spanning a vlan with 80 servers

Unanswered Question
Aug 7th, 2009

Hi Everyone

I want to create a port span for a vlan which has around 80 servers. I know monitoring a physical interface has virtually no performance effect on the switch but im not too sure about a vlan with 80 servers. The switch is a 4506. The output interface will be on the same switch which will go out to an IPS.

Would spanning a vlan have the same performance effect of say, monitoring 80 ports?


I have this problem too.
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jbrenesj Fri, 08/07/2009 - 09:11

Be careful, if traffic is enough then first you'll first fill up the port's buffer and then the group of 8 ports of the line card (in case you have lets say a WS-X4548-GB-RJ45). This is when it gets dangerous because the 4500 will try to allocate more buffer space on the linecard's controller (here is where performance of all ports of the line card will suffer). Since the 4500 will still try to put the excessive traffic somewhere if it uses the sups ASICs you get a lot of issues even high CPU.

I have seen it a couple of times.

As a rule, just to keep it safe, don't send more data to the SPAN destination port than what it will be able to handle.

In the 4500 line cards that is comething like 1Gb / 8

faghouri83 Sun, 08/09/2009 - 14:04

Thanks for the reply.

Do you think i could span say 10 ports to start with and then increase that slowly? Would this give me the same effect of say spanning a vlan with 80 servers?

I dont want to be bringing down a live network so i need to be 110% sure that i can go ahead with this.


Edison Ortiz Tue, 08/11/2009 - 09:40

SPAN won't affect switch performance and any exceeded traffic will be dropped silently in hardware without affecting the other ports.

You will notice when you've exceeded the SPAN transmit limits when your analyzer isn't capturing all the traffic from the source ports.





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