OSPF dropping default route

RouterOS Forwarding Protocols
1

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?

2

What are the settings for the default route in OSPF?

http://wiki.mikrotik.com/wiki/Manual:Routing/OSPF#Properties

3

i think this is limitation of mikrotik ospf. i run into this a lot where default gateway disappears even though its in LSA. disabling and reenabling the default route fixes it, but is not automatic by any means. i wish mikrotik would fix it.

4

Hello,

I haven’t seen such behavior ever.

As asked above, what are the settings on your default route via OSPF?
What version of RouterOS?
Could you share your configuration?
What devices are you using?

This is a serious issue since it’d mean we can’t trust 100% on OSPF default routes redistribution (which is a key point on a lot of designs), but as I said, I have never experienced something like this.