网络工程专业英语课件ch4 Network Layer.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Network Layer 4-* BGP basics Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP conctns: BGP sessions Note that BGP sessions do not correspond to physical links. 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session Network Layer 4-* Distributing reachability info With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute this new prefix reach info to all routers in AS1 1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session When router learns about a new prefix, it creates an entry for the prefix in its forwarding table. 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session Network Layer 4-* Path attributes BGP routes When advertising a prefix, advert includes BGP attributes. prefix + attributes = “route” Two important attributes: AS-PATH: contains the ASs through which the advert for the prefix passed: AS 67 AS 17 NEXT-HOP: Indicates the specific internal-AS router to next-hop AS. (There may be multiple links from current AS to next-hop-AS.) When gateway router receives route advert, uses import policy to accept/decline. Network Layer 4-* BGP route selection Router may learn about more than 1 route to some prefix. Router must select route. Elimination rules: Local preference value attribute: policy decision Shortest AS-PATH Closest NEXT-HOP router: hot potato routing Additional criteria Network Layer 4-* BGP messages BGP messages exchanged using TCP. BGP messages: OPEN: opens TCP connection to peer and authenticates sender 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 Network La