BROCADE IP PRIMER

Console49F Link 50F1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47Power1F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 4849F Link 50FPowerConsole1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 4849F Link 50FPowerConsole1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 4849F Link 50FPowerConsole1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48Chapter 11: Open Shortest Path First (OSPF)Notice that this was configured in the global OSPF config. Now, it will divide thebandwidth in Mbps into 10,000. This means that a 10 Mbps link will now cost1,000; a 100 Mbps link will cost 100; a gigabit link will cost 10; and a 10-Gigabitlink will cost 1. The number that you use for your reference bandwidth (inthis example 10,000) can be any number between 1 and 4,294,967.Now that we know how to adjust the costs, how do the costs play into our routingdecisions? Consider the following image.1 GbpsCost 101 GbpsCost 10Router BCost 100100 Mbps1 GbpsCost 10Cost 110 GbpsRouter DRouter ARouter Creference-band with 10000Router A has two paths to get to Router C. The most obvious path (visually)would be the directly connected path (the 100 Mbps line). This has a cost of100. However, if Router A were to go through to Router B (cost of 10) and fromB to Router C (cost of 10 again), it would be a total cost of 20. This cost wouldbe considerably less than 100 (the cost of its direct connection to C). It wouldchoose the path through Router B.Now, what about from Router A to Router D? There are more paths it couldtake. Router A to B (cost of 10) to D (cost of 10) would be a total cost of 20.Router A to B (cost of 10) to C (cost of 10) to D (cost of 1) would be a total costof 21. Router A to C (cost of 100) to D (cost of 1) would be a total cost of 101.Which route will Router A prefer? The path from A to B to D (total cost of 20).That's the cheapest route.Adjusting OSPF TimersBy default, all OSPF interfaces will send a Hello packet to their neighbors every10 seconds. This is defined as a 16-bit number, so any number of secondsfrom 1-65,535 may be used. I don't recommend you delay the Hello intervaltoo long, as this is a means for the router to be aware that his neighbor may bedown. This setting is customized to the individual interface. Here's an examplein which we change this interface's Hello interval to be 20 seconds, instead ofthe default 10:BR-Switch#conf tBR-Switch(config)#int e 5BR-Switch(config-if-e1000-5)#ip ospf hello-interval 20The Router Dead Interval is the amount of time a router will wait before declaringa neighbor dead. By default, if an interface does not receive a Hello packetfrom its neighbor in 40 seconds, it declares that neighbor down. This is configuredon the individual interface. This is also a 16-bit (0-65,535) number (inseconds).252 Brocade IP Primer