[c-nsp] HSRP issues on Cisco3550

[Sam Stickland]

Gert Doering wrote:
> Hi,
>
> On Wed, Nov 08, 2006 at 03:52:12PM +0000, Sam Stickland wrote:
>   
>> I'm not sure I understand this - wouldn't the standby router populate 
>> it's CAM table from the ARP replies? And it has to send out and ARP 
>> reply (and get a response), or the router can't populate the destination 
>> MAC address in frame.
>>     
> The problem is that the standby router will have an ARP timeout of 4 hours
> (default) while the switches in between will have a CAM timeout of 
> much less (default is 300 seconds, if I remember correctly).
>
> When the CAM tables timeout, the ARP tables are still valid for nearly
> 4 hours - and in that time, the switches will not see packets from 
> the end hosts (no ARP, and all normal packets go to the other router)
> so you'll end up with flooding.
>
> (Ethernet just sucks).
>   
Ah, gotcha - that makes senses. Having asked around here it turns out 
many of my colleges have worked on networks where the ARP timeout was 
reduced to the same as the CAM timeout (300 seconds), to 
prevent/minimise this sort of flooding.

The one question they weren't able to answer is why they are such wildly 
different times to start with though. Anyone?

S