First subnet of blackhole route not advertised in OSPF

RouterOS Forwarding Protocols
1

Hi,

I have a /22 blackhole route on my core router as a summary route, along with a bunch of smaller connected/static routes from the /22

My OSPF is setup to redistribute connected and static routes into the backbone area

However I have noticed the first route (/24 connected) from the /22 is not advertised to other routers in the backbone, but all other connected/static routes are, and if I disable the /22 blackhole route the /24 that was missing suddenly appears on the other routers

Is this expected behaviour and is there anyway to get it to advertise both the /22 and /24 (eg. 203.51.34.0/22 and 203.51.34.0/24)

Thanks :slight_smile:

2

Bump

3

Check the LSA entries for the routes that are being advertised so understand where they originate from. Summarised routes are announced to cover smaller ones so they may be originating from other routers in your network.

4

The summarised route is coming from the same router as the more specific routes, for example the below route set, everything is advertisting except the first /24

ASB 203.51.34.0/22
DAC 203.51.34.0/24 << only route not advertised out of the router on OSPF
DAC 203.51.35.16/30
DAC 203.51.35.20/30
DAC 203.51.35.24/30
DAC 203.51.36.0/24

5

Readvertise connected or static is not recommended. If running only OSPF you should define the subnet/s covering the interfaces you want to advertise and you can summarise smaller ranges by defining a summary route.

A more scalable way would be to only use OSPF for the router loopback IPs and interfaces leading to neighbour routers and then readvertise connected and static using BGP where you set next hop as self using the loopback interface. This way OSPF is generally super quick to reconverge.