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New Member

CAM agingtime

Hi,

I'm using a c6500 switch, CAM agingtime is set to default 300secs. Behaviour of the switch is that once agingtime timer expiries, then the whole of the CAM table is flushed, and then follows flooding of initial traffic till CAM table is built, so every 5mins we will have flooding on all ports. Is there a way to configure the switch, so that flooding doesn't occur for traffic which continuously run through the switch, I.E. this periodic flooding doesn't occur at the same time for every single CAM table entry.

Best Regards

Burim Bivolaku

  • Other Network Infrastructure Subjects
4 REPLIES
New Member

Re: CAM agingtime

The entire CAM table is not flushed every 300 seconds. Rather, each entry (MAC address) in the CAM table has a timer associated with it. Each time a frame is received by the switch, the timer for the source MAC address in the received frame is reset (or a new CAM table entry is created if none exists). If the switch does not receive a frame for a particular MAC address in the CAM table for 300 seconds, the entry in the CAM table is purged.

See the following URL for unicast flooding and techniques to mitigate it

http://www.cisco.com/en/US/customer/products/hw/switches/ps700/products_tech_note09186a00800a875c.shtml

Also there is a new feature is CatOS 7.5, that can be used to block unicast flooding. See the following URL

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_7_5/confg_gd/uniflood.htm

New Member

Re: CAM agingtime

What is the behaviour with multicast flooding on a switch which has igmp snooping enabled. I'm definitely seeing multicast flooding every 300 seconds on a port which is not subscribing to any multicast groups?

Once again, thanks a lot for your previous answer.

New Member

Re: CAM agingtime

What is/are the destination MAC addresses (and IP addresses, if applicable) of the flooded multicast packets? Note that IPMP Snooping does not apply to IP multicast packets in the range of 224.0.0.x.

Also note that PAgP packets are transmitted every 300 seconds to the destination MAC address 01-00-0c-cc-cc-cc with SNAP encapsulation of 0x0104. These packets are transmitted if the port channel mode is DESIRABLE or AUTO (the default).

New Member

Re: CAM agingtime

PMFJI. I appreciate your detail explanation on how the CAM flushing works on a switch. Do you happen to know if a switch sends the arp requests to learn these MAC addresses, similar to what the router does?

thanks.

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