Port Channels and trunking

[chiseledst]

Hello…

i was configuring an interface which was in trunk mode, i was adding allow vlan but the interface became suspended.

Th interface was also a part of a channel group. Channel group mode was on. I know I should have added the vlan to the port channel instead, but how come it would suspend the interface? I could not unsuspend the interface until i removed the channel group from the port. Can someone explain port channels and trunking to me? 

[balaji.bandi]

what model of the Switch and IOS code running, how is your config looks like ?

what device other side connected ?

can you post show EtherChannel x summary

[chiseledst]

I dont have exact model but it’s a Catalyst 4500 L3 switch, should be running IOS-XE. It was connected to a 5520 wireless controller. I will have to share the ether channel summary when I get back.

[chiseledst]

Here's the ether channel summary below. I removed the config "channel-group 100 mode on" from the interface. 

#show etherchannel 100 summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use N - not in use, no aggregation
f - failed to allocate aggregator

M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port

A - formed by Auto LAG

Number of channel-groups in use: 61
Number of aggregators: 61

Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
100 Po100(SD) -

[David Ruess]

Yes you are correct the config should be added to the port channel. Adding a separate config to an individual port or ports could casue issues when establishing the bundling of the ports. I suspect you had to remove the port channel config on the interface because you were using On mode. On mode has no negotiation so it may be more difficult to detect a change even if you remove it. 

Port-channels work pretty much exactly how you experienced it. Best practice is to remove config off interfaces (except port specific things like speed and duplex but even those need to be the same) and appply configs to the port channel that has been created.

EDIT: Also since you removed the port-channel config on the interfaces the output you provided is not useful as it contains no information now.

-David

[chiseledst]

Thanks David. What is the point of a port channel? Why can't the config just be on the interface? I looked this up but I think I need a real world example to understand fully. The port channel I removed was for a wireless controller, so it must have been for speed/performance? No other interface is part of the port channel which is why I don't understand why it had On Mode. The output I provided shows this, if there were other trunk ports, then they should show. Since there are none, this port channel is useless then?

[David Ruess · Accepted solution]

Port channel is used to load balance traffic "evenly". Take a L2 domain. If you have 2 links to the same switch one of them will be in the blocking state as STP needs to have a loop free topology for proper function. If you bundle these ports together in a port channel then STP sees it as 1 link and you will be able to forward traffic over both links while still maintaining loop free forwarding. There are 3 modes of Port channel:

PaGP - Cisco proprietary - needs to be configure don both sides

LaCP - Open standard - needs to be configured on both sides

On - needs to be configured only one one side

You had it configured for on which is mostly used for connecting to a device that does not support negotiation of a port-channel (required with LaCP and PaGP).

If you only had one link in the port channel then you are correct that it is a useless configuration. 

Ports configured for port channel also can be access ports, not just trunk although I dont see that as often. Maybe to a serve that has multiple ports connecting to the same switch and you can configure the switch to bundle them as well.

Keep the 2 main points in mind.

1. Configurations should go under the port channel interface created when configuring the individual ports

2. You don't get "increased BW". when you bundle ports. Its a common misconception. What I mean by that is if you bundle 2 - 1 Gig ports together you dont get a 2 GB connection. You get 2 - 1 Gig connections that work in sync to provide traffic flow in a unified manner. You're adding lanes to the freeway not increasing the speed limit of the traffic.

Hope that helps.

-David