1. CS 672 1 Summer 2003 Lecture 3

2. CS 672 2 Summer 2003 What is a BGP Path Attribute? BGP uses a set of parameters known as path attributes to characterize a route. BGP path attributes are similar in nature to the IGP metrics. BGP Update message contains variable number of path attributes.

3. CS 672 3 Summer 2003 BGP Path Attributes 1OriginWell-known, mandatory 2AS_PathWell-known, mandatory 3Next_HopWell-known, mandatory 4Multi_Exit_Disc (MED)Optional non-transitive 5Local_PrefWell-known, discretionary 6Atomic_AggregateWell-known, discretionary 7AggregatorOptional, transitive 8CommunityOptional, transitive) ……… Attribute NameAttribute Type Code Category

4. CS 672 4 Summer 2003 BGP Path Attributes (Well-known) Each Path Attribute belongs to one of following four categories: Well-known Mandatory  Attribute is recognized by all BGP implementations  Included in each Update message Well-known Discretionary  Attribute is recognized by all BGP implementations  It may or may not be present in an Update message

5. CS 672 5 Summer 2003 BGP Path Attributes (Optional) Well-known attributes are always transitive (i.e., to be propagated to peers). The Optional attributes can be transitive or non-transitive. Optional Transitive  Attribute is not recognized by all BGP implementations  It may or may not be present in an Update message  If present and transitive flag is set, the receiving BGP speaker should pass it on to other peers Optional Non-transitive  Attribute is not recognized by all BGP implementations  It may or may not be present in an Update message  If present and the non-transitive flag is set, the BGP speaker should NOT pass it on to other peers

6. CS 672 6 Summer 2003 Path Attributes Encoding Each Path Attribute is encoded as a triple or TLV format. Attribute Type field (2 byte) Contains subfields (1 byte each) namely Attribute Flags and Attribute Type Code The attribute Flags field is used for categorization of the attribute (e.g., Well-known Mandatory, Well-known Discretionary, …) The attribute type code value is used to identify a particular Path Attribute (e.g., type code = 1 means Origin, type code = 2 means AS_Path, …)

7. CS 672 7 Summer 2003 Path Attribute Encoding b0 = 0 means Well-known attribute b0 = 1 means Optional attributed b1 = 0 means the Optional attribute is non-transitive b1 = 1 means the Optional attribute is transitive Attribute Flags Attribute Type Code b0 b1 b2 b3 Unused bits 1.Origin (Attribute Type Code =1) 2.AS_Path (Attribute Type Code = 2) 3.Next_Hop (Attribute Type Code = 3) …

8. CS 672 8 Summer 2003 Routing Information Bases (RIBs) BGP stores its routes in RIB. BGP routing information bases can be further categorized as: Adj-RIBs-In  Stores the routes learned via Update messages from peers Adj-RIBs-Out  Stores the routes selected for advertisement to peers Loc-RIB  Stores routes that have been selected by the decision process BGP keeps its forwarding state in forwarding information base (FIB). FIB is the routing information that is used for forwarding.

9. CS 672 9 Summer 2003 BGP Decision Process BGP takes Adj-RIBs-In as input and carries out a three phase decision function for selecting and advertising the best routes. Phase I: determine a degree of preference (e.g., using Local_Pref path attribute or preconfigured local policy) for each route learned from neighbors Phase II: select the best route out of all available routes for each destination and install it in the Loc-Rib. All routes with unfeasible BGP next hop addresses (i.e., Next_Hop address) are excluded from the second phase decision function. Phase III: take Loc-RIB as input, processes it (e.g., filter, manipulate path attributes) to influence routing behavior, generate a corresponding entry in the Adj-RIBs-Out.

10. CS 672 10 Summer 2003 Adj-RIBs-Out Adj-RIBs-In Input filtering and Path attribute manipulation Output filtering and Path attribute manipulation Decision Process Loc-RIB FIB Locally originated routes Adj-RIBs-In Loc-RIB FIB Adj-RIBs-Out Input filtering and Path attribute manipulation Output filtering and Path attribute manipulation Decision Process Locally originated routes Advertise routes via Update message BGP Speaker 1 BGP Speaker 2 BGP Peers

11. CS 672 11 Summer 2003 AS100 AS200 eBGP IGPStatic Routes BGP IGP BGP IGP/static routes injected into BGP Update (NLRI, attributes)

12. CS 672 12 Summer 2003 BGP Route Selection BGP uses path attributes to select best routes and influence decision process of peers. The selection of a most preferable route is performed as follows: 1. From the routes available in Adj-RIBs-In, exclude routes with inaccessible next hop address. 2. Select the routes with largest local preference. 3. If local preference is same, select a locally (by the router) originated route. 4. If still multiple candidates, select the route with the shortest AS_Path. 5. If the AS_Path is same, select the route with lowest value of origin type (0=IGP, 1=EGP, 2=Incomplete). 6. If origin type same, select the route with lowest MED. 7. …..

13. CS 672 13 Summer 2003 Origin Attribute Origin (Attribute Type Code = 1) is a Well-Known path attribute that defines the origin of the routing information contained in the Update message. Origin attribute is created by the AS that originates routing information (i.e., NLRI) The receiving speakers pass along the Origin parameter in their UPDATE messages when advertising corresponding routing information to other speakers. BGP considers following origin types: (0=)IGP - Network Layer Reachability Information (NLRI) is internal to the originating AS (1=)EGP - NLRI is learned via EGP (e.g., BGP) in the originating AS (2=)INCOMPLETE – NLRI is learned by some other means in the originating AS.

14. CS 672 14 Summer 2003 Update (NLRI=128.32.0.0, Origin = IGP) 2 R1 R2 R3 128.32.0.0 eBGP 1 3 Update (NLRI=128.32.0.0, Origin = IGP) 1 R1 learns about prefix (128.32.0.0) via IGP 2 R1 originates this route with Origin attribute set to IGP 3 After going through BGP decision process, R2 decides to advertise this route to its eBGP peer R3. R2 propagates the Origin attribute associated with the NLRI to R3.

15. CS 672 15 Summer 2003 AS_Path Attribute AS_Path attribute is a Well-known mandatory attribute AS_Path attribute identifies the AS’s traversed by the NLRI contained in the Update message The traversed AS’s are recorded in two ways: Ordered sequence of AS numbers (i.e., AS_Sequence list), or Unordered sequence of AS numbers (i.e., AS_Set list) When an iBGP speaker originates a route, it includes an empty AS_Path attribute list. A BGP speaker includes its AS number in the AS_Path attribute when advertising or originating a route to an eBGP peer. When a BGP speaker advertises a route received from an eBGP peer to an iBGP peer, it does not modify the AS_Path attribute.

16. CS 672 16 Summer 2003 1 R6 originates a BGP route for 179.16.12.0/24 with empty AS_Path 2 R4 advertises this route for 179.16.12.0/24 with AS_Path = 200 3 R3 advertises this route to R1 with AS_Path = 200 4 R1 advertises this route to R7 with AS_Path = 300, 200. AS 300 AS 200 R1 R2 R3 R4 R6 R5 iBGP eBGP iBGP R7 eBGP 2 1 3 4

17. CS 672 17 Summer 2003 AS_Path Attribute BGP uses AS_Path attribute to construct a loop-free AS topology graph. For instance, BGP does not accept an Update message if its own AS number is included in the AS_Path attribute. BGP also uses the AS_Path attribute in its decision process for selecting a best route. For example, when there are several routes to chose from, BGP selects the route with the shortest AS_Path attribute. AS_Path attribute is also used identify routes for applying input/output route filtering. In summary, manipulation of AS_Path attribute enable BGP to influence certain inter-AS routing behavior.

18. CS 672 18 Summer 2003 AS 100 AS 200 AS 300 172.64.16.0/16 1 Update (172.64..16.0/16, AS_Path = {AS100}) Update (172.64..16.0/16, AS_Path = {AS100, AS200}) 1 Update (172.64..16.0/16, AS_Path = {AS100, AS200, AS300}) 3 4AS100rejects this Update message

19. CS 672 19 Summer 2003 AS_Path and Routing Behavior We know that in case of a tie, a route with shorter AS_Path length is preferred. Thus AS_Path attribute can be used to influence inter-AS routing behavior. For example, BGP speaker can insert dummy AS numbers in the AS_Path attribute to increase its length to make it less preferable during the BGP decision process.

20. CS 672 20 Summer 2003 Update (NLRI= 172.64.16.0/16, AS_Path{AS100,AS200}) AS 100 AS 200 AS 300 AS 500 DS3 OC3 ( 155 Mb/s) OC3 AS 400 OC3 172.64.16.0/16 Update (NLRI= 172.64.16.0/16, AS_Path{AS100,AS300, As400}}) Traffic flow

21. CS 672 21 Summer 2003 Update (NLRI= 172.64.16.0/16, AS_Path{AS100,AS100,AS100, AS00,AS200}) AS 100 AS 200 AS 300 AS 500 DS3 OC3 ( 155 Mb/s) OC3 AS 400 OC3 172.64.16.0/16 AS100 inserts dummy AS numbers Update (NLRI= 172.64.16.0/16, AS_Path{AS100,AS300, As400}}) Update (NLRI= 172.64.16.0/16, AS_Path{AS100,AS100, AS100}) Traffic Flow