3750 to 2950T FEC problem

[luke.cassar]

Hi all,

the story of what we are trying to do is simple.

We have a stack of 2 x 3750 switches (connected via stackwise) and a bunch of 2950T switches. What we are doing is making a port channel between the 3750 stack and each 2950T, in a manner that will provide a bit of fault tolerence and speed.

example. Port Channel 1 on the 3750 contains gig 1/0/1 and gig 2/0/1. Port Channel 1 on the 2950T contains gig 0/1 and gig 0/2. 1/0/1 on the 3750 connects to gig 0/1 on the 2950T and 2/0/1 on the 3750 connects to gig 0/2 on the 2950T.

The idea being that if a 3750 dies, there will still be an active link.

Now I would have assumed this was a simple operation. Create a port channel on 3750 and add 1/0/1 and 2/0/1 to it. Create a port channel on 2950 and add gig 0/1 and gig 0/2 to it. Plug in the cables and away we go.

This was not the case. While there is communication, whenever the second link in the trunk is brough online (plugged in) the following messages occur.

*Mar 1 01:33:46: %EC-5-CANNOT_BUNDLE2: Gi2/0/1 is not compatible with Gi1/0/1 a

nd will be suspended (trunk mode of Gi2/0/1 is access, Gi1/0/1 is trunk) (NENFA0

-3750-01-2)

000062: *Mar 1 01:33:48: %LINK-3-UPDOWN: Interface GigabitEthernet2/0/1, change

d state to up

000063: *Mar 1 01:33:51: %LINEPROTO-5-UPDOWN: Line protocol on Interface Gigabi

tEthernet2/0/1, changed state to up

I dont have a show run of either device, but it really is just a standard port. All it has is a description, port channel membership (all set to mode ON) and mdix auto enabled. Theres no vlan trunks or settings at all on those ports at either end.

Issuing a sh run on both devices, the pairs of ports look identical, so I have no idea where this problem is comming from.

If anyone can shed some light on the situation that would be great.

Thanks,

Luke

[a.manosca · Accepted solution]

"Mar 1 01:33:46: %EC-5-CANNOT_BUNDLE2: Gi2/0/1 is not compatible with Gi1/0/1 and

will be suspended (trunk mode of Gi2/0/1 is access, Gi1/0/1 is trunk) (NENFA0-3750-01-2)"

The info inside the parenthesis indicates the reason for incompatibility.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3750/12218se/3750smg/msg_desc.htm#53992

My guess is, the 2950 is negotiating a trunk because of its default switchport settings:

"switchport mode dynamic desirable"

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12119ea1/2950cr/cli2.htm#1631612

The 3750 will accept the trunk negotiation because the default switchport mode is:

"switchport mode dynamic auto"

If you are not using VLAN trunks, you might want to try configuring the switchport mode

to "access" on both ports of the 2 switches. What I usually do is to shutdown the ports

to be configured with Etherchannel before configuring them, then enable the ports after

the configuration.

[luke.cassar]

Thanks for the quick response there.

I think I tried setting the 3750 to switchport mode access and the link went off.

I will try setting all 4 ports involved to switchport mode access and see if that helps.

Just seems odd that the default configurations dont work properly with each other.

Thanks again,

Luke

[shovels3]

Luke,

on all ports that you want to channel set the

switchport mode on

You must force it to this otherwise it wont work

i.e. gi1/0/1, gi2/0/1 on the cat 3750

fa0/25, fa0/26

My configuartion is exactly as yours and it works very well

if you have any questions email me

shovels3@tiscali.co.uk