Hi,
Our network is multihomed, with one provider (ProviderA vi RouterA ) in one city and another provider (ProviderC via RouterC) in another city. There is one hop between the 2 routers (RouterB).
[ProviderC]<==>[RouterC]<==>[RouterB]<==>[RouterA]<==>[ProviderA]
Also, Routers A & C are distribution points for the 2 cities.
We are BGP peered with both providers advertising routes and receiving a default from each. We advertise 4 /24 routes to each. At ProviderC BlockD is advertised with no preprended-AS numbers, Blocks A-C has 5 AS Numbers prepended. At provider A the reverse is true. The network is also configured such that if the circuit between A & C fails, they stop advertising the routes that are NOT primary at their node. Our IGP is OSPF.
Today Provider C dropped traffic for 30 minutes. Since RouterC was learning its default via BGP, after the timers expired it dropped the default route. OSPF should have provided a second default route, however, RouterA stopped advertising its default route into the network at the same time. When we connected to RouterA and added a Static default(BGP provided Default was still in place) it began redistributing the default again even after I disabled the Static default.
So the problem turns out that is RouterC drops its default from the network, then RouterA drops its advertisement as well.
What could cause this?