Port-Channel Not Forming

[martino-cisco]

Hi All,

I'm experiencing a strange scenario with a Port-Channel between a 3750X and 3850. The channel doesn't form. On the 3750, the second interface shows as DOWN (D), and on the 3850 shows as 'Waiting to Be aggregated'(w). The configs are below.

SIDE-POINTS: I know there is no need to use lacp between two Cisco switches. Also, no need to config 'encapsulation dot1q'. Those commands are there because one of the switches was previously a HP. However, I haven't changed the config only because the same config works between a separate pair of stacks (and same models - same thing, one of them was previously HP) and I believe they should still work.

Config:

3850:

int gi1/1/1
switchport mode trunk
switchport trunk native vlan 999
switchport nonegotiate
channel-protocol lacp
channel-group 2 mode active

int gi2/1/1
switchport mode trunk
switchport trunk native vlan 999
switchport nonegotiate
channel-protocol lacp
channel-group 2 mode active

3750:
int gi1/1/1
switchport trunk encapsulation dot1q
switchport trunk native vlan 999
switchport mode trunk
switchport nonegotiate
channel-protocol lacp
channel-group 1 mode active

int gi2/1/1
switchport trunk encapsulation dot1q
switchport trunk native vlan 999
switchport mode trunk
switchport nonegotiate
channel-protocol lacp
channel-group 1 mode active

I ran debugs on the 3850 and got the below errors:

Mar 22 15:14:38.745: FEC: pagp_switch_agc_compatable: comparing GC values of Gi1/1/1 Gi2/1/1 flag = 1 1
*Mar 22 15:14:38.745: FEC: pagp_switch_port_attrib_diff: compare LACP modes for Gi1/1/2
*Mar 22 15:14:38.745: FEC: pagp_switch_port_attrib_diff: Gi1/1/1 Gi2/1/1 same
*Mar 22 15:14:38.745: FEC: pagp_switch_agc_compatable: EC - attrib incompatable for Gi1/1/1; 

Running debugs on the 3750 don't give anything so I suspect the issue is between the ports on the 3850.

Anyone experience something similar?

[Madhukrishnan Gopinathan Nair]

Try bringing up the channel with only 1 interface and see it forms. If it forms, then there is some mismatch possibly between the second port. Check for speed duplex,MTU etc..

Thanks

MAdhu

[Madhukrishnan Gopinathan Nair]

Also do a sh cdp neighbors to ensue they are indeed cabled correctly.

[Peter Paluch]

Hi Martin,

Please follow Madhu's advice. However, I would like to comment on a couple of your statements.

I know there is no need to use lacp between two Cisco switches.

This is not true. You should use LACP or PAgP whenever doing EtherChannels between Cisco switches. Using the on mode creates a very real threat of having a permanent switching loop, and should be avoided whenever possible.

Also, no need to config 'encapsulation dot1q'.

This is not true. On 3750 switches, there are two trunk encapsulations present: ISL and 802.1Q, and there is no default value for static trunks. The switchport mode trunk would not be accepted if it was not preceded with a switchport trunk encapsulation command.

Would you also mind posting the complete output of show etherchannel 1 detail command from the 3850?

Best regards,
Peter