BROCADE IP PRIMER

Chapter 13: Border Gateway Protocol (BGP)long time. It is not uncommon for Internet BGP routers to have sessions thathave not been reset for many months (even years). Every bit of communicationthat needs to be done between the two neighbors is done using that existingsession.Peering StatesWhen neighbors peer, they may go through six states. These often happen soquickly that most are not seen, but if you're troubleshooting a problem, youmay recognize some of these states:• Idle. Nothing's happened yet; the neighbor has been defined, but thepeering session hasn't started; you may see this state if a peering sessionhas been hard reset, or if a connection has been failing, and it is about totry again• Connect. The router has sent a TCP SYN on TCP port 179 to the neighbor;if it succeeds and the three-way handshake completes, we move to theOpenSent state; if it fails, the router will continue to listen on TCP 179 (forthe neighbor to initiate a session), and keep trying; if you see a neighborin Connect state, it is most likely a problem with Layer 3; try pinging theneighbor; if it can't reach the neighbor via Layer 3, it certainly isn't goingto reach it via Layer 4 (TCP)• Active. You'd think this is a favorable state to see; it isn't; Active meansthat the router has attempted to establish a TCP session with the neighborand it has failed for some reason; it also means that it is “actively”listening on TCP 179 for the neighbor to make a connection; if you see aneighbor hung in Active, you've either got a Layer 4 problem (e.g., a firewallbetween the router and the neighbor is not permitting TCP 179traffic) or perhaps the neighbors are not defined at both ends; you willalso see this if you are using an MD5 key, and the key is not the same onboth routers• OpenSent. The router is waiting for an OPEN message from the neighborit just established a TCP session with; it's the neighbor's way of saying,“I'm ready”; upon receiving the OPEN message, the router will send backa KEEPALIVE message to the neighbor as its way of saying, “Got it”; if yousee a router hung in this state, there is probably something wrong withthe neighboring router (perhaps under heavy load?); another possibility isa Layer 7 firewall that is, for some reason, allowing TCP 179, but notallowing BGP OPEN messages• OpenConfirm. The router has sent its own OPEN message to the neighborand is waiting for the KEEPALIVE message to come back; if a router ishung in this state, you've either got something wrong with your neighbor'srouter, or a bizarre Layer 7 firewall configuration in between the two peers• Established. Once the KEEPALIVE has been heard from the neighbor, weswitch to Established state, and the initial exchange of routes begins; thepeer will remain in this state, unless the session is somehow interrupted(e.g., link goes down, router is rebooted, etc.)278 Brocade IP Primer