I just finished upgrading our entire network from 5.25 to 6.4. The final device to be upgraded was our edge router. That upgrade killed an entire tower.

The edge router is a 493AH. It runs a half dozen point-to-point links to other towers, usually through pairs of SXTs set up as Level 2 WDS links as described here in the wiki, using a small /29 address space for each link. This configuration has never given me a problem. On my SXTs, it works in all releases.

However, one tower link is run off a radio card inside the 493AH itself, which then talks to an SXT at that tower, using the same WDS/L2 link configuration. When I upgraded the edge router to 6.4, this link broke. I still had good radio connection, but it would pass no L3 traffic.

From the edge router, I could not ping the IP address of the remote SXT or tower router port. Similarly, from the SXT, I could not ping the IP address of the edge router WDS bridge. However, ARP pings worked great in both directions, and on both the SXT and edge router, /tool ip-scan on the appropriate interface reported all the addresses that it should have.

Routing rules looked fine (the route for the /29 network is automatically generated anyway, so should have worked). Yet, no L3 traffic would pass. Traceroute across the link in either direction failed before the first hop.

I tried configuring the link as a plain level-3 routed link (no WDS), but got the same results. I checked the firewall, NAT rules, and anything else I could think of that might be blocking traffic.

In desperation, I downgraded back to 5.26. Boom, the link came up immediately and worked fine, just as it always has.

Does anyone have any suggestions as to what might be going on, and what to do about it?

(I didn't bother trying 6.5, which came out hours ago, because none of the fixes in the journal looked in any way related to anything in use here.)

Next time it happens first thing you need to do is to make supout.rif file and get it to support@mikrotik.com

Did you have any queues?

Yes, I have some queue trees. I suppose I could disable them temporarily to see if that changes anything.

I'm sure I can make this happen at will, so I can get a supout. I just have to schedule the upgrade/downgrade for 3 AM.