CCNP – Etherchannel trunk configuration

In my last post on Etherchannel, I reviewed how to setup an etherchannel for an access port configuration. When working with an inter-switch configuration or a virtual server configuration, you may occasionally need more than a 1 Gig connection when you dont have a 10 Gig network card available. One difference here is that you will mirror the configuration on an physical interface port when doing an ether-channel for a trunk connection also at the port-channel interface as well. Here is the most basic ether-channel config when needing to create a multi-port trunk connection -

interface port-channel 1
switchport trunk encapsulation dot1q
switchport mode trunk

interface gi0/24
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 1 mode on

Something that I havent talked about before is that there is a limit of 8 interfaces that can be in one port channel group. Maybe a future release will allow you go to beyond the current 8 port limit. If you haven’t already begun using a native vlan configuration for trunk ports, this would be a good time to start in that direction. The command to use is switchport trunk native vlan #. I normally use the management vlan as the vlan number to use for the native vlan #.

Even the increased amount of bandwidth from having multiple paths, there is no sense in letting that consume all of the links, if you havent already started doing some vlan pruning to maximize available bandwidth, use the switchport trunk allowed vlan command followed by a list of vlan’s that you want to traverse the trunk link(s). While this does slightly increase the administrative work on setting up a new trunk port or trunk etherchannel configuration, I think it is well worth the effort as one step in containing a potential broadcast storm by something that doesnt need to reach a given switch.

A Cisco employee CCIE that I have come to know made a very good suggestion – put spanning-tree guard root on the etherchannel configuration on the switch that is spanning-tree root for your network. This will help avoid an unplanned spanning-tree change on the network. Before hearing this suggestion, I had already made the change to my network by forcing the switch at the center of the network to be the root switch for all vlan’s on the network. I am still thinking about implementing it on the etherchannel configuration that I create.

I recently ran into a situation where I needed to migrate an inter-switch configuration from a single trunk port to a ether-channel based trunk configuration. While I couldnt find anything talking about how to do this, what I ended up finding it couldnt be simpler. What I did was to create the port-channel interface will the commands present on the trunk port that I was needing to add bandwidth on. All that was needed after that was to add the channel-group1 mode on command. If you have done any amount of reading on ether-channel, you are aware of the different modes. From past experience, I have found that mode on does what I need a majority of the time. When I added this command to the physical interface on one switch and not the other, the port didnt go down or show any other issues, so it would appear that you can make the change from single to multi trunk port configuration during day without any problems.

Here are some etherchannel troubleshooting commands that you may want to use when setting up a new etherchannel configuration. The first command will show you all of the ports assigned to a particular port-channel setup. You get a very good go/no go indication if all the ports are working correctly.

Switch#sh etherchannel 1 summ

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 f – failed to allocate aggregator

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

Number of channel-groups in use: 1
Number of aggregators: 1

Group Port-channel Protocol Ports
1 Po1(SU) – Gi0/25(P) Gi0/26(P)

This next command gives you a little more granularity on a port-channel setup by showing when any changes were made to the port channel, ports being added/removed from a particular port-channel, what protocol is being used, how long the port-channel has been up, etc.Switch#sh etherchannel 1 port-channel
Port-channels in the group:

Port-channel: Po1

Age of the Port-channel = 3d:5h:35m:30s
Logical slot/port = 1/0 Number of ports = 2
GC = 0×00000000 HotStandBy port = null
Port state = Port-channel Ag-Inuse
Protocol = -
Port security = Disabled

Ports in the Port-channel:

Index Load Port EC state No of bits
0 00 Gi0/24 On 0
0 00 Gi0/25 On 0

Time since last port bundled: 0d:36h:25m:11s Gi0/25You will only see this next line if an interface has been taken out of the etherchannel bundle since the bundle was first created.Time since last port Un-bundled: 2d:11h:4m:2s Gi0/24

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