BROCADE IP PRIMER

Console49F Link 50F1Power1F 2F 3F 4F325476981110131215141716191821202322252427262928313033323534373639384140434245444746 4849F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 4849F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 48Chapter 10: Routing Basics15. A hop count of 16 is interpreted as being impossible to reach. Typically, ifyou find yourself designing an infrastructure containing more than 16 routers,you're not going to want to use RIP anyway. We'll go more into that later.RIP actually utilizes this number to its advantage. For one, if a route goesdown, there's no way, in RIP, to tell your neighboring routers to remove a particularroute. Even if you don't readvertise it, they'll still have the one youoriginally advertised in their routing tables. RIP will advertise the route, but willgive it a hop count of 16. The routers will update their tables and consider theroute unreachable.Loop AvoidanceIn our example, you can actually see a physical loop. This means that it's theoreticallypossible for the routers to play an infinite game of “hot potato” withyour packets. In other words, Host A could send a packet to Switch A (destinedfor Host C). Switch A could say, “I know how to get to Host C- through Switch B”and hand it off to Switch B. Switch B says, “I know how to get to Host C-through Switch A” and hand it back to Switch A. Switch A will not rememberthat it originated the packet. It will simply see a packet coming in destined forHost C, and it will say, “I know how to get to Host C- through Switch B.” And thegame will continue forever (or until the upper-layer protocols decide it hastaken too long to get there).This is called a routing loop, and it's a bad thing. It doesn't usually abuse yourswitches like a Layer-2 loop does, but it is still a bugger to troubleshoot. Themain problem is that traffic does not get to its destination. It just bouncesbetween two or more routers, like a trapped pinball.RIP uses two methods to resolve this problem. The first (and default) is calledpoison reverse. Because Switch B will broadcast its entire routing table out allof its routing ports, Switch A might get back some routes that Switch B learnedfrom Switch A. What poison reverse does is keep track of which port a routewas learned from. When it advertises its routing table out that port, it changesthe routes that it learned from that port to have a hop count of 16(unreachable).e1192.168.1.1/30Switch Ave10172.16.0.1/24“Hey, I know how to getto 172.16.0.0/24,but it takes 16 hops.”e2192.168.3.1/30e2192.168.1.2/30“Hey, I know how to getto 172.16.0.0/24”Poison ReverseSwitch Bve20172.17.0.1/24Host B172.17.0.10/24e2192.168.2./30“Hey, I know how to getto 172.16.0.0/24”e2192.168.3.2/30e1192.168.2.1/30Switch Cve30172.18.0.1/24Host A172.16.0.10/24Host C172.18.0.10/24232 Brocade IP Primer