Asymetric Routing (timers)

[ccnaluna93]

I'm studying the Asymetric routing issue, Cisco recomends that we adjust the ARP timer to be lower than CAM timer in the FHRP device in order to not flood to all ports by CAM, but anyway the device will flood because of ARP timer, so what is the point with this if we are going to get a flood anyway?

Thanks in advance.

Regards!

[mlund]

Hi

If we don't adjust, the default timers for arp is 300 minutes and for cam 5 minutes. That will result in flooding of unknown mac in the cam for 255 minutes. If we adjust them to be nearly the same, the arp will flood ones every period you have set it to, and then cam will learn. So if you adjust the cam and arp both to 10 minute for example, you have one flood every 10 minutes instead of continous flood for 255 minutes.

/Mikael

[ccnaluna93]

Hi mlund,

Thanks for answering. I think there is something that I'm not understanding yet.

 That will result in flooding of unknown mac in the cam for 255 minutes.

But why would CAM flooding for 255 minutes if the timer is set to 5 minutes? It would be every 5 minutes, isn't it? 

[mlund]

see Joseph:s excellent explanation below, that is what is happening.

l

[Joseph W. Doherty]

". . . but anyway the device will flood because of ARP timer, so what is the point with this if we are going to get a flood anyway?"

How/why do you believe that?

If you asking about an ARP flood, that's normal for a broadcast. If you're asking about once the ARP and CAM timers have been properly adjusted, you shouldn't be seeing asymmetrically unicast flooding.

Have you read: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23563-143.html?dtid=osscdc000283 ?

[ccnaluna93]

Joseph thanks for answering.

My doubt is because of the flood process of ARP and CAM, even if you avoid one of them you will get a flood from the other one.

[Joseph W. Doherty]

Not if the timers are adjusted "correctly".

The underlying problem is a CAM that ages out before the ARP does. So, the switch no longer knows what port the MAC is on, but the ARP cache continues to send to it. If you insure the ARP cache ages out before the CAM, ARP will broadcast for the MAC and the switch will then be able to refresh its CAM (avoiding the need to flood [the unicast traffic]).