Fun with /24 connected to /30

phoeneous · April 2013

Turned up an mpls site this week that was out of footprint for my primary provider so it crossed the nni and interfaced with a third party lec. Because of how the service terminated, the lec's router was at the edge and I was behind them (cpe) by ethernet. Unbeknownst to me, their cpe side lan was a /24 despite the order saying it was a /30. Half-luckily they were .1 and I was .2. We saw massive packet loss, between 50-90%. Routes would be in the table on second and then disappear the next. We swapped cables, tried different ports, endless debugs and traceroutes. In the end, problems went away when they changed their config to /30 to match mine.

Why would this happen? Since logically we were in the same subnet with valid ip's, shouldn't everything be ok?

instant000 · April 2013

if that /24 overlapped with other networks, then I can see it causing major issues, as the other networks would be thought to be in the same subnet as the /24 interface, when the destination could actually be several hops away, thus blackholing lots of traffic

I have no way to determine if that is the case or not, but that's the first thought that comes to the mind when an improper mask is used, like in this case.

Hope this helps.

Jackace · April 2013

instant000 wrote: »

if that /24 overlapped with other networks, then I can see it causing major issues, as the other networks would be thought to be in the same subnet as the /24 interface, when the destination could actually be several hops away, thus blackholing lots of traffic

I have no way to determine if that is the case or not, but that's the first thought that comes to the mind when an improper mask is used, like in this case.

Hope this helps.

This is most likely what was happening. Also if the network was being advertised in 2 places on the service providers network then that explains why routes were showing up and going away. Routing updates were causing the route to that network to change over and over and over.

phoeneous · April 2013

The /24 was accidentally configured on the provider side facing me. Prior to us turning up that site, I didn't see that subnet in any of my routing tables. They chose the subnet so I have no way of knowing if it was overlapping on their end or not.

Fun with /24 connected to /30

Comments