So I have noticed something when it comes to OSPF broadcasting routes. When the router has a larger block it stops broadcasting out smaller blocks on the same IP address.

For example,

The router has 192.168.252.0/24 pointing to 192.168.1.25 which is reachable via ether1

If I then add a 192.168.252.0/22 and set the type to unreachable the router no longer hands out the 192.168.252.0/24 to its neighbors.

If I add 192.168.253.0/24, both the 192.168.252.0/22 and 192.168.253.0/24 get announced to the neighbors.

It seems that when two subnets share the same network address the router only shares the larger of the two blocks.

Is there a way I can get it to share all its blocks. I ask because I have 3 core routers. I am trying to null route all of our large /22 and /20 blocks using unreachable routes. I want to null the large blocks in each of the cores but when one of them stops broadcasting the smaller blocks, it breaks certain blocks from talking. This way we can avoid routing redirects until the TTL expires.

Any Ideas?

Just a side note. I have done the same test using ubiquiti edge routers and the the ospf works properly. The routers rebroadcast both the .0/30 and the .0/22 and even a .0/24 where the mikrotik seems to consolidate them.

I have reproduced this in the lab. I took two 750r2 and ran a basic setup. The same thing is happening.

I have the following config on router 1
/ip route
add distance=1 dst-address=192.168.252.0/24 gateway=192.168.1.25
add distance=1 dst-address=192.168.252.0/30 gateway=192.168.1.25
add distance=1 dst-address=192.168.253.0/24 type=unreachable


Router 2 learns all the routes with no issue. I then add distance=1 dst-address=192.168.252.0/22 gateway=192.168.1.26 to router 1.
Router 2 then unlearns both of the 192.168.252.0 routes and replaces them with 192.168.252.0/22. (192.168.253.0/24 still remains in router 2.)
If i remove the 192.168.252.0/22 route from router 1, router 2 never relearns the smaller blocks until they are disabled and re-enabled in router 1.

I have tested this with both redistribute as type 1 and as type 2.

Response from Mikrotik is that is is a known problem.

Confirmed.

It looks like Mikrotik is automatically summarizing redistributed routes. If the router was properly doing ASBR summarization via configured summary prefixes, the second behavior (failing to advertise sub-prefixes when the master prefix goes away) would not be an issue because only the summary route should ever be advertised, and then only if an active sub-prefix exists in the routing table.

I've definitely felt that an explicit ASBR sumary route feature should be there in ROS (and not as an apparently-broken automatic feature that cannot be deactivated)

I had a similar issue with 2 subnets, one included in the other sharing the same "wire".
I remember getting an error in the log stating something about the prefix not matching.
From that point on, all OSPF messages regarding those subnet got ignored, both matching and non-matching prefixes.
IMHO it should accept matching prefix subnets, even if there was a wrong packet received before.

I have had other issues where changing routes or changing from a /30 to /29 or vise versa, the mirotik will not update the routes until rebooted or routes are removed and added again.

I understand consolidating routes is not a massive issue but the way we are trying to null routes across multiple routers requires all routes to broadcast.