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Community Member

2950 port monitoring - can I turn off VLAN tagging?

On a 2950 switch, when monitoring an access port via another access port all packets are getting VLAN tagged. While SnifferPro understands this, LANwatch and our other packet sniffing tools don't. They see Ethernet frame type 8100 [VLAN tagged frames] but cannot decode the packets.

Can I not have frames VLAN tagged while monitoring?

IOS version is 12.1(9)

3 REPLIES
Cisco Employee

Re: 2950 port monitoring - can I turn off VLAN tagging?

There is a brief mention of this in the doc:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/1219ea1/scg/swspan.htm#xtocid4

It is not clear to me whether it is saying this is always true or only from a trunk. But your experience indicates it is true regardless of the source/dest configuration.

Perhaps someone that has a 2950 to test with can confirm, but sounds like this is a limitation. :(

Community Member

Re: 2950 port monitoring - can I turn off VLAN tagging?

IT is not possible to turn it off. use Ethereal which is another free product which supports it. it is acutally a useful feature if you think abt it.

Bronze

Re: 2950 port monitoring - can I turn off VLAN tagging?

I asked TAC about this a few weeks ago and was told that a future IOS release would either reverse this behavior or at least provide a command that will do so. It does indeed occur whether or not the port is a trunk, and I'm somewhat at a loss as to the logic behind this.

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