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New Member

CDP and native VLAN

i would like to know over what vlan are cdp packets send. Actually i had a layer 2 network in wich i change the native vlan in the trunks. The 2950 saw each other and saw the 3550 too trough that trunks, but the 3550 can't saw any of the 2950 but the 2950 can saw it. I have debug cdp and the packets are sent correctly by the 2950 and by the 3550, they share a trunk and the native vlan is correctly configured in both sides, but the 3550 doesn´t receive that packets???

Any help wellcome, thanks.

6 REPLIES
Cisco Employee

Re: CDP and native VLAN

CDP is always sent on Native VLAN. Are the ports Layer 2 or Layer 3 ports?

New Member

Re: CDP and native VLAN

They are Layer 2 ports.

The config in the 2950 is:

interface GigabitEthernet0/1

switchport trunk native vlan 201

switchport trunk allowed vlan 101-105,201-205,301-305,401-405

switchport mode trunk

no ip address

mls qos trust dscp

rmon native

rmon collection stats 13 owner config

And in the 3550 is:

interface GigabitEthernet0/8

switchport trunk encapsulation dot1q

switchport trunk native vlan 201

switchport trunk allowed vlan 101-105,201-205,301-305,401-405

switchport mode trunk

no ip address

mls qos trust dscp

rmon native

rmon collection stats 8 owner config

There is an interface vlan201 (the native one) in the 2950 and a fastethernet interface configured as switchport access vlan 201, but is down, there is nothing connected to that port.

There is too an interface vlan201 in the 3550.

The 2950 can saw another 2950 through the other giga interface.

If I add the vlan 1 to the switchport trunk allowed vlans in the 3550 then it saw the 2950, without add that vlan 1 in the 2950 side of the trunk.

Thanks for your interest.

Re: CDP and native VLAN

IMHO, you are not correct.

"CDP, VTP, and PAgP updates are always forwarded on trunks with a VLAN 1 tag. This is the case even if VLAN 1 has been cleared from the trunks and

is not the native VLAN. Clearing VLAN 1 for user data has no impact on control plane traffic that is still sent using VLAN 1. "

See VLAN1 part of http://www.cisco.com/warp/customer/473/103.html#cat_control for other details.

Regards,

Milan

New Member

Re: CDP and native VLAN

So, if vlan1 is always used to send CDPs even if it's not the native vlan and it has been cleared from the trunk why the 3550 doesn't see the 2950 if it has not vlan1 allowed in its side of the trunk but

saw it if i allow vlan1 in the 3550 side of the trunk, considering that, in both cases, vlan1 is not allowed in the 2950 side of the trunk and that each 2950 saw others 2950 having disallowed vlan1 in both sides of the trunk beetwen them.

Maybe a bug with the 3550 ios version 12.1(12c)EA1?

Regards,

Luis.

Re: CDP and native VLAN

Yes, this is a bug in Cat3550

"If VLAN 1 or VLANs 1002 to 1005 are removed from a trunk port, the switch no longer receives CDP or VTP frames. VLAN 1 minimization is not supported on the Catalyst 3550 switch. You cannot remove VLAN 1 or VLANs 1002 to 1005 from the allowed VLAN list. (CSCdz22629)"

See Release Notes for the Catalyst 3550 Multilayer Switch Cisco IOS Release 12.1(12c)EA1.

(Unfortunately, using Bug Toolkit and searching "3550 CDP" doesn't show this bug, you have to use searching by bug ID.)

Regards,

Milan

New Member

Re: CDP and native VLAN

Thanks Milan, your response resolve absolutely my problem.

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