AR2200 OSPF External Routes Are Selected Randomly

Publication Date:  2015-11-02 Views:  592 Downloads:  0
Issue Description
1. Network Topology

LSW1, RouterD, RouterA, and RouterB run in OSPF Area 0. LSW1 is the core switch, while RouterD, RouterA, and RouterB are headquarters' access routers and connect to the access router RouterC of the branch. These routers have static routes configured for interworking.

The headquarters and branch communicate through RouterA and RouterC. After the link between RouterA and RouterC becomes Down, the headquarters and branch communicate through RouterB and RouterC. After the link between RouterB and RouterC becomes Down, the headquarters and branch communicate through RouterC and RouterD.



2. Fault Symptom

When the link between RouterA and RouterC becomes Down, the headquarters and branch can communicate normally. However, traffic from the headquarters to the branch is transmitted along the link LSW1-RouterD-RouterC instead of the link LSW1-RouterB-RouterC.
Handling Process
Step 1 The core switch LSW1 has only one OSPF external route anytime. Only one of the three access routers, RouterD, RouterA, and RouterB, imports OSPF external routes.

Step 2 When the link between RouterA and RouterC becomes Down, RouterB cannot advertise the OSPF external route of RouterC into an OSPF process after learning the OSPF external route advertised by RouterD.Similarly, RouterB cannot advertise the OSPF external route of RouterD after learning the OSPF external route advertised by RouterC.

Step 3 RouterB and RouterD have only one LSA advertised into an OSPF process. Consequently, the core switch LSW1 learns the OSPF external route of RouterD or RouterB randomly.

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Root Cause
The headquarters' three access routers import only one Type 5 external route in OSPF Area 0 at a time. RouterD and RouterB determine whether they import their own static route according to the sequence in which they learn the Type 5 LSA of each other. As a result, after the link between RouterA and RouterC becomes Down, LSW1 selects the OSPF external route of RouterD or RouterB randomly, failing to meet customer requirements.
Solution
1. Set the static route priority of RouterD, RouterA, and RouterB to smaller than 150 (lower than the OSPF external route priority). Subsequently, LSW1 can learn the three Type 5 LSAs advertised by the headquarters' three access routers and select the optimal route based on the costs of the three OSPF external routes.

Type 5 LSAs are generated by an ASBR to describe AS external routes and will be advertised to all the areas except stub areas and NSSAs.

2. Ensure that all traffic of the headquarters is forwarded through the core switch LSW1 because the paths for incoming and outgoing traffic may be different.

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