span port question

[f.fnlaspect]

If you have the same vlan that exists on two switches that are connected together, do you need a source port for the span on both switches or will a port on one of the switches collect all traffic for that vlan?

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[John Blakley]

If you're going to cross switches by trying to capture traffic on 1 switch to a destination port on the other switch, you need to use RSPAN to do it. Normal SPAN will have a source/destination port on a single switch and RSPAN uses a special vlan to carry your monitored information over. Otherwise, you'd set your source port on your primary switch and you can collect information for all ports in a certain vlan, and make your destination the RSPAN vlan. On the other switch, you'd make your source your RSPAN vlan and then your destination port is whatever port your monitoring device is configured on.

HTH,

John

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[f.fnlaspect]

Thanks John, however i think perhaps i didnt make it clear what i mean. On the other hand perhaps i just havent understood.

I am asking whether the vlan that is on both switches will be all captured by monitoring the vlan on one switch, so is all the traffic for the vlan captured by specifying 'vlan' on one switch - is one switch cogniscent of all the vlan traffic even though it exists on the other directly connected switch?

Thanks:)

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[John Blakley]

Here's a link to help you out with the config:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_13_ea1/configuration/guide/swspan.html

Even if you have a single vlan trunked across 2 switches, you'll only be able to capture traffic on the single switch unless you're using rspan. You won't be able to see the vlan, even though the same, on the other switch.

HTH,

John

[f.fnlaspect]

Thanks John, you answered my question:)

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