OT: VSS + MEC - port-channel dynamically cloned?

Leland Vandervort leland at taranta.discpro.org
Tue Nov 24 00:51:29 CST 2009


Hi all.. this one seems to have stumped even the community forums on
CCO ;)  Anyone else seen this behaviour?  (It's not actually a problem
as such since the MEC port-channels are actually working fine, just
unexpected the way that it has done it...)

We have a few paris of VSS running, and having brought the down-stream
access-switches online using MEC we note that the active port-channel
that they bind under isn't actually the one configured, but rather a
CLONE of the configured one which appears to be dynamically generated.

 

In our case, all ports and port-channel configurations are set to dot1q
encapsulation and trunk mode active.

 

For example, we have about 20 3560s MEC'd to the VSS:

 

the MEC comes up, but under a slightly different port-channel
identifier:

 

Here's the 3560 side:

interface status:

Gi0/49                       connected    trunk      a-full a-1000
1000BaseSX SFP
Gi0/50                       connected    trunk      a-full a-1000
1000BaseSX SFP
Gi0/51                       connected    trunk      a-full a-1000
1000BaseSX SFP
Gi0/52                       connected    trunk      a-full a-1000
1000BaseSX SFP

 

#sh int po1 | i Member
  Members in this channel: Gi0/49 Gi0/50 Gi0/51 Gi0/52

 

Here's the VSS side:


Interface status:

Po11         CISCO C3560 A01    notconnect   1            auto   auto 
Po11A                           connected    trunk      a-full a-1000
^^^^
  Po11A is not actually configured, but appears to be cloned from Po11
 

#sh int po11A | i Member
  Members in this channel: Gi1/1/33 Gi1/1/34 Gi2/1/41 Gi2/1/42

 

#sh cdp nei | i hacc01-d
hacc01-d         Gig 2/1/42        140              S I   WS-C3560G Gig
0/50
hacc01-d         Gig 2/1/41        140              S I   WS-C3560G Gig
0/49
hacc01-d         Gig 1/1/34        161              S I   WS-C3560G Gig
0/52

hacc01-d         Gig 1/1/33        161              S |   WS-C3560G Gig
0/51

 

Essentially, for all of the MEC connections, the VSS has created a clone
of the configured port-channel to bind the actual physical connections,
rather than binding them under the configured port-channel (and suffixed
the port-channel number with A or B depending on which chassis was first
to bind).

 

Config for port-channel and interfaces all match:

 

VSS SIDE:

interface Port-channel11 
description CISCO C3560 A01
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 40-48,51,400
switchport mode trunk
mtu 9216
end

Physical ports are the same config, with channel-group 11 mode active
(and repeated for 20 different port-channels each with 4 ports) 

 

ACCESS Side:

interface Port-channel1
description DIST-VSS-EQX1
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 40-48,51,400
switchport mode trunk
mtu 9216
end

Similarly, physical ports are the same with channel-group 1 mode active
 

I'm not overly worried about it since the MEC is working with the
dynamically generated port-channel, and traffic is flowing correctly...
just curious as to why it uses a clone of the configured port-channel,
rather than the actual configured port-channel itself.  Just find that a
bit strange.

 

 

Thanks,

 

Leland






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