ARP Question , clearing and time

[ccoletti]

Hello Everyone.

Two question if I may.

The Arp Age time on a cisco router ( 4000 and higher 7000 or rsp1) , is this expressing how long the node has been connected to the router or how long ago the end device was last updated between the router and end device.

Second Question,

Clearing the Arp table, does this have and ill effect on the router to end node connection?

ST1#sh arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet x.x.x.x - 0000.3012.c141 SNAP TokenRing1/2

Internet x.x.x.x - 0000.3012.c1c1 SNAP TokenRing1/3

Internet x.x.x.x 158 0008.28c4.3ed0 SNAP TokenRing1/0

Internet x.x.x.x - 0000.3012.c11e SNAP TokenRing0/0

Internet x.x.x.x 160 0008.28c4.3e50 SNAP TokenRing0/0

Thank you

Charlie

[Richard Burts]

Charlie

The age time is the amount of time that this entry has been in the ARP table (so basically it reflects when was the last update).

In general clearing the ARP table will not have ill effect on communication with end stations. When you clear the table, Cisco clears the entry and immediately sends an ARP request. If the station is still on line it will respond and the table entry is rebuilt. The implication is that when you clear the table you will create some extra traffic on the segments connected to the router. And there is the possibility that while the router has cleared the table and is waiting for the ARP response that it will receive a packet which is to be forwarded to that end station. If the router receives a packet to forward and is not able to find an entry in the ARP table it will discard the packet. So there is some (very) small chance that when you clear the table some packets might not get delivered.

So if you ask in absolute terms if there is any impact on end station communicatin the answer would be yes. If the question is whether there is significant impact I would say that the answer is no there is not significant impact on end station communication when you clear the ARP table.

HTH

Rick

[ashok_boin]

Hi Rick,

I feel there will be significant impact for clients connectivity for eg you had more than 200 hosts in your LAN.

Is there any option in Cisco routers to clear only one ARP entry for testing?

Thanks in advance...

Regards...

-Ashok.

[Richard Burts]

Ashok

I am not sure why you think there would be significant impact for clients connectivity. The ARP cache on the client stations is not affected so they can continue to send without interruption. When you clear the ARP cache on the router it immediately sends an ARP request to all of the addresses that it is clearing and should pretty quickly get responses. It looks to me that worst case the router might have to drop one packet per client (probably for a few clients) before it gets the ARP responses.

I do not believe that there is a way to clear a single entry. It is possible to limit the clearing of ARP cache to the entries associated with a single interface so if your router had several LAN interfaces you could clear a part but not all of the ARP cache.

HTH

Rick

[ashok_boin]

Hi Rick,

Thank you for clarification.

I have found only "clear arp-cache" command with no other option (to clear by the interface). I would be grateful if you confirm me whether it is hidden option for this command.

-Ashok.

[Richard Burts]

On the router that I checked it on it is not a hidden command. If you enter clear arp-cache ? it should indicate that you can to clear the entire cache or you can put an interface and clear only the entries associated with that interface.

HTH

Rick

[ashok_boin]

Hi Rick,

Thank you for your patience.

Cleared.

Regards...

Ashok.

[ccoletti]

Richard, thank you for the update, i issues the command and i was able to work out my issues with the router.

Thank you

Rq,

Charlie

[ccoletti]

Hello Rick ,

Very Cool, Thank you so much for taking time out

to update my question. Have a great weekend.

Charlie