Using RSPAN

Unanswered Question
Aug 29th, 2008

Hi, We have an issue with spanning a voice vlan to an rspan vlan on some 3750 switches. We are doing this to record voice calls from multiple voice vlans, so i have a question. Does the config need to be entered on each switch stack, or does it need to be set up only on the switch where the voice recording servers are connected. i.e. the servers are in a rspan vlan & the monitor session is set up for the voice vlans on that switch or does a monitor session need to be configured on each switch where there are ip phones connected. The reason i ask, is that on one of the 3750 stacks, we have an issue, when the traffic from the voice vlan is spanned to either of the rspan vlans, all communication for that vlan across a trunk stops. We have an identical 3750 switch stack in the same building (same ios & hardware) where this does not cause a problem. We have checked the config & even tried anothe 3750 with a different version of IOS (same problem). This only occurs if we make the destination vlan an rspan vlan. If i use a standard vlan there are no issues. Thanks in advance Colin

I have this problem too.
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Giuseppe Larosa Fri, 08/29/2008 - 09:26

Hello Colin,

I hope I have understood your scenario.

switches in the middle must have the remote vlan configured and declared as remote-span.

switches in the middle should have:

an incoming link trunk or access port

an outgoing link trunk or access port

The overall path should be a point-to-point via source switch - middle switches - destination switch

this is because the effect of declaring a VLAN as a remote span vlan is to disable the MAC address learning function on that vlan so that all frames are carried on the RSPAN switched path (treated as unknown unicast) regardless of MAC destination address.

Ideally I think you should use one RSPAN instance for each switch.

OR

a single RSPAN if all access switches are connected directly to core switch but take care on what trunks the remote span vlan is permitted in order to build a point (core) to multipoint (access switches) but I'm not sure this can work I'm afraid traffic could be sent also from branch to branch and this would be a terrible mess.

Hope to help

Giuseppe

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