Port-channel Question

PeraHoman used Ask the Experts™
If you configure an interface (g0/1) as a trunk to allow VLANs 20,30 and a port-channel interface (port-ch1), which g0/1 & gx/x will be apart of (channel-group) to allow VLANs 30,40, would there be an issue with traffic being forwarded from VLANs 20 and 40 since they are only configured on one side (physical side-g0/1 vs logical side-portchannel). I'm not putting this into production, I'm just curious to see if it works.  VLAN 30 should be fine since its configured on both interfaces, but what about VLAN 20&40?
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Ernie BeekSenior infrastructure engineer
Top Expert 2012
Wouldn't work. I think you would even get an error when adding the interface to the port channel. Either that or the interface would be adjusted to match the port channel settings.
Brian MurphySenior Information Technology Consultant
Ingress or Egress?

Where are you tagging the traffic.

I just recently solves a big Layer 2 issue for a customer where they had 1 of 4 ports set to trunk and it was not supposed to be set to trunk.

For example 1-1, 1-2, 1-3, 1-4 was set to tag the frame with VLAN ID X.  Each port was a different VLAN but port 1-4 was set to trunk and 1-3 had two VLAN ID's not one.

Point being.  When trunking you need something that is tagging.

When I configure Netscaler and Cisco I tag the packets on the egress from the Netscaler to Cisco.

If the Netscaler has 10 ports (9700) but 4 in use let's call those VLAN 10, 20, 30, 40.

I tag the traffic to Cisco and on the Cisco set it to allow 10,20,30,40 on port 1-1, 1-2, 1-3, 1-4.

Netscaler is tagging the packets not the Cisco Switch.

This configuration eliminates MAC moves on the Netscaler and prevents "Muted" interface.

Just one example.


In this case, egress.

The infrastructure is Cisco switch to switch, and will be tagged via dot1q.  From my example, I was just curious if traffic would still be forwarded in VLAN 30 since its configured on both physical and logical interfaces, where VLANs 20 & 40 wouldn't be able to forward traffic through the 'port-channel' since both are configured on only one side (VLAN 20 only on physical trunk, VLAN 40 only on port-channel).  

If anything would VLAN 20 would still forward traffic through the physical trunks even though the physical interfaces are part of the channel group where it VLAN 20 isn't configured within the port-channel's trunk?  I can see VLAN 40 not forwarding, but I'm more curious about the situation of VLAN 20.  

Also, would VLAN 30 forward traffic even though VLAN 20 & 40 are misconfigured, or is this an 'all-or-nothing' situation.  

Port-channel 1
 switchport mode trunk
 switchport trunk allowed vlan 30

inter g1/1 - x
 switchport mode trunk
 switchport trunk allowed vlan 20,30,40
 channel-group 1 mode on

Another way I'd ask is, "is the switch smart enough to still forward traffic if its misconfigured, or would it just break, and not do anything until its corrected?"

Top Expert 2015
If the physical ports are not configured identically, the port channel will not function.  The physical ports will be in an err-disable state.  And while the port channel interface may be created, if it is, it will also be err-disabled.

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