BROCADE IP PRIMER

BI4XGBI24CT X RX T X RX T X RX T X RXBI4XGBI24CT X RX T X RX T X RX T X RXBI24C6 12 18 2430 36 42 48BI24CBigIron RX-8NETWORKS1 7 13 1925 31 37 43BI24CBI24CBI24CAC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMBI24CBI4XGBI4XGT X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RXEJECT SYSBI4XGBI4XGBI24CBI24CBI24C16T X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RX12BigIron RX-8NETWORKS7131819242530313637424348AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMBI24CBI24CBI24CBI24CBI24CBI4XGBI4XGT X RX T X RX T X RX T X RXT X RX T X RX T X RX T X RXEJECT SYSChapter 13: Border Gateway Protocol (BGP)Consider this illustration:INTERNETISP A2.2.2.1/30AS 1234AS 12344.0.0.0/8eB GP2.2.2.2/30AS 400iB GP1.1.1.1/301.1.1.2/30Router ARouter BISP A has advertised a route for 4.0.0.0/8 which is in its own AS (1234). ISP Atells Router A that the next hop for this network is 2.2.2.1. Router A has aniBGP peer with Router B. Router A sends the route to Router B: 4.0.0.0/8 in AS1234 has a next-hop of 2.2.2.1. Great, right?Router B has a problem. He doesn't know how to get to 2.2.2.1! BGP makes noassumptions. Just because he received the route from Router A doesn't meanthat Router A knows how to get there either. And it doesn't matter. Router Blooks in its routing table and finds no entry for a 2.0.0.0/8 (or any subaddresses) anywhere! This becomes a dead route.Somehow, we need Router A to relabel the route to show that it is Router A thatis the next hop. In Router A's BGP config, we need the following:BR-RouterA(config)#router bgpBR-RouterA(config-bgp-router)#neighbor 1.1.1.2 remote-as 400BR-RouterA(config-bgp-router)#neighbor 1.1.1.2 next-hop-selfThe “next-hop-self” tells Router A that when it advertises its eBGP routes toRouter B (neighbor 1.1.1.2), it should show the next hop router as itself (RouterA). With this addition, Router B will see the route for 4.0.0.0/8 in AS 1234, andit will have a next-hop of 1.1.1.1 (Router A). Next-hop-self is almost always awise move when you're using iBGP.eBGP - multihopWhat happens when you have an Internet router that is not powerful enough torun BGP? Or maybe you've got a nice BigIron to run your eBGP, but you want toput a firewall in front of it? While some firewalls can run BGP, generally speaking,BGP is not a firewall's area of expertise. It's designed for packet analysis,not routing.282 Brocade IP Primer