Thousands of ARP entries on outside interface.

[Jeffrey Simon]

Hello, I have been reading the forums for about an hour but I can't pin down anyone who has this exact problem.  I have a Cisco 2821 with gig0/0 configured for DHCP as my outside interface.  I noticed that my ARP table is enormous.  I reloaded the router but the problem is starting to happen again.  Currently I have 17,533 ARP entries and the router has only been up for 1 day, 21 hours, 16 minutes. All of my other routers configured for DHCP on an outside facing internet have a few ARP entries, but certainly not thousands.  Can anyone shed some light on this and tell me how to fix the problem?

r2821a-fios#show arp summary

Total number of entries in the ARP table: 17533.

Total number of Dynamic ARP entries: 17529.

Total number of Incomplete ARP entries: 0.

Total number of Interface ARP entries: 4.

Total number of Static ARP entries: 0.

Total number of Alias ARP entries: 0.

Total number of Simple Application ARP entries: 0.

Total number of Application Alias ARP entries: 0.

Total number of Application Timer ARP entries: 0.

Interface              Entry Count

GigabitEthernet0/0           17523

GigabitEthernet0/1.200           1

GigabitEthernet0/1.20            5

GigabitEthernet0/1.10            4

[Richard Burts · Accepted solution]

Jeffrey

This is a classic symptom of what happens when you point a static route at an Ethernet interface. So I am going to guess that this router has a default route configured that looks like ip route 0.0.0.0 0..0.0.0 Gig0/0.

The issue with this is that when the router attempts to forward a packet toward some Internet destination using interface Gig0/0 as the exit interface it needs to have a destination MAC address and since it does not have one in the ARP table then it sends an ARP request for that address and adds its MAC into the table.

I suggest that you change the default route to something like ip route 0.0.0.0 0.0.0.0 dhcp

HTH

Rick

[Jeffrey Simon]

Hi Richard,

You are 100% correct.  I am making the change now and hopefully it will resolve the problem.  Thank you for the fast feedback!

[Richard Burts]

Jeffrey

I am glad that my explanation helped to solve your problem. Thank you for using the rating system to mark this question as answered. It makes the forum more useful when people can read about a problem and can know that a solution was found. Your marking has contributed to this process.

HTH

Rick