Subject: RE: Cisco 2610 ARP table problem

From: CARDOSO Jorge Miguel <[email-masked]>
Date: Fri, 1 Jun 2001 19:58:02 +0100

It can be a result of config mismatch, present or past config:

A) Some of the possible configuration mismatch that could generate this kind
of output:

1* !interface ethernet 0/0
   !ip address 10.10.1.15 <subnetmask> secondary

2* manually assigned entry @ the arp-cache table
  (config)#arp 10.10.1.15 00d0.c0fc.81c0 arpa alias

3* its also possible to change ethernet interface mac-address!But this is
not usually used!
   * you can check if bia (burn in mac address) address is the one that is
assigned to the interface, using the commmand "router#show interface
ethernet 0/0"

4* HSRP config. also generate special ARP entrys.

On the collumn interface you see ethernet 0/0, it could be case (1). You
have a secondary ip address configured on router or you had some time ago a
secondary ip address assigned to ethernet 0/0.

B) I supose that you have already check properly the config. and router is
working properly. This means that someone has assigned an invalid MAC
address to another system! isolate it and disconnect it from the lan, until
mac-address is corrected!

regards,
j0rge card0s0

-----Original Message-----
From: Hagop Karaoghlanian [mailto:[email-masked]]
Sent: Sexta-feira, 1 de Junho de 2001 18:46
To: [email-masked]'
Cc: Mason Livingston
Subject: Cisco 2610 ARP table problem

Hello All, I have alittle problem, my cisco 2610, which is in a production
environment, is running NAT on ISDN, this router only has one thernet
interface, and its address has been assigned 10.10.1.1 255.255.255.0, and
when I do a show arp on it, it thinks that has 10.10.1.1 and 10.10.1.15?????
why is this.  I have tried to clear the arp table, and the nat table, but to
o avail..heres what I get from show arp...

Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.1.1               -   00d0.c0fc.81c0  ARPA   Ethernet0/0
Internet  10.10.1.15              -   00d0.c0fc.81c0  ARPA   Ethernet0/0
Internet  10.10.1.100             3   00b0.d0c1.e9f2  ARPA   Ethernet0/0
Internet  10.10.1.103             3   00b0.d0f0.f21f  ARPA   Ethernet0/0
<snip>

thanks everyone....

---------------
Hagop R. Karaoghlanian, CCNP
Network Specialist
AT&T Canada IES
905 King St West
416-341-5876
[email-masked]

"Gold leads into gold, then into restlessness, and finally into crushing
misery."     Kahlil Gibran

---

From: "Wang Xianzhu" <[email-masked]>
Date: Mon, 4 Jun 2001 09:02:35 +0800

> mac-address is corrected!

But how can the invalid MAC be permanent in the router?
So I think that is still a configuration problem.
'clear arp-cache' doesn't affect permanent arp entries.
Try to use 'no arp 10.10.1.15' in configuration mode.

Regards,
Wang Xianzhu