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Webcast-Catalyst9k
New Member

Cisco Routers & ARP

If a host on a LAN is down, but a Cisco router keeps getting IP packets destined for the host that is down, will it try to send out an ARP request for every IP packet is receives for that host?

How will that effect the router CPU and is there a potential for the router to crash?

Is there a way to explicitly configure a Cisco router to NOT send out ARP requests for a specific IP address on a LAN?

Thanks

1 ACCEPTED SOLUTION

Accepted Solutions
Silver

Re: Cisco Routers & ARP

In global configuration:

arp

Cisco wont arp for this IP any more.

Thanks.

3 REPLIES
Silver

Re: Cisco Routers & ARP

If the host is down, ARP requests will time out and router will send ICMP error "Host unreachable". ARPs are not sent for every packet received for the unknown destination. I think they are repeated every five seconds and after few attempts the router timesout.

Too many ARPs, though not healthy for a network or the router, usually don't crash it.

If you statically add an ARP for some IP address in the ARP table, router wont ARP request for that entry, as that entry will be treated as permanent in ARP cache table.

Hope this helps.

New Member

Re: Cisco Routers & ARP

What is the Cisco router command to statically configure an IP address-to-MAC address so that ARP won't be needed?

Silver

Re: Cisco Routers & ARP

In global configuration:

arp

Cisco wont arp for this IP any more.

Thanks.

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