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New Member

Catalyst 2924XL VLAN Question

I have a Catalyst 2924 (inherited) which has an interesting code feature. The switch pretty much has a default out-of-the-box configuration for VLANs. None of the ports have had "switchport mode", or asociated commands configured to place it in any other than the default VLAN.

My predecessors enabled the ports for several servers. The communications is happening, but with an interesting effect: The server is seeing traffic for everything else on the switch in addition to its own unicast traffic. I have verified this with a sniffer.

This works for other ports just enabled as well unless I configure it for a specific VLAN. Then I get broadcast, multicast, and unknown unicast traffic as one would expect.

Is anyone familiar with this feature? Although I have an idea what is happening here, I would like to get a straight explanation.

4 REPLIES
Cisco Employee

Re: Catalyst 2924XL VLAN Question

By default all port on the switch will be configured for trunking.

You should configure "switchport mode access" and "switchport access vlan xx" on all port that you don't want to be trunking.

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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New Member

Re: Catalyst 2924XL VLAN Question

By default, on the 2900XL, all ports are on the same vlan and set to mode access. Your best bet is to get a sniffer trace and check what the destination MAC is. Then, verify why the switch has not learned that MAC in its bridge table. Possible, could be due to assymetrical design.

Example:

show mac-address address aabb.ccdd.1234

2900XL#sh int f0/1 switchport

Name: Fa0/1

Switchport: Enabled

Administrative mode: static access <-- HERE

Operational Mode: static access

Administrative Trunking Encapsulation: isl

Operational Trunking Encapsulation: isl

Negotiation of Trunking: Disabled

Access Mode VLAN: 10 (VLAN0010)

Trunking Native Mode VLAN: 1 (default)

Trunking VLANs Enabled: NONE

Pruning VLANs Enabled: NONE

Priority for untagged frames: 0

Override vlan tag priority: FALSE

Voice VLAN: none

Appliance trust: none

2900XL#

New Member

Re: Catalyst 2924XL VLAN Question

I will check later today.

What do you mean by asymmetrical design, in this case?

FYI, the switch in question is connected to another 2924 some distance away by a SM media converter. No trunking is set between them. The VLAN on the "problem" switch is even different than the one feeding it.

New Member

Re: Catalyst 2924XL VLAN Question

You should look at the destination MAC in the flooded traffic and not the source/destination IP pair. Cause remember, IP header does not change between transient devices. Below is a link that explains assymetrical issues. So, possible that you are not running into it.

http://www.cisco.com/warp/public/473/62.shtml#t8

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