Creating a Not-So-Stubby Area

Problem

You want to configure a stub area for an area that has an ASBR to connect it to an external network. This area already sends AS External LSAs to the backbone area and doesn need to receive them from the backbone.

Solution

Configure the area as an NSSA. Include this configuration on each router in the NSSA area.

[edit protocols ospf] aviva@RouterJ# set area 0.0.0.3 nssa

Discussion

The configuration for NSSA is similar to that for stub areas. Include the nssa statement when configuring each area. Once you commit the configuration, you can see that the router is in an NSSA:

aviva@RouterJ> show ospf interface fe-0/0/1.0 detail Interface State Area DR ID BDR ID Nbrs fe-0/0/1.0 BDR 0.0.0.3 192.168.16.1 192.168.17.1 1 Type: LAN, Address: 10.0.8.1, Mask: 255.255.255.240, MTU: 1500, Cost: 1 DR addr: 10.0.8.2, BDR addr: 10.0.8.1, Adj count: 1, Priority: 128 Hello: 10, Dead: 40, ReXmit: 5, Stub NSSA Auth type: None

Before you configure the stub area, the OSPF link-state database contains two Type 5 (AS External) LSAs for the external routes that are coming from the ASBR, which is RouterE (192.168.15.1):

aviva@RouterJ> show ospf database area 0.0.0.3 OSPF link state database, area 0.0.0.3 Type ID Adv Rtr Seq Age Opt Cksum Len Router 192.168.15.1 192.168.15.1 0x80000004 163 0x2 0x4156 60 Router 192.168.16.1 192.168.16.1 0x80000007 49 0x2 0xbd74 84 Router *192.168.17.1 192.168.17.1 0x80000004 290 0x2 0x47ea 36 Network *10.0.8.1 192.168.17.1 0x80000002 699 0x2 0x4e86 32 Summary *10.0.0.0 192.168.17.1 0x80000003 289 0x2 0xe7ac 28 Summary *10.0.0.1 192.168.17.1 0x80000002 289 0x2 0x5830 28 Summary *10.0.1.0 192.168.17.1 0x80000007 22 0x2 0xdeaf 28 Summary *10.0.2.0 192.168.17.1 0x80000002 289 0x2 0xd3bf 28 Summary *10.0.16.0 192.168.17.1 0x80000002 289 0x2 0xb1c7 28 Summary *192.168.13.1 192.168.17.1 0x80000002 289 0x2 0x9982 28 Summary *192.168.17.1 192.168.17.1 0x80000002 289 0x2 0x68fc 28 Summary *192.168.18.1 192.168.17.1 0x80000002 289 0x2 0xe939 28 Summary *192.168.19.1 192.168.17.1 0x80000002 289 0x2 0xde43 28 OSPF AS SCOPE link state database Type ID Adv Rtr Seq Age Opt Cksum Len Extern 0.0.0.0 192.168.15.1 0x80000001 590 0x2 0x67f1 36 Extern 172.19.121.0 192.168.15.1 0x80000001 590 0x2 0x859a 36

With the NSSA configured, these Type 5 routes are injected into the area as Type 7 (NSSA) LSAs:

aviva@RouterJ> show ospf database area 0.0.0.3 OSPF link state database, area 0.0.0.3 Type ID Adv Rtr Seq Age Opt Cksum Len Router 192.168.15.1 192.168.15.1 0x80000002 29 0x0 0x6338 60 Router 192.168.16.1 192.168.16.1 0x80000004 2 0x0 0xe155 84 Router *192.168.17.1 192.168.17.1 0x80000003 1 0x0 0x6dc5 36 Network *10.0.8.1 192.168.17.1 0x80000001 1 0x0 0x6e69 32 Summary *10.0.0.0 192.168.17.1 0x80000001 55 0x0 0xa8e 28 Summary *10.0.0.1 192.168.17.1 0x80000001 55 0x0 0x7813 28 Summary *10.0.1.0 192.168.17.1 0x80000002 1 0x0 0x78e 28 Summary *10.0.2.0 192.168.17.1 0x80000001 55 0x0 0xf3a2 28 Summary *10.0.16.0 192.168.17.1 0x80000001 55 0x0 0xd1aa 28 Summary *192.168.13.1 192.168.17.1 0x80000001 55 0x0 0xb965 28 Summary *192.168.17.1 192.168.17.1 0x80000001 55 0x0 0x88df 28 Summary *192.168.18.1 192.168.17.1 0x80000001 55 0x0 0xa1c 28 Summary *192.168.19.1 192.168.17.1 0x80000001 55 0x0 0xfe26 28 NSSA 0.0.0.0 192.168.15.1 0x80000001 29 0x8 0xf2e4 36 NSSA 172.19.121.0 192.168.15.1 0x80000001 29 0x8 0x118d 36 OSPF AS SCOPE link state database Type ID Adv Rtr Seq Age Opt Cksum Len Extern 0.0.0.0 192.168.15.1 0x80000001 711 0x2 0x67f1 36 Extern 172.19.121.0 192.168.15.1 0x80000001 711 0x2 0x859a 36

The output shows that the Type 7 NSSAs still originate from the ASBR, RouterE (192.168.15.1).

As with stub areas, you can configure the ABR in the NSSA to not flood Type 3 (Network Summary) LSAs to the routers in the stub area:

[edit protocols ospf] aviva@RouterJ# set area 0.0.0.3 nssa no-summaries

Looking at a router in Area 0.0.0.3, where before its link-state database would have contained Type 3 (Summary) LSAs:

aviva@RouterE> show ospf database OSPF link state database, area 0.0.0.3 Type ID Adv Rtr Seq Age Opt Cksum Len Router *192.168.15.1 192.168.15.1 0x80000004 429 0x0 0x4167 60 Router 192.168.16.1 192.168.16.1 0x8000000b 7 0x0 0x94d3 72 Router 192.168.17.1 192.168.17.1 0x80000006 8 0x0 0xd8ca 48 Network 10.0.8.2 192.168.16.1 0x80000006 7 0x0 0xbd6 32 Summary 10.0.0.0 192.168.17.1 0x80000002 8 0x0 0x8552 28 Summary 10.0.1.0 192.168.17.1 0x80000002 8 0x0 0x8451 28 Summary 10.0.2.0 192.168.17.1 0x80000002 8 0x0 0x795b 28 Summary 192.168.18.1 192.168.17.1 0x80000001 17 0x0 0x91d3 28 NSSA *10.0.15.0 192.168.15.1 0x80000001 429 0x8 0xcaf3 36 NSSA *172.19.121.0 192.168.15.1 0x80000001 429 0x8 0x118d 36 NSSA *192.168.15.1 192.168.15.1 0x80000001 429 0x8 0x91cc 36

The Summary LSAs are no longer present:

aviva@RouterE> show ospf database OSPF link state database, area 0.0.0.3 Type ID Adv Rtr Seq Age Opt Cksum Len Router *192.168.15.1 192.168.15.1 0x80000004 338 0x0 0x4167 60 Router 192.168.16.1 192.168.16.1 0x80000008 283 0x0 0x9ad0 72 Router 192.168.17.1 192.168.17.1 0x80000004 278 0x0 0xdcc8 48 Network 10.0.8.2 192.168.16.1 0x80000004 283 0x0 0xfd4 32 NSSA *10.0.15.0 192.168.15.1 0x80000001 338 0x8 0xcaf3 36 NSSA *172.19.121.0 192.168.15.1 0x80000001 338 0x8 0x118d 36 NSSA *192.168.15.1 192.168.15.1 0x80000001 338 0x8 0x91cc 36