BROCADE IP PRIMER

BI4XGBI4XGBI24CBI24CBI24C16T 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 SYSBI4XGBI24CT 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 SYSNeighbor PeeringINTERNETAS 1234ISP AAS 65000AS 65001AS 65002All three of these customers are peering with ISP A using a different private ASnumber. Each of these three ASs are Stub ASs. They only have one link to theinfrastructure outside of their own AS (in this case, the Internet). But here's aunique thing. Each of the private ASs are depending on the same public AS. Inessence, their kind of a subdivided AS to the bigger AS (AS 1234). When multipleStub ASs are under the control of a single, larger AS, that's called aconfederation.Now, you may be looking at the Stub AS, and thinking, “Why are they evenusing BGP? Couldn't you just use a statically-configured default gateway, andhave ISP A advertise your IP addresses for you?” The answer is yes, and veryoften, a default gateway is all that's done. Probably the number one reason forcreating a BGP session for a Stub AS is for planning ahead. Sure, you're onlyusing one link and one ISP now, but who knows what the future will bring? Ifyou use a static default gateway, and you want to start multihoming, you'll haveto make an intrusive change (e.g., you'll experience a temporary loss in service)to implement it. But if you're already using BGP, adding another peer isoften not intrusive at all. Another reason may have to do with having controlover how your IP addresses are presented to the Internet, but that goesbeyond the scope of this book.Neighbor PeeringWith OSPF, the protocol discovered your neighbors for you (remember theneighboring process?). BGP does not using a self-discovery mechanism. Youhave to specifically define your neighbors (or, as they are sometimes called,peers). The neighbor definition must be configured on both sides (e.g., bothrouters), or the session will not succeed.Once the neighbors are defined on both routers, a TCP session (using TCP port179) will be created between the two routers. They will exchange their routes(that they have been configured to exchange; not necessarily their entire routingtable). Now, they will keep the same TCP session open, and use it toexchange updates as they happen. This initial TCP session is maintained for aBrocade IP Primer 277