Vlan interface status and trunking

Unanswered Question
Jul 6th, 2007

I have following scenario:

There are two cat6509 swtiches, lets say cat1 and cat2.

Vlan 10 is configured on cat1 with layer 3 information and is functioning as gateway for vlan10.

There is no physical access port on cat1 that is part of vlan10. There is 802.1q trunk

between cat1 and cat2 (and vlan10 is one of the trunked vlan).

There are some access ports on cat2 that are member of vlan10.

cat1(vlan10)-----trunk-----cat2(vlan10)----layer2-access(for vlan10)

My question is that if let's say all the ports on cat2 that are part of vlan10 go down

(for example if they are on the same line card and that line card goes down),

what will be the status of vlan interface on cat1? Shoud it show "down down" or remain

up?

I have this problem too.
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Edison Ortiz Fri, 07/06/2007 - 15:57

It will stay up/up since Vlan10 is in forwarding state under the trunk interface.

You can verify by typing

show int trunk

after all access-ports are down/down.

sundar.palaniappan Fri, 07/06/2007 - 17:24

My guess is the vlan interface would go down.

Here's the reason for that.

VTP pruning is enabled by default on Cisco switches.

When there are no active ports in vlan 10 on Cat2 VTP pruning would cause Cat1 to prune vlan10 on the trunk dynamically. This would cause vlan 10 interface on Cat1 to go down.

Let us know what you see.

HTH

Sundar

Edison Ortiz Fri, 07/06/2007 - 18:35

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12225see/scg/swvtp.htm#wp1035139

VTP Pruning

VTP pruning increases network available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to reach the destination devices. Without VTP pruning, a switch floods broadcast, multicast, and unknown unicast traffic across all trunk links within a VTP domain even though receiving switches might discard them. VTP pruning is disabled by default.

VTP pruning blocks unneeded flooded traffic to VLANs on trunk ports that are included in the pruning-eligible list. Only VLANs included in the pruning-eligible list can be pruned. By default, VLANs 2 through 1001 are pruning eligible switch trunk ports. If the VLANs are configured as pruning-ineligible, the flooding continues. VTP pruning is supported with VTP Version 1 and Version 2.

royalblues Sat, 07/07/2007 - 03:10

i totally agree with Edison that the vlan interface will not go down as long as the trunk is up between the 2 switches and is passing that vlan over that

HTH

Narayan

mohammedmahmoud Sat, 07/07/2007 - 03:29

Hi,

I've tested this before, and VTP Pruning is disabled by default, and i totally agree with both Edison and Narayan that the interface will remain UP.

HTH,

Mohammed Mahmoud.

sundar.palaniappan Sat, 07/07/2007 - 05:51

Alright guys. If VTP pruning is disabled, which it looks like is the default setting, then the vlan int would be up. On the other hand if pruning was enabled by default then vlan int would go down.

HTH

Sundar

abidkhanusa Sat, 07/07/2007 - 13:59

If cat1 is in vtp transparent mode (with vtp pruning disabled) and if cat2 has vtp pruning enabled, then if cat2 prunes vlan10 (upon access ports failure), will vlan10 port suppose go into down status in this case?

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