Right way of configuring higher MTU over a Port Channel

[Florin Barhala]

Hi guys,

I have a running critical Port-Channel between two locations.

Here's the config

SW1:

!
interface Port-channel2
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
end

!

interface GigabitEthernet1/45

 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-protocol lacp
 channel-group 2 mode active
end

!

interface GigabitEthernet1/46
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-protocol lacp
 channel-group 2 mode active
end

SW2

!
interface GigabitEthernet1/1
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-protocol lacp
 channel-group 2 mode passive
end

!
interface GigabitEthernet1/2
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-protocol lacp
 channel-group 2 mode passive
end

!
interface Port-channel2
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
end

Now I need to increase the MTU from default value to 9198. What the right way to do it and avoid any connectivity loss, PortChannel restart.

Does it matter what switch I start first?

Thanks!

L.E. both SW are WS-C4948

[luckymike33]

Hi,

Because you are using layer 2 interfaces - there is no fragmentation support at layer 2, and interfaces receiving frames which have an unsupported size will be dropped.

I think the best way for you to proceed is to lab this up; and verify what happens - it may be that you need to make changes on switches at either end of the channel within a very short time frame to prevent too large an outage.

 When you are ready to maike your change - think the best way to do this is to use the interface range command, and apply the 'mtu' command to all the interfaces in this range. I don't think it matters which switch you apply this change to first, and I don't believe if you are hinting at the 802.3ad (controlled by system-priority) decision maker, that it makes any difference.

HTH

Mike

[Florin Barhala]

Hi Mike,

I am planning to change the MTU on the entire path from

Server A - SW1 - SW2-etherchannel-SW3 - SW4 - Server B.

My only concern is if changing MTU on the Etherchannel can affect/disrupt in any way the PortChannel. I can't afford to have it down, not even for 1ms.

[luckymike33]

Hi,

I think you will be lucky not to have an outage of even 1ms, like I say I would lab this up, although this is only going to prove the segment between the 2 switches, unless you have 2 identical servers with which you can lab.

It may be better to start asking some questions, like:

What sort of traffic is the network carrying?

Are these links a bottleneck at the moment?

is it possible to add further links into these bundles

How is the load balancing over the links configured? src-mac, src-dst-ip etc

HTH

Mike

[Florin Barhala]

There is no bottleneck as we speak. I just need to transport some iSCSI traffic and there's the MTU increase requirement.