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Knowledge Base

Aggregated Routes Advertised by EBGP Flap Frequently Because of Unreasonable Planning of Routing Protocol Priorities

Publication Date:  2012-07-27  |   Views:  141  |   Downloads:  0  |   Author:  SU1000866616  |   Document ID:  EKB0000325941


Issue Description

For the networking diagram, see the attachment.
In the MAN, two NE80E routers serve as the core routers. The route planning in the MAN is that the two NE80E receive internet routes and advertise the internet routes in the MAN according to network plus static blackhole routes. Besides, provincial routers set EBGP route suppression to the routes advertised by the MAN. Some traversal traffic controlled with MED exists over the interconnected links between the core routers. To prevent that, the IP addresses of the connected interfaces between the NE80E routers are used to set up IBGP neighbors.
When the interconnected link between the NE80E routers frequently shifts between the UP and Down status because of board or link faults, internet services in the entire MAN are interrupted. According to the feedback information form the provincial network, the routes advertised by the MAN are suppressed.


Alarm Information


Handling Process

1. Check logs on the device and confirm that related policies are not changed and routes are not deleted/added manually.
2. No action of deleting or adding routes should exist because the NE80E routers advertise routes based on network plus blackhole routes. Display information about the aggregated routes shows that the routes have exit for a long period and have not been interrupted.
3. On the two NE80E routers, IBGP and EBGP priorities are set to 20 and the priority of blackhole routes is set to 60. Therefore, when both static routes and IBGP routes exist, the NE80E routers prefer IBGP routes when they advertise aggregated routes; when the IBGP neighbor relationship between the two NE80E routers flaps, the aggregated routes advertised by the NE80E routers are necessary to flap.
3. Solution
Adjust the priority of BGP routes and static routes so that the priority of IBGP routes is lower than that of static routes.

Root Cause

The route flapping may be due to the following:
1. Related policies including policies on the local and the peer devices are changed.
2. Routes, especially the aggregated routes, are added and deleted continuously twice.
3. Priorities of static and dynamic protocol routes are unreasonable. As a result, BGP aggregated routes are not completed advertised based on network plus blackhole routes.


In theory, route flapping should not exist because the NE80E routers advertise routes based on network plus blackhole routes. Here, the route flapping is mainly caused by unreasonable configuration of routing protocol priorities and thus the NE80E routers advertise routes not according to the configured method.