as the best in the BGP table “>” (offered to the IP routing table) and advertised to its BGP neighbors. 35 R1(config)#router bgp R1(config-router)# maximum-paths 2

Configuring BGP 36

Entering BGP Configuration Mode Use the router bgp autonomous-system global configuration command autonomous-system identifies the local AS.  AS determines whether IBGP or EBGP neighbors. The router bgp command alone does not activate BGP on a router.  Must enter at least one subcommand. Only one instance of BGP can be configured on a router 37 RTA(config)#router bgp 100 RTA(config-router)# RTB(config)#router bgp 200 RTB(config-router)#

Defining BGP Neighbors and Activating BGP Sessions neighbor {ip-address | peer-group-name} remote-as autonomous-system This address must be reachable (for TCP session) and exchanging BGP updates. autonomous-system field of the neighbor remote-as:  EBGP: different AS numbers  IBGP: same AS number Note: We will discuss peer groups later. 38 RTA(config)#router bgp 100 RTA(config-router)#neighbor remote-as 200 RTB(config)#router bgp 200 RTB(config-router)#neighbor remote-as 100

39 EBGP and fully meshed IBGP router bgp neighbor remote-as neighbor remote-as 65101

Shutting Down a BGP Neighbor  To disable (administratively shut down) an existing BGP neighbor or peer group  This command not only terminates the session but also removes all associated routing information. Use the “no neighbor…” to enable a neighbor previously shutdown If you want to implement major policy changes to a neighboring you must:  Administratively shut down the neighboring router  Implement the changes (attributes, route filtering, etc.)  Administratively bring the neighboring router back up 40 Router(config)# router bgp as Router(config-router)# neighbor {ip-address | peer-group-name} shutdown

Defining the Source IP Address The BGP neighbor statement tells the BGP process the destination IP address of each update packet. The source IP address must match the address in the corresponding neighbor statement on the other router. BGP does not accept unsolicited updates. 41 RTA(config)#router bgp 100 RTA(config-router)#neighbor remote-as 200 I will only accept BGP updates with a source IP address of

Router D has the configuration above. If Router A is sending the BGP packets to Router D via Router B  The source IP address of the packets will be Router D:  Does not recognize the sender of the BGP packet ( )  is not configured as a neighbor of Router D.  The IBGP session between Router A and D will not be established. 42 RouterD(config)#router bgp RouterD(config-router)#neighbor remote-as

Solution: Establish the IBGP session using a loopback interface when there are multiple paths between the IBGP neighbors. Causes the router to use the address of the specified loopback interface as the source address for BGP connections to this neighbor. Need to make sure loopback address is reachable (TCP session) 43 Router(config)#router bgp as Router(config-router)# neighbor {ip-address | peer-group-name} update-source loopback interface-number RouterA(config)#router bgp RouterA(config-router)#neighbor remote-as RouterA(config-router)#neighbor update-source loopback0 RouterD(config)#router bgp RouterD(config-router)#neighbor remote-as RouterD(config-router)#neighbor update-source loopback0

Router B has Router A as an EBGP neighbor  Only reachable via directly connected address Router C has Router D as an EBGP neighbor  Only reachable via directly connected address

However, Router B has multiple paths to reach Router C (IBGP neighbor) Router B and C: All networks are reachable via IGP EIGRP including loopback interfaces Neighbor relationship between Routers B and C are loopback interfaces If one of the links between Router B and C goes down the BGP peering is not affected. Without the loopbacks as source interfaces If one of the links between Router B and C goes down the BGP neighbor relationship would be lost. 45

EBGP Multihop Because IGP routing information is not exchanged with external peers:  The router must point to a directly connected address for external neighbors.  A loopback interface is never directly connected. So, if you want to peer with a loopback interface instead:  Must add a static route to the loopback pointing to the physical address of the directly connected network (the next-hop address)  Must also enable multihop EBGP, with the router configuration command: neighbor {ip-address | peer-group-name} ebgp-multihop [ttl] This command increases the default of one hop for EBGP peers by changing the default Time to Live (TTL) value of 1 and therefore allowing routes to the EBGP loopback address. By default, the TTL is set to 255 with this command. This command is of value when redundant paths exist between EBGP neighbors. 46

Alternative but less desirable Router A in AS has two paths to Router B in AS  Primary  Backup Router A can use two neighbor statements pointing to and on Router B. Disadvantage: Router A will send duplicate (two) BGP updates to Router B because of the two neighbor statements. 47 EBGP Multihop RTA(config)#router bgp RTA(config-router)#neighbor remote-as RTA(config-router)#neighbor remote-as 65101

EBGP Multihop Each router instead:  Uses its loopback address as the source IP address for its BGP updates  Points to the loopback address of the other router IGP is not used between autonomous systems, so neither router can reach the loopback of the other router without assistance. Assistance: Each router needs to use two static routes:  A path to reach the loopback address of the other router. neighbor ebgp-multihop: Must also be configured to change the default setting of BGP and inform the BGP process that this neighbor IP address is more than one hop away (2 hops in this example). 48 EBGP Multihop

Changing the Next-Hop Attribute An internal protocol, such as RIP, EIGRP, or OSPF, always uses the source IP address of a routing update as the next-hop address for each network from that update that is placed in the routing table. IBGP: The next-hop is carried into IBGP unchanged. neighbor {ip-address | peer-group-name} next-hop-self router configuration command  Used to force BGP to use the source IP address of the update as the next hop for each network it advertises to the neighbor  Rather than using the next-hop AS IP address (default). 49

50 EBGP: The next-hop is the IP address of the neighbor that sent the update.  Router A advertises (EBGP) to Router B, with a next hop of  Router C advertises (EBGP) to Router B, with a next hop of IBGP: The next-hop is carried into IBGP unchanged.  Router B advertises (IBGP) to Router C, with a next hop of Router B uses as the next-hop attribute to get to and Router C uses as the next-hop attribute to get to Router C’s next hop to reach is not one of it’s local interfaces Very important that Router C knows how to reach the :  IGP  Static route Otherwise, Router C will drop packets destined for , because it will not be able to get to the next-hop address for that network. Must either:  Advertise the next-hop network, the network between AS’s, ( ) into the AS  Change the next-hop address to Router B’s local interface

The neighbor next-hop-self command to change the default BGP next-hop settings. Router B advertises a next hop of (the IP address of its loopback interface) to its IBGP neighbor  set with the neighbor update- source command

Defining the Networks That BGP Advertises Two options available to advertise networks into BGP:  network command  Redistributing routes from an IGP into BGP  Not recommended because any change in IGP routes (link goes down) might cause a BGP update  Could result in unstable BGP tables  If redistribution:  Only local routes to the AS are redistributed  Problem if you redistribute routes learned from other AS’s into the IGP and the redistribute those routes back into BGP because it could result in a routing loop. 52 RTA(config)#router bgp as RTA(config-router)#network network-number

network network-number [mask network-mask] [route-map map-tag] Permits BGP to advertise a network if it is present in the IP routing table No mask: Classful default mask is assumed. Different than IGP network command:  Determines which networks this router advertises.  Does not determine which interfaces send/receive routing protocol updates  Sole purpose of the network command is to notify BGP which networks to advertise If the mask is not specified router announces only the classful network address  At least one subnet of the specified major network must be present in the IP routing table for BGP to announce the classful network as a BGP route. If you specify the network-mask:  An exact match to the network (both address and mask) must exist in the routing table for the network to be advertised. 53 RTA(config)#router bgp as RTA(config-router)# network network-number [mask network-mask] [route-map map- tag]

With Cisco IOS 12.1(8)T – Default is no auto-summary auto-summary – All redistributed subnets are summarized to their classful boundaries in the BGP table.  When a subnet is redistributed from an IGP into BGP, only the classful network route is injected into the BGP table. 54 RTA(config)#router bgp as RTA(config-router)# no auto-summary Auto-summary

BGP Neighbor Authentication BGP neighbor authentication on a router so that the router authenticates the source of each routing update packet that it receives Exchange of an authenticating keys (sometimes referred to as a passwords) To enable MD5 authentication on a TCP connection between two BGP peers, use the string router configuration command: neighbor {ip-address | peer-group-name} password 55

synchronization - Router configuration command to enable BGP synchronization  Router will not advertise routes in BGP until it learns them in an IGP.  No longer the default as of Cisco IOS 12.2(8)T BGP synchronization rule states: A BGP router should not use or advertise to an external BGP neighbor a route learned by IBGP, unless that route is directly connected or learned from the IGP. 56 BGP Synchronization I learned about via IBGP from Router B. I will not advertise to Router E via EBGP unless I see this network in my routing table leaned via an IGP (OSPF). ? IBGP AS networks Into OSPF OSPF Note: There is not a physical link b/t A and B RTA(config)#router bgp RTA(config-router)# synchronization

no synchronization - Router configuration command disables synchronization Default as of Cisco IOS 12.2(8)T Best practice is to no longer redistribute BGP networks into the IGP. Instead, all routers in the AS should be fully meshed IBGP. 57 No BGP Synchronization ? IBGP AS networks Into OSPF OSPF I learned about via IBGP from Router B. I will not advertise to Router E via EBGP unless I see this network in my routing table leaned via an IGP (OSPF). I learned about via IBGP from Router B. I can advertise to Router E via EBGP even if this network in NOT in my routing table leaned via an IGP (OSPF). RTA(config)#router bgp RTA(config-router)# no synchronization

Resetting BGP Sessions So we can focus on other items, please read this section on your own. 58

Resetting BGP Sessions BGP can potentially handle huge volumes of routing information. Cisco IOS Software applies changes on only those updates received or transmitted after the BGP policy configuration change has been performed. If the network administrator wants the policy change to be applied on all routes, he or she must trigger an update to force the router to let all routes pass through the new filter.  Outgoing information, the router has to resend its BGP table through the new filter  Incoming information, the router needs its neighbor to resend its BGP table so that it passes through the new filter There are three ways to trigger an update:  Hard reset  Soft reset  Route refresh 59

Hard Reset of BGP Sessions clear ip bgp * or clear ip bgp {neighbor-address} Resetting a session is a method of informing the neighbor or neighbors of a policy change. All routes from that session are invalidated and removed from the BGP table. The remote neighbor detects a BGP session down state:  Also invalidates the received routes. After a period of 30 to 60 seconds, the BGP sessions are reestablished automatically The BGP tables are exchanged again, but through the new filters. However, resetting the BGP session disrupts packet forwarding. 60 RTA(config)#router bgp RTA(config-router)# no synchronization

Router A:  Has eight neighbors  Each neighbor sends Router A the full Internet table  assume that is about 32 MB in size If the clear ip bgp * command is issued on Router A:  All eight routers resend their 32 MB table at the same time.  Router A will need 256 MB of RAM  Router A will have to process all of this information.  This will take a considerable number of CPU cycles  This will further delaying the routing of user data clear ip bgp neighbor-address - one neighbor is reset at a time is less severe 61 RTA(config)#router bgp RTA(config-router)# clear ip bgp * Or RTA(config-router)# clear ip bgp neighbor-address

Soft Reset of BGP Sessions Outbound clear ip bgp {* | neighbor-address} [soft out] Note: soft keyword is optional Causes BGP to do a soft reset for outbound updates. Does not reset the BGP session Router creates a new update and sends the whole table to the specified neighbors. Does not have any memory overhead. This command is highly recommended when you are changing an outbound policy  But does not help if you are changing an inbound policy. 62 RTA# clear ip bgp * soft out Or RTA# clear ip bgp neighbor-address soft out

Soft Reset of BGP Sessions Inbound There are two ways to perform an inbound soft reconfiguration:  Stored routing update information  Dynamically Inbound Soft Reset Using Stored Information First: Enter the neighbor {ip-address} soft-reconfiguration inbound router configuration command to inform BGP to save all updates that were learned from the neighbor specified.  The BGP router retains an unfiltered table of what that neighbor has sent. Next, the inbound policy is changed Then, use clear ip bgp {* | neighbor-address} soft in privileged EXEC command causes the router to use the stored unfiltered table to generate new inbound updates; the new results are placed in the BGP forwarding database. Thus, if you make changes, you do not have to force the other side to resend everything. 63 RTA(config)#router bgp RTA(config-router)# neighbor {ip-address} soft-reconfiguration inbound RTA# clear ip bgp {* | neighbor-address} soft in

Route Refresh: Dynamic Inbound Soft Reset provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent on stored routing table update information. This new method requires no preconfiguration The clear ip bgp {* | neighbor-address} [soft in | in] privileged EXEC command is the only command required for this dynamic soft reconfiguration. The soft in option generates new inbound updates without resetting the BGP session, but it can be memory intensive. BGP does not allow a router to force another BGP speaker to resend its entire table. If you change the inbound BGP policy and you do not want to complete a hard reset, use this command to cause the router to perform a soft reconfiguration. The soft in option generates new inbound updates without resetting the BGP session, but it can be memory intensive. BGP does not allow a router to force another BGP speaker to resend its entire table. If you change the inbound BGP policy and you do not want to complete a hard reset, use this command to cause the router to perform a soft reconfiguration. 64

When a BGP session is reset using soft reconfiguration, the following commands can be useful for monitoring the BGP routes received, sent, or filtered: show ip bgp neighbors {address} received-routes: displays all received routes (both accepted and rejected) from the specified neighbor. show ip bgp neighbors {address} routes: displays all routes that are received and accepted from the specified neighbor.; this output is a subset of the output displayed by the received-routes keyword. show ip bgp: displays entries in the BGP table. show ip bgp neighbors {address} advertised-routes: displays all BGP routes that have been advertised to neighbors 65

BGP Configuration Examples 66

Basic BGP 67

68 IBGP and EBGP router bgp neighbor remote-as neighbor remote-as neighbor update-source loopback0 neighbor next-hop-self network mask network network no synchronization router eigrp 10 network Router B EBGP peer to Router A IBGP peer to Router C’s loopback IBGP-RTC: Use Lo0 as source IBGP-RTC: Changes next-hop attribute to its own Lo0 Advertises LAN /24 via BGP Advertises serial link /24 via BGP Advertises serial link /24 via BGP Advertises Lo /32 via EIGRP so RTC can reach it with it’s neighbor statement. RTC will advertise /32 May be default if IOS 12.2(8)T or later. Must be fully-meshed IBGP

Verifying and Troubleshooting BGP You can verify BGP operation using show EXEC commands, including the following: show ip bgp—Displays entries in the BGP topology database (BGP table). Specify a network number to get more specific information about a particular network. show ip bgp rib-failure—Displays BGP routes that were not installed in the routing information base (RIB), and the reason that they were not installed. show ip bgp neighbors—Displays detailed information about the TCP and BGP connections to neighbors. show ip bgp summary—Displays the status of all BGP connections. Use the show ip bgp ? command on a router to see other BGP show commands. debug commands display events as they happen on the router. For BGP, the debug ip bgp privileged EXEC command has many options, including the following:  dampening—BGP dampening  events—BGP events  keepalives—BGP keepalives  updates—BGP updates 69

show ip bgp show ip bgp - Displays the BGP topology database (the BGP table). The origin codes are shown at the end of each line. The status codes are shown at the beginning of each line of output In this output, most of the rows have an asterisk (*) in the first column.  Means that the next-hop address is valid.  The next-hop address is not always the router that is directly connected to this router. (IBGP)  A locally originated route has a next hop of

71 Please review on your own… s: indicates that the specified routes are suppressed (usually because routes have been summarized and only the summarized route is being sent). d: for dampening, indicates that the route is being dampened (penalized) for going up and down too often.  Although the route might be up right now, it is not advertised until the penalty has expired. h: for history, indicates that the route is unavailable and is probably down; historic information about the route exists, but a best route does not exist. r: for RIB failure, indicates that the route was not installed in the RIB.  The reason that the route is not installed can be displayed using the show ip bgp rib-failure command, as described in the next section. S: for stale, indicates that the route is stale (this is used in a nonstop forwarding-aware router

72 A greater-than sign (>) in the second column indicates the best path for a route selected by BGP This route is offered to the IP routing table.  asterisk (*) - means that the next-hop address is valid.

73 Third column is either blank or has an i:  Blank: BGP learned that route from an external peer (EBGP)  i: BGP learned that route from an internal peer (IBGP).

74 The fourth column lists the networks that the router learned. The fifth column lists all the next-hop addresses for each route. This next-hop address column might contain , which signifies that this router originated the route. The next three columns list three BGP path attributes associated with the path:  metric (MED)  local preference  weight.

75 “Path” header may contain a sequence of autonomous systems in the path. The first AS listed is the adjacent AS from which this network was learned. The last AS this network’s originating AS. If the path column is blank, the route is from the current AS (this AS)

76 The last column signifies how this route was entered into BGP on the original router (the origin attribute). i: The original router probably used a network command to introduce this network into BGP. e: The original router learned this network from EGP (legacy, not likely). ?: The original BGP process cannot absolutely verify this network’s availability, because it is redistributed from an IGP into the BGP process

show ip bgp rib-failure show ip bgp rib-failure - Displays BGP routes that were not installed in the RIB (IP routing table) and the reason that they were not installed. 77 Displayed routes were not installed because a route(s) with a better administrative distance already existed in the RIB.

show ip bgp summary show ip bgp summary – Verifies BGP neighbor relationships and other information 78

79 BGP Router ID Increases in increments when the BGP table changes Last version of BGP database that was Injected into the main routing table The IP address, used in the neighbor statement, with which this router is setting up a relationship BGP version Neighbors AS Number of BGP messages received from this neighbor Number of BGP messages sent to this neighbor The last version of the BGP table sent to this neighbor The number of BGP messages from this neighbor waiting to be processed The number of BGP messages queued and waiting to be sent to this neighbor. TCP flow control prevents a router from overwhelming its neighbor The amount of time this neighbor has been in the current BGP state (established, active or idle) When the session is in the established state, this value represents the number of BGP network entries received from this neighbor The current BGP state. If established, not shown, instead a value is in the Pfx/Rcd

debug ip bgp updates 80 After the neighbor adjacency is reestablished, Router A creates and sends updates to Update sent about network /24, with a next hop of , which is Router A’s address Update sent about network /24, with a next hop of , which is the address of one of Router A’s EBGP neighbors. Router A later receives updates from containing a path to two networks, /24 and /24

show ip bgp neighbors show ip bgp neighbors – Displays information about the BGP connections to neighbors. The BGP state is established, which means that the neighbors have established a TCP connection and the two peers have agreed to use BGP to communicate. 81

82 CharacterDescription ^ Matches the beginning of the input string. $ Matches the end of the input string. _ Matches a space, comma, left brace, right brace, the beginning of an input string, or the ending of an input stream. Matches any single character * Matches 0 or more single- or multiple- character patterns. Regular Expressions A regular expression is a pattern to match against an input string.

83 RouterC#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i *> i *> i RouterC# show ip bgp regexp ^300 Match beginning of input string, AS_PATH, = 300 Last prepended AS was 300: Routes matched: and

84 RouterC#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i *> i *> i RouterC# show ip bgp regexp ^200 Match beginning of input string, AS_PATH, = 200 Last prepended AS was 200: Routes matched : none

85 RouterC#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i *> i *> i RouterC# show ip bgp regexp 300$ Match end of input string, AS_PATH, = 300 Originating AS = 300: Routes matched :

86 RouterC#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i *> i *> i RouterC# show ip bgp regexp 200$ Match end of input string, AS_PATH, = 200 Originating AS = 200: Routes matched :

87 AS50#show ip bgp Network Path *> i *> i *> i *> i *> i *> I AS50#show ip bpg regexp 100 Match input string, AS_PATH, containing 100, including 1000 Routes matched : , ,

88 AS50#show ip bgp Network Path *> i *> i *> i *> i *> i *> I AS50#show ip bpg regexp ^100_ Match beginning of input string, AS_PATH, = 100 Last prepended AS was 100: Routes matched : ,

89 AS50#show ip bgp Network Path *> i *> i *> i *> i *> i *> I AS50# show ip bgp regexp _400$ Match end of input string, AS_PATH, = 400 Originating AS = 400: Routes matched :

90 AS50#show ip bgp Network Path *> i *> i *> i *> i *> i *> I AS50#show ip bpg regexp _400_ Match anywhere in input string, AS_PATH, 400 Routes matched : ,

91 AS50#show ip bgp Network Path *> i *> i *> i *> i *> i *> I AS50#show ip bgp regexp ^300$ Match input string that starts and ends at 300 Routes that originated from directly connected AS 300 customer Routes matched :

Configuring BGP Attributes 92

Step 1 Prefer the route with the highest weight. (Recall that the weight is Cisco-proprietary and is local to the router only.) Step 2 If multiple routes have the same weight, prefer the route with the highest local preference. (Recall that the local preference is used within an AS.) Step 3 If multiple routes have the same local preference, prefer the route that was originated by the local router. (network command) Step 4 If none of the routes were originated by the local router, prefer the route with the shortest AS-path. Step 5 If the AS-path length is the same, prefer the lowest origin code (IGP < EGP < incomplete). Step 6 If all origin codes are the same, prefer the path with the lowest MED. (Recall that the MED is exchanged between autonomous systems.) The MED comparison is done only if the neighboring AS is the same for all routes considered, unless the bgp always-compare-med router configuration command is enabled. Step 7 If the routes have the same MED, prefer external paths (EBGP) over internal paths (IBGP). Step 8 If synchronization is disabled and only internal paths remain, prefer the path through the closest IGP neighbor. This means that the router prefers the shortest internal path within the AS to reach the destination (the shortest path to the BGP next-hop). Step 9 For EBGP paths, select the oldest route, to minimize the effect of routes going up and down (flapping). Step 10 Prefer the route with the lowest neighbor BGP router ID value. Step 11 If the BGP router IDs are the same, prefer the route with the lowest neighbor IP address. 93 The Route Selection Decision Process (ROUTE) WLam Weight Local Preference as path med

Changing the Weight Weight attribute influences only the local router. When there is more than one route to the same destination. Cisco attribute Local to the router and it is not propagated in routing updates. Default is Higher weight is preferred when there are multiple routes to the same destination 94 WLam Weight Local Preference as path med

Routing policy dictates the selection of AS as the primary way out of AS for the traffic destined to any network originated by the AS R1

This route map set-weight is linked to neighbor (R2) as an inbound route map. The match condition is defined by the match as-path path-list-number  The path-list-number parameter is the number of the AS access-list (10)  Checks the AS-path attributes of networks to see which are permitted by AS access list 10. as-path AS access-list 10 permits networks whose:  AS-path attribute ends with 65020; these are networks originating in AS set weight 150: The route map sets these networks to a weight of AS _65020$ Weight=150 R1

Statement 20 does not have any match statements so all remaining networks are permitted. These remaining networks have their weight set to 100, with the set weight 100 command. When Router R1 receives updates from (R2):  It processes them through the set-weight route map  Sets the weight accordingly as the networks are placed in Router R1’s BGP table 97 AS _65020$ Weight=150 R1

Accepting only a default/Announcing only your locally source routes (non-transit) The local AS is only added after processing outbound AS path access lists. ^$: Will check for an empty path if we want to announce only our locally sourced routes, then AS will be added. 98 router bgp 24 no synchronization bgp log-neighbor-changes network neighbor remote-as 7777 neighbor description To ISP 1 neighbor weight 1000 neighbor prefix-list DEFAULTONLY in neighbor route-map LOCALONLY out neighbor remote-as 8888 neighbor description To ISP 2 neighbor weight 2000 neighbor prefix-list DEFAULTONLY in neighbor route-map LOCALONLY out ip as-path access-list 99 permit ^$ route-map LOCALONLY permit 10 match as-path 99 ip prefix-list DEFAULTONLY seq 5 permit /0

99 E1(config)#router bgp 100 E1(config-router)# neighbor weight 2000 E1(config-router)# neighbor weight 2000 E1(config-router)# neighbor weight 1000 E1(config-router)# neighbor weight 1000 E1 AS 1 (ISP1) 2000 AS 100 (E2) 1000 Higher weight preferred Routes from ISP1 get a weight of 2000 Routes from E2 and EX get a weight of 1000 Larger weight preferred Can also set the weight in the neighbor statement (for all routes received from that neighbor)

Setting Local Preference Local preference is used only within an AS between IBGP speakers to determine the best path to leave the AS to reach an outside network. Default is 100 Higher values are preferred bgp default local-preference value router configuration command changes the default local preference to the value specified  All BGP routes that are advertised include this local preference value. The value can be set to a number between 0 and WLam Weight Local Preference as path med

Local preference for all routes on Router B to 500 and on Router A to 200 All BGP routers in AS send all traffic destined for the Internet to Router B, causing its outbound utilization to be much higher and the utilization out Router A to be reduced to a minimal amount. This change is probably not what the network administrator intended. We should use route maps to:  Set only certain networks to have a higher local preference through Router B 101

Each network has two paths (< best path) All routes have:  Weight of 0  Default local preference of 100 So, BGP uses the shortest AS-path to select the best routes 102

: shortest AS-path is via , Router X ( ) : shortest AS-path is via , Router Y (65005) : shortest AS-path is via , Router Y ( ) 103

A traffic analysis reveals the following:  The link going through:  Router B to is heavily used  Router A to is hardly used at all  The three largest-volume destination networks on the Internet from AS are: , , and  30% of Internet traffic is going to network (via Router B)  20% of Internet traffic is going to network (via Router B)  10% of Internet traffic is going to network (via Router A)  40% of Internet traffic is going to other destinations You have decided to divert traffic to network and send it out Router A to the next hop of  Better load balancing 104

Router A: The route map is linked to neighbor as an inbound route map.  Receives updates from  Processes them through the local_pref route map  Sets the local preference accordingly and adds them to the BGP table. The first route map statement  Match condition checks all networks to see which are permitted by access list 65.  Access list 65 permits network  set local-preference 400 sets these networks to a local preference of 400 The second route map statement does not have any match or set statements.  Similar to a permit any statement in an access list.  Because there are no match conditions for the remaining networks, they are all permitted with their current settings.  So, the local preference for networks and stays set at the default of Router A LocPref=400

Router C learns about the new local preference value (400) coming from Router A for network Only change: the new best route to network is via Local preference = 400; higher than the default local preference of 100 via The AS-path through is still shorter than the path through , but AS-path but local preference takes precedence LocPref=400 WLam Weight Local Preference as path med

Setting AS Path AS Prepending It is complicated to influence other autonomous systems to select a particular path for traffic that is returning to a specific AS (can use MED) Virtually impossible to influence another AS to select the desired path based on the weight and local preference attributes  These require configuration changes in the neighboring AS  Need to have the network administrator for that AS(‘s) make the change By default, if no BGP path selection tools are configured to influence traffic flow, BGP uses the shortest AS path, regardless of available bandwidth. 107 By default, the preferred way (shortest path) to reach networks in AS65010 to reach is via R1

One way that an AS can attempt to influence incoming traffic flow is by sending out EBGP updates with an extended AS-path attribute for undesired paths  Multiple copies of the AS number of the sender  Known as AS-path prepending Makes it less likely that the receiver of the update will select this as best path. To avoid BGP loop prevention mechanisms, no other AS number, except that of the sending AS, should be prepended to the AS-path attribute. 108

This route map is linked to neighbor as an outbound route map. Therefore, as Router R1 sends updates to , it processes them through the set-AS-path route map Route map called set-AS-path has only one statement, a permit statement with a sequence number of 10. All updates sent to neighbor are prepended three times (plus the original AS number) with the AS number of the sender (65040), making that path less preferable for the returning traffic. 109 AS Path = I now prefer the three AS hop via I also include this complete AS path in my BGP updates to AS 65020

Setting the MED MED is used to decide how to enter an AS when multiple paths exist between two autonomous systems and one AS is trying to influence the incoming path from the other AS. Default MED is WLam Weight Local Preference as path med

To change this value, use the default-metric number router configuration command. To reach networks in AS all routers in AS see:  A MED of 1001 through the next hop of Router A  A MED of 99 through the next hop of Router B If AS has no overriding policy, all routers in AS choose to exit their AS through Router Y to reach the networks in AS This could result in suboptimal bandwidth utilization. 111

Using route maps we want the preferred exit points to be:  Router A to reach networks /24 and /24  Router B to reach network /24 Remember, lower MED is preferred! 112 Setting the MED using Route Maps

Router A: Outbound route map named med_65004 is linked to neighbor Router B: Outbound route map named med_65004 is linked to neighbor Router A Router B

Router A:  Networks permitted by ACL 66 ( /24 and /24)  Set MED to 100  All other networks ( /24)  Set MED to Router A Router B

Router B:  Networks permitted by ACL 66 ( /24)  Set MED to 100  All other networks ( /24 and /24)  Set MED to Router A Router B

Router Z BGP table: Indicating the networks learned from AS

117 SVLSAC LAXRVR SVL-Agg1 LAX-Agg1 L2PAIX CUDI Level 3 Transit Cogent Transit CENIC (non-transit AS) Google Commodity Peer Hurricane Elec Commodity Peer Comcast Commodity Peer Customer Prefixes Internet 2 Research Peer NLR Research Peer Bi-lateral Settlement Free Peer Note: This diagram is by no means accurate but only a very simplified representation of an AS. Cabrillo UCSC UCLA Cuesta Public IP Address AS and Public IP Address AS and Public IP Address Public IP Address Static Default Static Default EBGP Fully-meshed IBGP EBGP MEDLocPref IGP: IS-IS used for next-hop reachability EBGP Buy transit per Mb/s per month for routes not know from non-transit AS’s Non-transit. You can get to our prefixes and we can get to your prefixes. No charge. Note: Non-customer prefixes (networks connecting peers) are not included in EBGP updates to peers. IGP is not redistributed into IS-IS and IS-IS is not redistributed into BGP. Redistribute static Non-transit. You can get to our prefixes and we can get to your prefixes. No charge.

Peer Groups Filtering BGP Updates Additional Attribute Examples These sections are fairly straight forward. Please read this material on your own. 118

Peer Groups In BGP, many neighbors are often configured with the same update policies (for example, they have the same filtering applied). On a Cisco Systems router, neighbors with the same update policies can be grouped into peer groups to simplify configuration and, more importantly, to make updating more efficient and improve performance. Peer group – A group of BGP neighbors of the router being configured that all of the same update policies. A neighboring router can only be a part of one peer group. When you have many peers, this approach is highly recommended. You can use neighbor peer-group-name peer-group or neighbor peer- group command, the neighbor ip-address peer-group peer-group-name 119

AS has four routers running IBGP. All of these IBGP neighbors are peering with each others’ loopback 0 interface Router C has an outbound distribution list associated with each IBGP neighbor. the distribute-list command when used for BGP, it is linked to a specific neighbor. The ISP behind Router C might be announcing private address space to Router C, and Router C does not want to pass these networks to other routers running BGP in AS If Router C receives a change from AS 65101:  It must generate an individual update for each IBGP neighbor and  Run each update against distribute-list 20.  If Router C has a large number of IBGP neighbors, the processing power needed to inform the IBGP neighbors of the changes in AS could be extensive. 120 Router C

Router C is using a peer group called internal. These commands are all linked to peer group internal, which in turn is linked to each of the IBGP neighbors.  neighbor remote-as  neighbor update-source  neighbor next-hop-self  neighbor distribute-list 20 out If Router C receives a change from AS  It creates a single update and  Processes it through distribute-list 20 once.  The update is replicated for each neighbor that is part of the internal peer group. Saves processing time in generating the updates for all IBGP neighbors. Peer groups can improve efficiency when processing updates for BGP neighbors that have a common outbound BGP policy. 121

Filtering BGP Updates 122

BGP may receive a high number of routing updates. To optimize BGP configuration, route filtering may be applied. Filter lists, prefix lists, and route maps can be applied to either incoming or outgoing BGP information, or in any combination.  Incoming prefix list, filter list, and route map must all permit the routes that are received from a neighbor before they will be accepted into the BGP table.  Outgoing routes must pass the outgoing filter list, prefix list, and route map before they will be transmitted to the neighbor. Redistribution from an IGP into BGP, the routes must successfully pass any prefix list or route map applied to the redistribution process before the route is injected into the BGP table. 123

BGP Filtering Using Prefix Lists prefix-list ANY-8to24-NET is applied to the incoming advertisements from the BGP neighbor Permits routes from:  /0: any network  ge 8 le 24: with a mask length from 8 to 24 bits 124

BGP Filtering with Route Maps To apply a route map to filter incoming or outgoing BGP routes, use: neighbor ip address route-map name {in | out} 125

Router A is configured for BGP with two neighbors: and Both neighbors are configured with the neighbor route-map command to filter the incoming routing update traffic according to the route-map named filter. The route map filter:  Matches default route AND from AS  Assigned a weight value of 150  Matches default route (from all other AS’s)  Assigned a weight value of 100 Since a higher weight value is preferred, the link to ISP AS is preferred 126 Matches all routes /0 Weight= /0 Weight=

Other Attribute Examples (FYI) 127

128 neighbor weight command Modify WEIGHT E1 so it prefers the longer AS_PATH through ISP1. E2 and EX will still prefer the shorter AS_PATH through ISP2 because of equal WEIGHTs but shorter AS_PATH.

129 E1#show ip bgp Network Next Hop Metric LocPrf Weight Path * i * i *>i i E1#show ip route B /8 [200/0] via , 00:39:01 Before modifying WEIGHT. With WEIGHTs equal, E1 prefers ISP2 because of shorter AS_PATH

130 E1(config)#router bgp 100 E1(config-router)# neighbor weight 2000 E1(config-router)# neighbor weight 2000 E1(config-router)# neighbor weight 1000 E1(config-router)# neighbor weight 1000 E1 AS 1 (ISP1) 2000 AS 100 (E2) 1000 Higher weight preferred Routes from ISP1 get a weight of 2000 Routes from E2 and EX get a weight of 1000 Larger weight preferred

131 E1#show ip bgp Network Next Hop Metric LocPrf Weight Path * i *> i * i i *> / i * i i s> / i s> / i * i *> i *>i i E1#show ip route B /24 [200/0] via , 00:00:12 B /8 [20/0] via , 00:00: /16 is variably subnetted, 3 subnets, 2 masks O /24 [110/2] via , 00:48:24, FastEthernet0/0 C /24 is directly connected, FastEthernet0/0 B /19 [200/0] via , 00:00:12, Null /30 is subnetted, 2 subnets C is directly connected, Serial0/0 C is directly connected, Serial0/1 B /16 [20/0] via , 00:00:14 Only the BGP routes from E2 have the WEIGHT of The BGP routes from ISP1 have a WEIGHT of 1000.

132 E2#show ip bgp BGP table version is 11, local router ID is Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path * i i *> i * i / i *> i s> / ? s> / ? *>i i *> i E2#show ip route B /8 [20/0] via , 00:49:40 WEIGHT is not included in BGP updates. E1 prefers longer path (larger WEIGHT) via ISP1 but… E2 and EX are unchanged with WEIGHTs remaining equal preferring shorter AS_PATH.

133 LOCAL PREFERENCE Attribute Configuration of WEIGHT removed. Multiple routes to the same destination ( ), AS 100 Enterprise routers prefer shorter AS_PATH to ISP2 via E2. The weight attribute was local to the router on which it is assigned, and it is not propagated in routing updates. Multiple routes to the same destination ( /8), we want ALL AS 100 Enterprise routers to prefer ISP1 - not just E1 (WEIGHT). The path with the higher preference is preferred (the default value of the local preference attribute is 100) Local Preference

134 E1(config)#router bgp 100 E1(config-router)#bgp default local-preference 200 E2(config)#router bgp 100 E2(config-router)#bgp default local-preference

135 E1#show ip bgp Network Next Hop Metric LocPrf Weight Path * i *> i * i / i *> i s> / i s> / i * i *> i *>i i E1#show ip route B /24 [200/0] via , 00:01:01 B /8 [20/0] via , 00:01: /16 is variably subnetted, 3 subnets, 2 masks O /24 [110/2] via , 00:18:33, FastEthernet0/0 C /24 is directly connected, FastEthernet0/0 B /19 [200/0] via , 00:18:34, Null /30 is subnetted, 2 subnets C is directly connected, Serial0/0 C is directly connected, Serial0/1 B /16 [20/0] via , 00:18:21

136 E2#show ip bgp Network Next Hop Metric LocPrf Weight Path * i *>i i *> / i * i i s> / ? s> / ? *>i i *> i E2#show ip route B /24 [20/0] via , 00:01:46 B /8 [200/0] via , 00:01: /16 is variably subnetted, 3 subnets, 2 masks C /24 is directly connected, FastEthernet0/0 O /24 [110/2] via , 00:19:23, FastEthernet0/0 B /19 [200/0] via , 00:01:31, Null /30 is subnetted, 1 subnets C is directly connected, Serial0/0 B /16 [200/0] via , 00:01:47

137 EX#show ip bgp Network Next Hop Metric LocPrf Weight Path *>i i * i / i *>i i *>i i *>i i EX#show ip route B /24 [200/0] via , 00:02:32 B /8 [200/0] via , 00:03: /16 is variably subnetted, 3 subnets, 2 masks C /24 is directly connected, FastEthernet0/1 C /24 is directly connected, FastEthernet0/0 B /19 [200/0] via , 00:20:06 B /16 [200/0] via , 00:19:51

138 MED (Multi-Exit Discriminator) Attribute New Topology No ISP2 ISP1 prefers path to /7 via E1 AS 100 wants ISP1 to prefer path to /7 via E2 WLam Weight Local Preference as path med

139 MED (Multi-Exit Discriminator) Attribute The multi-exit discriminator (MED) attribute is a hint to external neighbors about the preferred path into an AS when there are multiple entry points into the AS. A lower MED value is preferred over a higher MED value. The default value of the MED attribute is 0. Unlike local preference, the MED attribute is exchanged between AS’s, but a MED attribute that comes into an AS does not leave the AS. WLam Weight Local Preference as path med

140 ISP1#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i * / i *> i *> i ISP1#show ip route C /8 is directly connected, Loopback /19 is subnetted, 1 subnets B [20/0] via , 01:01: /30 is subnetted, 2 subnets C is directly connected, Serial0/0 C is directly connected, Serial0/1 C /16 is directly connected, Loopback180 Currently…

141 E1(config)#router bgp 100 E1(config-router)#neighbor route-map SETMEDOUT out E1(config)#route-map SETMEDOUT permit 10 E1(config-route-map)#set metric 50 E2(config)#router bgp 100 E2(config-router)#neighbor route-map SETMEDOUT out E2(config)#route-map SETMEDOUT permit 10 E2(config-route-map)#set metric 10 MED 50 MED 10 The multi-exit discriminator (MED) attribute is a hint to external neighbors about the preferred path into an AS when there are multiple entry points into the AS.

142 ISP1#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i * / i *> i *> i ISP1#show ip route C /8 is directly connected, Loopback /19 is subnetted, 1 subnets B [20/10] via , 00:00: /30 is subnetted, 2 subnets C is directly connected, Serial0/0 C is directly connected, Serial0/1 C /16 is directly connected, Loopback180 Note: If you want MED attributes from neighbors in other AS’s to be compared, you must configure the bgp subcommand bgp always-compare- med. MED is sometimes referred to as the metric MED 50 MED 10

143 AS_PATH and Prepending an AS We want to make it look as if the /8 in AS 14 network can be reached via ISP1 and ISP2. We will add this network to both routers and prepend some AS numbers to make it look like it originated in AS 14 from several AS’s away. Sometimes used by ISPs to prepend their own AS number several times to make a path look less desirable /8 AS WLam Weight Local Preference as path med With all else equal shortest AS_PATH is best

144 ISP1(config)#inter loop 99 ISP1(config-if)#ip add ISP1(config)#router bgp 1 ISP1(config-router)#network ISP1(config-router)#neighbor route-map set-99 out ISP1(config-router)#neighbor route-map set-99 out ISP1(config)#route-map set-99 permit 10 ISP1(config-route-map)#match ip address prefix-list only-99 ISP1(config-route-map)#set as-path prepend ISP1(config)#route-map set-99 permit 20 ! All other routes sent as normal ISP1(config)#ip prefix-list only-99 seq 5 permit /8

145 ISP2(config)#inter loop 99 ISP2(config-if)#ip add ISP2(config)#router bgp 1 ISP2(config-router)#network ISP2(config-router)#neighbor route-map set-99 out ISP2(config)#route-map set-99 permit 10 ISP2(config-route-map)#match ip address prefix-list only-99 ISP2(config-route-map)#set as-path prepend ISP2(config)#route-map set-99 permit 20 !All other routes sent as normal ISP2(config)#ip prefix-list only-99 seq 5 permit /8

146 E1#show ip bgp Network Next Hop Metric LocPrf Weight Path * i * i *>i i * i / i *> i s> / i s> / i *> i * i *>i i E1#show ip route B /24 [200/0] via , 00:39:01 B /8 [200/0] via , 00:39: /16 is variably subnetted, 3 subnets, 2 masks O /24 [110/2] via , 00:39:18, FastEthernet0/0 C /24 is directly connected, FastEthernet0/0 B /19 [200/0] via , 00:39:12, Null /30 is subnetted, 2 subnets C is directly connected, Serial0/0 C is directly connected, Serial0/1 B /16 [20/0] via , 00:38:54 E1 does not send this longer path to E2 and EX because it heard better path from both. Best Path via E2

147 E2#show ip bgp Network Next Hop Metric LocPrf Weight Path *> i * i / i *> i s> / ? s> / ? *>i i *> i E2#show ip route B /24 [20/0] via , 00:39:30 B /8 [20/0] via , 00:39: /16 is variably subnetted, 3 subnets, 2 masks C /24 is directly connected, FastEthernet0/0 O /24 [110/2] via , 00:39:40, FastEthernet0/0 B /19 [200/0] via , 00:39:40, Null /30 is subnetted, 1 subnets C is directly connected, Serial0/0 B /16 [200/0] via , 00:39:26

148 EX#show ip bgp Network Next Hop Metric LocPrf Weight Path *>i i * i / i *>i i *>i i *>i i EX#show ip route B /24 [200/0] via , 00:18:20 B /8 [200/0] via , 00:18: /16 is variably subnetted, 3 subnets, 2 masks C /24 is directly connected, FastEthernet0/1 C /24 is directly connected, FastEthernet0/0 B /19 [200/0] via , 00:18:21 B /16 [200/0] via , 00:18:21 EX#

CIS 185 CCNP ROUTE Ch. 6 Border Gateway Protocol Solution for ISP Connectivity – Part 2 Rick Graziani Cabrillo College Last Updated: Fall 2010