FEX configs are retained

The N5k maintains the config of the FEXs, even when they’re removed (and even though it’s not visible to us). In this article, we’re going to look into this a little further… We’ll start with a working FEX, using Po100 (with Eth1/19-20 being the physical bundle members).

We can see the config for eth100/1/1:

Now let’s shut down the Port-Channel:

Now look for the eth100/1/1 config:

Oh no, we’re not in Kansas anymore!!! Don’t worry – it’s there. Let’s bring up the FEX now and see it come back, automagically:

Ah, life’s great! We’re back in business.

I was curious about this, if there’s a private config file or something that’s used to maintain these invisible configurations. Poking around, I looked at:

It could be that the config is a hidden/system file that I can’t see, although I don’t see any flags for the dir command. Anyway, it’s obvious that it keeps the config, just that it’s hidden until the fabric extender comes back online.

There is a caveat – if the FEX is removed, the config is blown away (lost):

The moral of the story is to not remove the FEX from the Nexus 5000 if you don’t want the config of the fabric extenders (FEXs – 2148Ts at this point) to be lost. I haven’t had a chance to experiment with swapping out 2148Ts, to see if a change in the serial number would make a difference. I don’t think it would (as long as the s/n isn’t specified so that a different FEX can’t peer with the parent switch) – I might try that scenario another day.

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