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Knowledge Base

Improper Default Routes Delivered for the NSSA on the AR28 Cause a Loop

Publication Date:  2012-07-27  |   Views:  115  |   Downloads:  0  |   Author:  SU1001891224  |   Document ID:  EKB0000352427

Contents

Issue Description

Network topology:
172.0.0.9/30 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 1.1.1.1, the IP address of the loopback interface of R2 is 2.2.2.2, and so on.
A loop appears between R2 and R3.
Use the tracert command on R2 to trace R4. The external route of R4 (100.1.1.1) is a loop.
<R2>tracert 100.1.1.1
traceroute to 100.1.1.1(100.1.1.1) 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 
 

Alarm Information

Null

Handling Process

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
#
ospf 1
area 0.0.0.1
network 10.1.0.8 0.0.0.3
nssa no-summary
#
area 0.0.0.0
network 2.2.2.2 0.0.0.0
network 172.0.0.8 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 3.3.3.3
Routing Tables

Routing for Network

0.0.0.0/0 2 SNet 10.1.0.10 2.2.2.2 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
#
ospf
area 0.0.0.1
network 4.4.4.4 0.0.0.0
network 192.168.0.8 0.0.0.3
nssa default-route-advertise
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. 
 

Root Cause

Loops are usually caused by improper configurations. Therefore, the configurations should be checked. 

Suggestions

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.