Configuring a Multiarea OSPF Network

Problem

Your OSPF network has a number of physical and departmental boundaries, and, for administrative purposes and for scalability, you want to divide the network into a number of areas.

Solution

Configure a second OSPF area on the router:

[edit protocols] aviva@RouterG# set ospf area 0.0.0.1 interface fe-1/0/0.0

Discussion

To create additional areas in the OSPF network, configure the second area on the routers interface (or interfaces) that connects to the second area. In this recipe, we configure the second area, Area 0.0.0.1, on our router. This router acts as the ABR between the backbone area and Area 0.0.0.1. See Figure 12-2.

Check that the interface is configured:

aviva@RouterG> show ospf interface Interface State Area DR ID BDR ID Nbrs fe-0/0/1.0 DR 0.0.0.0 192.168.19.1 192.168.18.1 1 fe-1/0/1.0 BDR 0.0.0.0 192.168.17.1 192.168.19.1 1 fe-1/0/0.0 BDR 0.0.0.1 192.168.42.1 192.168.19.1 1

You see that interface fe-1/0/0 is up and is in Area 0.0.0.1.

Again, check that the neighbor is active:

aviva@RouterG> show ospf neighbor detail Address Interface State ID Pri Dead 10.0.1.1 fe-0/0/1.0 Full 192.168.18.1 128 33 area 0.0.0.0, opt 0x42, DR 10.0.1.2, BDR 10.0.1.1 Up 01:11:30, adjacent 01:11:30 10.0.0.2 fe-1/0/1.0 Full 192.168.17.1 128 39 area 0.0.0.0, opt 0x42, DR 10.0.0.1, BDR 10.0.0.2 Up 01:11:30, adjacent 01:11:25 10.0.16.1 fe-1/0/0.0 Full 192.168.42.1 128 31 area 0.0.0.1, opt 0x42, DR 10.0.16.1, BDR 10.0.16.2 Up 00:00:55, adjacent 00:00:55

In addition to showing that the neighbor is up (State is Full), the detail version of the show ospf neighbor command shows how long the interface and adjacency have been up, as well as the area number and the router IDs of the DR and BDR for the area.

Checking on RouterA at the other end of the connection, you see the connection to our router, RouterG, and that it is the designated router for Area 0.0.0.1:

aviva@RouterA> show ospf neighbor detail Address Interface State ID Pri Dead 10.0.16.2 fe-0/0/0.0 Full 192.168.19.1 128 39 area 0.0.0.1, opt 0x42, DR 10.0.16.1, BDR 10.0.16.2 Up 00:03:56, adjacent 00:03:56 10.0.21.2 se-0/0/3.0 Full 192.168.12.1 128 33 area 0.0.0.1, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0 Up 03:24:41, adjacent 03:24:41

OSPF collects topology information for each of the areas in their respective databases:

aviva@RouterG> show ospf database OSPF link state database, area 0.0.0.0 Type ID Adv Rtr Seq Age Opt Cksum Len Router 192.168.17.1 192.168.17.1 0x800000d0 1481 0x2 0xebd2 60 Router 192.168.18.1 192.168.18.1 0x80000083 1481 0x2 0xbd4a 60 Router *192.168.19.1 192.168.19.1 0x800000a1 78 0x2 0x45a3 60 Network *10.0.0.1 192.168.19.1 0x80000010 551 0x2 0x9b2e 32 Network *10.0.1.2 192.168.19.1 0x80000030 851 0x2 0x5353 32 Network 10.0.2.1 192.168.18.1 0x80000005 1949 0x2 0x993b 32 Summary *10.0.16.0 192.168.19.1 0x80000002 77 0x2 0xa81b 28 Summary *10.0.21.0 192.168.19.1 0x80000001 77 0x2 0xebc7 28 Summary *10.0.22.0 192.168.19.1 0x80000001 77 0x2 0x594d 28 Summary *172.19.121.0 192.168.19.1 0x80000001 77 0x2 0x69e 28 Summary *172.100.1.0 192.168.19.1 0x80000001 319 0x2 0x6368 28 Summary *192.168.12.1 192.168.19.1 0x80000001 77 0x2 0x1646 28 Summary *192.168.42.1 192.168.19.1 0x80000001 77 0x2 0x52f7 28 OSPF link state database, area 0.0.0.1 Type ID Adv Rtr Seq Age Opt Cksum Len Router 192.168.12.1 192.168.12.1 0x80000054 1223 0x2 0x1540 72 Router *192.168.19.1 192.168.19.1 0x80000002 77 0x2 0x771a 48 Router 192.168.42.1 192.168.42.1 0x8000005b 78 0x2 0x870d 96 Network 10.0.16.1 192.168.42.1 0x80000001 78 0x2 0x5147 32 Summary *10.0.0.0 192.168.19.1 0x80000001 77 0x2 0x5b79 28 Summary *10.0.1.0 192.168.19.1 0x80000001 77 0x2 0x5083 28 Summary *10.0.2.0 192.168.19.1 0x80000001 77 0x2 0x4f82 28 Summary *192.168.17.1 192.168.19.1 0x80000001 77 0x2 0x66fc 28 Summary *192.168.18.1 192.168.19.1 0x80000001 77 0x2 0x5b07 28

Notice that the link-state database now contains summary advertisements, which summarize routing information from one area into another.

The OSPF routing table now shows destinations in Area 0.0.0.1:

aviva@RouterG> show ospf route Prefix Path Route NH Metric NextHop Nexthop Type Type Type Interface addr/label 192.168.12.1 Intra Router IP 13 fe-1/0/0.0 10.0.16.1 192.168.17.1 Intra Router IP 1 fe-1/0/1.0 10.0.0.2 192.168.18.1 Intra Router IP 1 fe-0/0/1.0 10.0.1.1 192.168.42.1 Intra Router IP 1 fe-1/0/0.0 10.0.16.1 10.0.0.0/24 Intra Network IP 1 fe-1/0/1.0 10.0.1.0/24 Intra Network IP 1 fe-0/0/1.0 10.0.2.0/24 Intra Network IP 2 fe-0/0/1.0 10.0.1.1 fe-1/0/1.0 10.0.0.2 10.0.16.0/24 Intra Network IP 1 fe-1/0/0.0 10.0.21.0/24 Intra Network IP 13 fe-1/0/0.0 10.0.16.1 10.0.22.0/24 Intra Network IP 25 fe-1/0/0.0 10.0.16.1 172.19.121.0/24 Intra Network IP 2 fe-1/0/0.0 10.0.16.1 172.100.1.0/24 Intra Network IP 2 fe-1/0/0.0 10.0.16.1 192.168.12.1/32 Intra Network IP 13 fe-1/0/0.0 10.0.16.1 192.168.17.1/32 Intra Network IP 1 fe-1/0/1.0 10.0.0.2 192.168.18.1/32 Intra Network IP 1 fe-0/0/1.0 10.0.1.1 192.168.42.1/32 Intra Network IP 1 fe-1/0/0.0 10.0.16.1

These routes are also in the routers routing table:

aviva@RouterG> show route table inet.0 inet.0: 19 destinations, 20 routes (19 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both 0.0.0.0/0 *[Static/5] 5w4d 20:51:01 > to 172.19.121.1 via fe-0/0/0.0 10.0.0.0/24 *[Direct/0] 3d 01:53:17 > via fe-1/0/1.0 10.0.0.1/32 *[Local/0] 3d 01:53:17 Local via fe-1/0/1.0 10.0.1.0/24 *[Direct/0] 3d 01:53:17 > via fe-0/0/1.0 10.0.1.2/32 *[Local/0] 3d 01:53:17 Local via fe-0/0/1.0 10.0.2.0/24 *[ OSPF/10] 00:05:31, metric 2 > to 10.0.1.1 via fe-0/0/1.0 to 10.0.0.2 via fe-1/0/1.0 10.0.16.0/24 *[Direct/0] 3d 01:53:17 > via fe-1/0/0.0 10.0.16.2/32 *[Local/0] 3d 01:53:17 Local via fe-1/0/0.0 10.0.21.0/24 *[ OSPF/10] 00:05:31, metric 13 > to 10.0.16.1 via fe-1/0/0.0 10.0.22.0/24 *[OSPF/10] 00:05:31, metric 25 > to 10.0.16.1 via fe-1/0/0.0 172.19.121.0/24 *[Direct/0] 5w4d 20:51:01 > via fe-0/0/0.0 [OSPF/10] 00:05:31, metric 2 > to 10.0.16.1 via fe-1/0/0.0 172.19.121.119/32 *[Local/0] 5w4d 20:51:04 Local via fe-0/0/0.0 172.100.1.0/24 *[OSPF/10] 00:07:48, metric 2 > to 10.0.16.1 via fe-1/0/0.0 192.168.12.1/32 *[OSPF/10] 00:05:31, metric 13 > to 10.0.16.1 via fe-1/0/0.0 192.168.17.1/32 *[OSPF/10] 00:05:31, metric 1 > to 10.0.0.2 via fe-1/0/1.0 192.168.18.1/32 *[OSPF/10] 00:05:31, metric 1 > to 10.0.1.1 via fe-0/0/1.0 192.168.19.1/32 *[Direct/0] 3d 01:51:49 > via lo0.0 192.168.42.1/32 *[OSPF/10] 00:05:31, metric 1 > to 10.0.16.1 via fe-1/0/0.0 224.0.0.5/32 *[OSPF/10] 2d 21:03:25, metric 1 MultiRecv