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.
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.