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How to disable learning MAC address process on Catalyst2950 ?

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Sep 20th, 2003
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I'm setting port mirror on Catalyst2950. But there're a lot of MAC addresses in network so I'm afraid that CAM table on switch may be full.

I heard that Cisco can disable process of learning source MAC address then it's not kept in CAM table. I just want to do this on source port of SPAN. When data arrives, SPAN will send to destination port. So it's not necessary to keep those source MAC addresses in table. Please advise.


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m-villas Sat, 09/20/2003 - 19:59
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mac address-table aging-time 0


mac-address-table aging-time 0 (Oldesr versions)

but I dont understand your logic to disable MAC learning.......If you disable MAC learning, switch perforamance significantly reduces and effectively it becomes a Hub. I dont think in that case we can justify the SPAN need in switch........All traffic on all ports can be seen by any port int the switch, so there is no need to configure SPAN in that case.

Why are you worried about CAM size? How many hosts u have on SPAN source port?

Switch becomes a L2 device only due to CAM if you disable that it is L1 device like Hub.

Correct me if I am wrong in above.


boonkeng Sun, 09/21/2003 - 00:21
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MAC aging 0 is not solution because it will not remove entry from table. That means I will have all MAC addresses from SPAN in table, but I don't want to keep those entries.

Actually, I want to do RSPAN. Because of I have only one IDS device but I want to capture traffic from two switches. As I know, RSPAN and local SPAN can't mirror traffic to same port which connects to IDS. So I have to to do local SPAN on both switches. By first switch mirror traffic from port 1-49 to port 50. Then I connect that port 50 to port 1 of second switch and mirror traffic from port 1 to port 50 which has IDS. I also mirror port 2-49 of second switch to IDS.

You will see that second switch has to keep all MAC addresses from two switches into it. That made me concern about its performance. So I want to disable MAC learning process on port 1 of second switch ONLY.


m-villas Sun, 09/21/2003 - 17:39
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Maximum Mac addresses which 2950G/2950 can suport in CAM is 8000 as per CCO. Do you think u reach those many MACs?

If your LAN is completely switched network then I dont think you exceed this limit.

Hope above helps.

m-villas Sun, 09/21/2003 - 17:45
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I dont think it is possible to disable MAC learning on port basis. VLAN basis is suported in 3550 but not sure in 2950.

dave.keith Fri, 02/27/2004 - 12:32
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  • Bronze, 100 points or more

I also have an application where we'd like a few ports (in a VLAN) to not learn any MAC addresses, essenatially turning these switch ports into hub-like ports (flood everything). This is to accomodate several instances of the Microsoft Network Load Balancing 'feature' across a trunk. We're planning o moving one of the load-balanced servers in each cluster to another building, and would like to use VLANs on 2950's. The problem is that we need these ports to act as a hub, no learning.

The note by Mike above suggests that the 3550 can support this function, however I have not been able to discover how by looking through manuals/searching COO. Any pointers ?




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