Border Gateway Protocol (BGP) W.lilakiatsakun

BGP Basics (1) BGP is the protocol which is used to make core routing decisions on the Internet It involves a table of IP networks or "prefixes" which designate network reachability among autonomous systems (AS). RFC 4271 BGP version 4 is a De facto standard for exterior gateway protocol Run over TCP port 179

BGP Basics (2) The Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets configured by a network administrator. The major enhancement in version 4 was support for Classless Inter-Domain Routing and use of route aggregation to decrease the size of routings.

BGP Messages (1) BGP Messages 1 - OPEN 2 - UPDATE 3 - NOTIFICATION 4 - KEEPALIVE

BGP Messages (2) OPEN Message – After a TCP connection is established, the first message sent by each side is an OPEN message. – If the OPEN message is acceptable, a KEEPALIVE message confirming the OPEN is sent back.

BGP Messages (3) KEEP ALIVE – BGP does not use any TCP-based, keep-alive mechanism to determine if peers are reachable. Instead, KEEPALIVE messages are exchanged between peers often enough not to cause the Hold Timer to expire. – A reasonable maximum time between KEEPALIVE messages would be one third of the Hold Time interval. – KEEPALIVE messages MUST NOT be sent more frequently than one per second.

BGP Messages (4) NOTIFICATION – A NOTIFICATION message is sent when an error condition is detected. – The BGP connection is closed immediately after it is sent.

BGP Messages (5) UPDATE – UPDATE messages are used to transfer routing information between BGP peers. – The information in the UPDATE message can be used to construct a graph that describes the relationships of the various Autonomous Systems.

BGP Messages (6) UPDATE (con’t) – An UPDATE message is used to advertise feasible routes that share common path attributes to a peer, or to withdraw multiple unfeasible routes from service – An UPDATE message MAY simultaneously advertise a feasible route and withdraw multiple unfeasible routes from service.

BGP Operations (1) Learns multiple paths via internal and external BGP speakers Picks the best path and installs in the forwarding table Best path is sent to external BGP neighbors Policies applied by influencing the best path selection

BGP Operations (2) BGP neighbors, called peers, are established by manual configuration between routers to create a TCP session on port 179. A BGP speaker sends 19-byte keep-alive messages every 30 seconds to maintain the connection. Among routing protocols, BGP is unique in using TCP as its transport protocol.

BGP Operations (3) When BGP runs between two peers in the same autonomous system (AS), it is referred to as Internal BGP (iBGP) When it runs between different autonomous systems, it is called ExternalBGP (eBGP) The main difference between iBGP and eBGP peering is in the way routes that were received from one peer are propagated to other peers

BGP Operations (4) For instance, new routes learned from an eBGP peer are typically redistributed to all other iBGP peers as well as all eBGP peers (if transit mode is enabled on the router). However, if new routes were learned on an iBGP peering, then they are re-advertised only to all other eBGP peers. These route-propagation rules effectively require that all iBGP peers inside an AS are interconnected in a full mesh.

eBGP & iBGP BGP used internally (iBGP) and externally (eBGP) iBGP used to carry some/all Internet prefixes across ISP backbone ISP’s customer prefixes eBGP used to exchange prefixes with other Ases implement routing policy

External BGP Peering (eBGP) Between BGP speakers in different AS Should be directly connected Never run an IGP between eBGP peers

Configuring eBGP

Internal BGP (iBGP) BGP peer within the same AS Not required to be directly connected – IGP takes care of inter-BGP speaker connectivity iBGP speakers need to be fully meshed – they originate connected networks – They do not pass on prefixes learned from other iBGP speakers

Internal BGP peering

Configuring iBGP

BGP Attributes (1) Well-known attributes – must be supported by every BGP implementation Mandatory attributes – must be included with every route entry. If one attribute is missing, it will result in an error message – Ex: ORIGIN, AS_PATH, NEXT_HOP, LOCAL_PREF

BGP Attributes (2) Discretionary attributes – every BGP router must recognize, but they don’t have to be present with every route entry – Ex. ATOMIC_AGGREGATE Optional attributes – not necessarily supported by all BGP implementations. It can be either transitive or non-transitive. – Ex. AGGREGATOR, COMMUNITY, MULTI_EXIT_DISC

BGP Attributes (3) Origin AS-Path Next Hop Multi_Exit_Disc Local Preference Atomic_aggregrate Aggregrator

Origin ORIGIN is a well-known mandatory attribute. The ORIGIN attribute is generated by the speaker that originates the associated routing information. Three values: IGP, EGP, incomplete – IGP generated by BGP network statement – EGP generated by EGP – Incomplete redistributed from another routing protocol

AS_Path (1) This attribute identifies the autonomous systems through which routing information carried in this UPDATE message has passed.

AS_Path (2) AS_Path is Used for – Loop detection – Path metrics where the length of the AS Path is used as in path selection

AS_Path (3) AS_Path Loop Detection

AS_Path (4) When a BGP speaker propagates a route it learned from another BGP speaker’s UPDATE message, it modifies the route’s AS_PATH attribute based on the location of the BGP speaker to which the route will be sent only when a given BGP speaker advertises the route to an external peer.

Next Hop (1) The NEXT_HOP defines the IP address of the router that SHOULD be used as the next hop to the destinations listed in the UPDATE message Well known mandatory attribute

Next Hop (2) The IP address to reach the next AS – Router A advertise /16 and /16 to router B in eBGP with next hop (Change it to own IP) – Router B will update Router C in iBGP keeping the next hop unchanged

Next Hop (3) IOS default is for external next-hop to be propagated unchanged to iBGP peers – This means that IGP has to carry external next- hops ISP Best Practice is to change external next- hop to be that of the local router – neighbor x.x.x.x next-hop-self

Multi_Exit_Disc (1) The MULTI_EXIT_DISC is an optional non- transitive attribute that is intended to be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four-octet unsigned number, called a metric. All other factors being equal, the exit point with the lower metric SHOULD be preferred.

Multi_Exit_Disc (2) MED

Multi_Exit_Disc (3) For prefix /24 Router B send MED 1000 and router A send MED 2000 to eBGP neighbor Incoming traffic from AS200 will choose Router B since lowest MED will win

Local Preference (1) Local preference is used to advertise to IBGP neighbors only about how to leave their AS (Outbound Traffic). Paths with highest preference value are most desirable Local preference attribute is well-known and discretionary and is passed only within the AS Cisco Default Local Pref is 100

Local Preference (2) For destination /16 Router A advertise local pref 500 and Router B advertise local pref 800 in iBGP 800 will win best path (Router B)

Atomic_aggregate (1) If an aggregate excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD include the ATOMIC_AGGREGATE attribute

Atomic_aggregate (2) Conveys the IP address of the router/BGP Speaker generating the aggregate route Useful for debugging purposes Does not influence best path selection

Aggregator AGGREGATOR is an optional transitive attribute, which MAY be included in updates that are formed by aggregation A BGP speaker that performs route aggregation MAY add the AGGREGATOR attribute, which SHALL contain its own AS number and IP address. The IP address SHOULD be the same as the BGP Identifier of the speaker.

Path Selection (1) If the NEXT_HOP attribute of a BGP route depicts an address that is not resolvable, or if it would become unresolvable if the route was installed in the routing table, the BGP route MUST be excluded from the decision function. If the AS_PATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the decision function. – AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path.

Path Selection (2) Step 1: Prefer highest weight (local to router) Step 2: Prefer highest local preference (global within AS) Step 3: Prefer route originated by the local router Step 4: Prefer shortest AS path Step 5: Prefer lowest origin code (IGP < EGP < incomplete)

Path Selection (3) Step 6: Prefer lowest MED (from other AS) Step 7: Prefer EBGP path over IBGP path Step 8: Prefer the path through the closest IGP neighbor Step 9: Prefer oldest route for EBGP paths Step 10: Prefer the path with the lowest neighbor BGP router ID