BROCADE IP PRIMER

Configuring OSPFIf you use a loopback address, the Router ID doesn't change. It's also certainlynot contingent on whether one of the physical interfaces is up or down. Theloopback is the IP address of the device, not an individual interface.Common practice would be to choose a unique /30 network (some even use a/32 network), assign an IP address to a loopback interface, and assign thatinterface to be a member of a particular area.BR-Switch#conf tBR-Switch(config)#int lo 1BR-Switch(config-if-lbif-1)#ip address 192.168.100.1/30BR-Switch(config-if-lbif-1)#ip ospf area 0Now a single interface can only be a member of one OSPF area. What do youdo if you're configuring an ABR? Simple, create another loopback interface.Most switches will allow up to four.CostLet's talk a little bit more about cost. For any given link, OSPF advertises thecost of the link. The cost is 100 divided by the bandwidth of the link (in Mbps).A 10 Mbps link would have a cost of 10 (100 ÷ 10 = 10). A DS3 (45 Mbps) linewould have a cost of 3 (100 ÷ 45 = 2.222; this will be “rounded” up to 3). A100 Mbps link would have a cost of 1 (100 ÷ 100 = 1). As you can see, withthis system, a higher capacity link has a lower cost, and therefore a higherdesirability.But what about a gigabit link? It would also have a cost of 1 (100 ÷ 1,000 =0.1; this will be rounded up to 1). Now we have a problem. If our network is allEthernet, all of the links (100 Mb or more) will have the same cost! We wouldwant our gigabit and 10-gigabit links to be more highly favored over our 100Mb links. There are two ways to take care of this.One way is to manually configure the cost. This is done at the interface level.Instead of letting OSPF calculate the cost, assign one yourself:BR-Switch#conf tBR-Switch(config)#int e 5BR-Switch(config-if-e1000-5)#ip ospf cost 20We chose a cost of 20 here, but you can choose any 24-bit number (1-16,777,215). The lower the cost, the more desirable the link. Engineers willoften assign a manual cost to make a link more (or less) desirable, even ifthere's a higher bandwidth alternative. It just depends on your infrastructure.That may be okay if you've just got a couple of links to distinguish, but what ifyou have a lot? Wouldn't it be easier to just to tell OSPF to use a larger numberthan 100 to calculate cost?BR-Switch#conf tBR-Switch(config)#router ospfBR-Switch(config-ospf-router)#auto-cost reference-bandwidth10000Brocade IP Primer 251