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ARP Cache problem. Replacing an old catalyst.

Posted on 2005-04-09
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Last Modified: 2010-07-27
I’ve got two Catalyst switches, I am changing one switch out soon and need to clear the arp cache on the other remote Catalyst;  there is no other route/link to the remote catalyst. Unfortunately I can't send a tech or drive to this remote location in a reasonable amount of time.

If I run 'clear arp' and then immediately unplug the fiber link from the local switch would the remote catalyst already have the old arp entry?

Could I reboot the remote catalyst right before I install the core switch... would this clear the arp cache?

How can I change the arp aging time in a catalyst 5000, If I could temporarily set the time-out on arp caching for say 5 mins, then I could sync the two catalysts after the time-out occurs, I think the default time out is 4 hours!?!

Anyone, Any ideas...  Please help.



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Question by:QsSnrEng
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by:Don Johnston
ID: 13746834
The only time the ARP cache would be an issue on a Catalyst switch would be when you are communicating with the switch itself. Such as when you telnet to it or ping it.

I'm guessing you must be referring to the MAC-address table. The default aging time of MAC entries is 300 seconds (5 min). But even then, when an interface comes up, the switch goes through two transitional spanning-tree states. One of them is learning mode where all address entries are aged out after only 15 seconds.

Unless, that is, I don't understand the question.

-Don
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by:QsSnrEng
ID: 13747581
I was told "Once you swap out the switch that connects directly to the router, you may have a delay before everything starts working right - until the arp cache times out, or until you manually clear the arp cache."  

In my case its two Cisco Catalyst's switches, I would assume I will have the same problem as stated above, correct?

If I understand you correctly when I unhook the fiber from the old Catalyst and plug it in to the new Catalyst, the remote Catalyst will sync up within 15 secs due to STP?

I have three Catalysts, two are in production and the new Catalyst waiting to be installed.

What is the difference between the MAC-address table and the arp table?  I thought they were updated the same way?

I have confirmed that "set cam agingtime 1,5-8,10-14,20,1003,1005 300" is setup and also my "set spantree fwddelay 15 " is setup for vlan1
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by:Don Johnston
ID: 13747892
>I was told "Once you swap out the switch that connects directly to the router, you may have a delay before everything
> starts working right - until the arp cache times out, or until you manually clear the arp cache."  

I guess you could say that. The only time the arp cache entries will cause a problem is when you change the IP address or MAC address of a destination device. I think this person was thinking about spanning tree convergence issues.

>If I understand you correctly when I unhook the fiber from the old Catalyst and plug it in to the new Catalyst, the
> remote Catalyst will sync up within 15 secs due to STP?

Depends on your defination of "sync up". ;-)
Assuming 802.1d Spanning tree, the switches will will move the port into learning mode within 15 seconds. At that point, the switch will set the aging timer on existing entries to 15 seconds and begin learning MAC addresses. After 15 more seconds, the port will move into forwarding mode and start moving traffic.

>What is the difference between the MAC-address table and the arp table?

The MAC-address table is used by the switch to make forwarding decisions on all frames it recieves. The ARP table is used by the switch when it (the switch itself) needs to create frames to a destination IP address.

>I have confirmed that "set cam agingtime 1,5-8,10-14,20,1003,1005 300" is setup and also my
> "set spantree fwddelay 15 " is setup for vlan1

These are the default settings. The MAC-address table (referred to as the CAM table on CatOS platforms) is set to 300 seconds for all VLANs that exist and the forward delay timer is set at 15 seconds.

-Don
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Author Comment

by:QsSnrEng
ID: 13748587
Since I am trying to minimize down time (less then 15 mins) and keep the network changes transparent to the remote catalyst, I have configured the new Catalyst with the same ip address as the catalyst I am about to remove, I want to make sure that the remote Catalyst will pass data to the new catalyst. (The ip address will be the same but the mac address of course will be different).  Does this sound like it is possible, "hook up the new cartalyst and leave the remote catalyst alone?  Thanks Don for your help, I really apreciate it.  This may or may not help but my old Catalyst 5000 is running CatOS and my new Catalyst is 4506 which is running IOS.
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Don Johnston earned 2000 total points
ID: 13748625
Shouldn't be any problem at all.

Passing data from other devices will have nothing to do with the ARP cache at all.

When the link between the switches is disconnected, the arp cache entries that are associated with that port should be removed immediately. When the new switch is connected, the remaining switch will need to send an ARP request to re-learn the MAC for the new switch. This, of course, will only impact traffic originating from the existing switch going to the new switch. User traffic will be moving within 30 seconds.

-Don
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Expert Comment

by:lrmoore
ID: 13749011
You should have no problems.
The ARP cache is layer 3 IP address to MAC address mapping. This has absolutely nothing to do with switch-switch layer 2 communications. That's all done with the mac-address tables.
Yes, it can cause temporary issues with a router directly connected, but that's only because the router and only the router keeps that arp cache.
Have no fear !
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by:mikebernhardt
ID: 13756514
My experience is that the issue of clearing the arp cache when swapping out a switch is that the ROUTER needs it's arp cache cleared. Just get your switches swapped out and then on the router, type "clear arp" and you should be good to go. Switches learn MAC addresses from the source at Layer 2 and ARP is not an issue on them.
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Author Comment

by:QsSnrEng
ID: 13758031
I didn't have a problem at all.  After swapping each cable, traffic starting coming back around 15 secs later.... Just as hoped!  Thanks for the reassurance!  I really appreciate it.  Thanks again.
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