18.104.22.168/30 10.1.0.10/30 192.168.0.9/30
R1 eth1/0----------e0/0 R2 e1/0---------e0/0 R3 e1/0----------e2/0/0 R4
Area0 ABR NSSA ASBR
|--------- Area1 --------------------------|
The IP address of the loopback interface of R1 is 22.214.171.124, the IP address of the loopback interface of R2 is 126.96.36.199, and so on.
A loop appears between R2 and R3.
Use the tracert command on R2 to trace R4. The external route of R4 (188.8.131.52) is a loop.
traceroute to 184.108.40.206(220.127.116.11) 30 hops max,40 bytes packet
Press CTRL_C to break
1 10.1.0.9 3 ms 1 ms 2 ms
2 10.1.0.10 3 ms 1 ms 1 ms
3 10.1.0.9 3 ms 2 ms 2 ms
4 10.1.0.10 4 ms 3 ms 3 ms
5 10.1.0.9 5 ms 4 ms 4 ms
6 10.1.0.10 4 ms 4 ms 3 ms
Check the routes configured on R2, and you can find that R2, functioning as the ABR, distributes the Type 3 default route for the NSSA, but R2 receives the Type 7 default route distributed by R4.
<R2>disp c c ospf
network 10.1.0.8 0.0.0.3
network 18.104.22.168 0.0.0.0
network 22.214.171.124 0.0.0.3
In the Type 7 default route distributed by R4, the next hop is R3.
<R2>disp ip routing-table
Routing Table: public net
Destination/Mask Protocol Pre Cost Nexthop Interface
0.0.0.0/0 O_NSSA 150 1 10.1.0.9 Ethernet1/0
Check the routes on R3, and you can find that R3 has a default route to R2, and this default route is distributed by R2.
<R3>disp ospf routing
OSPF Process 1 with Router ID 126.96.36.199
Routing for Network
0.0.0.0/0 2 SNet 10.1.0.10 188.8.131.52 0.0.0.1
The default route distributed by R2 is the NSSA Type 3 default route.
R2, however, receives the NSSA Type 7 default route with R3 as the next hop. Thus, a loop appears.
Check the routes of the ASBR (R4).
<R4>disp c c ospf
network 184.108.40.206 0.0.0.0
network 192.168.0.8 0.0.0.3
A default route with a high priority exists on the ASBR.
ip route-static 0.0.0.0 0.0.0.0 NULL 0 preference 5
This is the reason that the NSSA Type 7 default route is successfully distributed by the ASBR.
To sum up, in the NSSA, the ABR distributes the NSSA Type 3 default route, but the ASBR (R4) distributes the NSSA Type 7 default route; the ABR then learns the default route distributed by the ASBR (R4); R3 selects the Type 3 default LSA route and generates the default route with R2 as the next hop. Thus the loop appears.
Loops are usually caused by improper configurations. Therefore, the configurations should be checked.
When distributing the Type 3 or Type 7 default route, the ABR in the NSSA also learns the Type 7 default route issued by the ASBR. Therefore, improper configurations can cause loops.
As shown in the preceding network topology, the NSSA is configured on ABR as the total NSSA, and therefore, the ABR will automatically distribute the Type 3 default route in the NSSA. The ASBR is configured with a static route destined for an external area and is configured to send the Type 7 default route with the default-route-advertise command.
When receiving the Type 3 default route and the Type 7 default route at the same time, R3 adds the Type 3 default route to its routing table. Therefore, the next hop of default route 0.0.0.0 in the routing table of R3 is the IP address of the ABR.
When receiving the Type 7 default route distributed by the ASBR, the ABR adds this Type 7 default route to its routing table. The next hop of default route 0.0.0.0 in the routing table of the ABR is R3 because the next hop from the ABR to the ASBR is R3,.
Therefore, a loop caused by default route 0.0.0.0 appears between the ABR and R3, and data sent from the ABR and the RTB to the external area cannot be forwarded normally.