I am using an F5 LTM and FEX and I see some instability but can't figure out where its coming from.

If both interfaces are on different VLANs and you're not bridging through the BigIP it should be fine. 2k's don't like spanning tree (i.e. they'll block the port) and it's not really supported to connect them to another switch, but a BigIP acting as a router or end device is usually fine.

Check the logs on the upstream switch that's controlling the 2k's (9k/7k/5k?) and just verify you don't see any errors there.

Any errors in the TMM or LTM logs on the BigIP? I take it all interfaces came up at the full 1000/TX? (I'm Assuming it's 1Gb copper connectivity on a 2248 or similar, you don't say if it's 1Gb or 10Gb).

Oh. The 2248TP's (If you're running those) also have very little buffer space... To the point where even moderate multicast is too much for them, and I know of people in the past who've had to replace them with something better. You could be running into something similar if you have a moderate amount of traffic. 2248TPE's are much better (Bigger buffers).

Sorry, might have diverged a bit there, but more detail when asking open ended questions is often useful

H