Border Gateway Protocol

What does the BGP do?

allows subnet to advertise its existence, and the destinations it can reach, to rest of Internet

What does BGP Provide each AS as a means to?

• obtain destination network reachability info from neighbouring ASes (eBGP)

• determine routes to other networks based on reachability information and policy

• propagate reachability information to all AS-internal routers (iBGP)

• advertise (to neighbouring networks) destination reachability info

What is a BGP Session?

two BGP routers (“peers”) exchange BGP messages over semi-permanent TCP connection

advertising paths to different destination network prefixes

What are some BGP Messages?

• OPEN - opens TCP connection to remote BGP peer and authenticates sending BGP peer

• UPDATE - advertises new path (or withdraws old)

• KEEPALIVE - keeps connection alive in absence of UPDATES; also ACKs OPEN request

• NOTIFICATION - reports errors in previous msg; also used to close connection

What is a BGP Advertised Route?

prefix + attributes

prefix - destination being advertised

attributes - AS-PATH - list of ASes through which prefix advertisement has passed ,NEXT-HOP - indicates specific internal-AS router to next-hop AS

What is Policy Based Routing?

• gateway receiving route advertisement uses import policy to accept/decline path (e.g., never route through AS Y)

• AS policy also determines whether to advertise path to other neighbouring ASes

What is Path Advertisment?

• AS2 router 2c receives path advertisement AS3,X (via eBGP) from AS3 router 3a

• based on AS2 policy, AS2 router 2c accepts path AS3,X, propagates (via iBGP) to all AS2 routers

• based on AS2 policy, AS2 router 2a advertises (via eBGP) path AS2, AS3, X to AS1 router 1c

What is Hot Potato Routing?

• 2d learns (via iBGP) it can route to X via 2b or 2c

• choose local gateway that has least intradomain cost (e.g., 2d chooses 2c over 2b)

What is the process for Route Selection?

1. select the route with the highest local preference value: (the local preference is configured by an AS to implement local policy decisions)

2. if two routes have the same local preference, then select the one with the shortest AS-PATH

3. if there is still a tie (same local preferences and same AS-PATH lengths) then choose the closest NEXT-HOP router: hot potato routing

Why does a Router selects one Inter-Domain Route per Destination?

• avoiding forwarding loops - using multiple unrelated AS paths risks creating inter-domain forwarding loops because BGP lacks the consistency guarantees of intra-domain routing

• stability and convergence - selecting a single best path prevents oscillations and ensures fast, stable convergence