Nexus 5k / 7k Loop

Hello Everyone,

We have an odd situation that I canbt quite understand - a
L2 loop is happening.

We have a production 5k environment, and a new 7k lab
environment.B When I connect the two
together, the 5kbs immediately report
a loop and disables learning. B While this happens, traffic in the production
vlan is intermittent, i.e. some pings go though, others dropped.

Both the
5kbs and the 7kbs are in a back to back vPC
configuration.B There is a
single link
between 5k04 and 7k02.B At first, we
set up that single link in a
port-channel and vPC on both the 7k and 5k.B We then removed the port-channel
and vPC
config for the link on both sides, and set the link as a standard
trunk.B B Both scenarios produced the loop b with the
detection showing on
the 5k that the same MAC was learned coming from the 7k,
and on the
port-channel going to the upstream 5k.

My guess is that this is an
unsupported config.B We do plan do add a second link from the 7k
ot the 5k,
this way we can run back to back vPC between the 7k and 5k.B Ibm thinking
this will solve the issue.

Question is why would the single link cause a
loop?

Mac address flooding coming in from the upstream 5k goes through the
link to the single 7k02. B The flood goes out the port-channel (to both
downstream 7k03 AND 7k04) and across the vPC peer link, correct? B From there,
each switch (other 7k01, and both downstream 7k03, 7k04) receive the flood on
a vPC port-channel, so it should not flood back, either over the same
port-channel, or through the downstream vPC peer link?

5k01 B ---- B 5k02
B
B | B B B B \ / B B B |
5k03 B ---- B 5k04
B B B B B B B B B
B B B |
B B B B B B B B B B B B |
7k01 B ---- 7k02
B B | B B
B \ B / B B B |
7k03 B --- 7k04

Each horizontal set is a vPC peer, and all
cross links are
port-channels in a vPC.B The single
connection is a standard
trunk.B The loop
errors occur on the 5k04 that is connected to the 7k02.
B 5k03 also reports the errors - the duplicate mac is seen on the vPC
peer-link from 5k04, and the port-channel to the upstream 5ks.

David

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Received on Tue Aug 19 2014 - 06:38:55 ART