Wed 18 Oct 2006
larger the priority number, the less likely the router will be elected the designated router. Give your most powerful router the lowest priority number. A designated router is used to reduce the size of the link-state database and thus the complexity of calculating the Dijkstra graph of the area. The designated router treats all other routers in the area as neighbors, but all other routers treat only the designated router as a neighbor. To understand how this reduces the size of the link-state database, think of a network of five routers. Without a designated router, all five routers advertise four neighbors, for a total of 20 neighbors in the database. With a designated router, only that router advertises four neighbors. The other four routers advertise one neighbor for a total of eight neighbors in the database. The larger the network, the more important it is to use a designated router. The router ospf statement enables the OSPF protocol. In Listing 7.13, the router ospf statement contains two clauses. The ospf router-id clause defines the router identifier used for OSPF advertisements. Normally, this is set to the primary address of one of the interfaces used for OSPF. On this sample router, only one network interface is being used for OSPF, so the address of that interface is used as the router identifier. The network clause identifies the network over which OSPF routes will be exchanged. In the ospfd.conf file, the network on the network clause is always defined by an IP-address/network-mask pair. The network clause also identifies the area of which the network is part. Remember that OSPF divides the autonomous system into areas, and every OSPF router must connect to some area. As mentioned earlier, the area that interconnects all other areas within the routing domain is called the backbone area. The number assigned to the backbone area is 0. Therefore, the network clause in Listing 7.13 specifies that this router is connected to the backbone area. A simple OSPF configuration, such as the one shown in Listing 7.13, should be adequate for any Linux system that needs to run OSPF. Much of the configuration information will come from the network designer who defines your routing hierarchy. The area you connect to, the type of authentication used, the authentication password, and the priority number of your system are all design decisions that will be made before your network even begins to run OSPF. As a final example of configuring the Zebra suite, we configure the router that attaches the OSPF network to the outside world via BGP. Running bgpd The zebra.conf file and the ospfd.conf file used on the BGP router are almost identical to the files shown in Listings 7.11 and 7.13, as the following cat commands show: [root]# cat /etc/zebra/zebra.conf hostname externalgw password BILL&ted enable password 4138doc log file /var/log/zebra.log ! interface eth0 ip address 172.16.1.5/24 multicast ! interface eth1 ip address 26.10.105.4/8 ! 216
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