I have a problem with ARP which I've been trying to resolve for a couple of days now and it's doing my head in.
Router (cisco 7200) -> Head Switch (cisco 3750 / VLAN 613 configured for customer) -> Customer's Switch -> Customer's LAN (172.16.2.0/24 on VLAN 613)
The customer is reporting arp issues when trying to reach another machine on the same LAN segment. When he pings this particular host (eg: 172.16.2.26) on his LAN segment, our core router for some odd reason answers to the ARP query (see tcpdump below).
13:53:16.020327 arp who-has 172.16.2.26 tell 203.x.x.58
13:53:16.020485 arp reply 172.16.2.26 is-at 00:04:61:46:c1:42
13:53:16.051428 arp reply 172.16.2.26 is-at 00:0c:86:74:f0:1b
13:53:18.036330 arp who-has 172.16.2.26 tell 203.x.x.58
13:53:18.036468 arp reply 172.16.2.26 is-at 00:04:61:46:c1:42
13:53:18.036753 arp reply 172.16.2.26 is-at 00:0c:86:74:f0:1b
00:04:61:46:c1:42 is the actual machine with IP address 172.16.2.26
00:0c:86:74:f0:1b is the MAC address of the router's sub interface.
Router#sh int g0/1.613
GigabitEthernet0/1.613 is up, line protocol is up
Hardware is BCM1250 Internal MAC, address is 000c.8674.f01b (bia 000c.8674.f01b)
There's no routes on the router for 172.16.2.0/24 so we're a bit puzzled why the router thinks it has a route to get to 172.16.2.26 and why it keeps replying back with it's MAC address.
Router#sh ip route 172.16.2.26
% Subnet not in table
Router#sh ip route | inc 172.16.2
To make this more interesting, I plugged my latop into the customer's lan segment and can not replicate the issue when I ping 172.16.2.26 (I'm only getting one arp reply from the actual machine). The customer has reported that the double arp replies is not happening on all machines as well.
Is there anyway to track down why the router is responding to arp requests it has no route for and how can we go about fixing this?
Cisco Internetwork Operating System Software
IOS (tm) 7200 Software (C7200-P-M), Version 12.3(22), RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by cisco Systems, Inc.
Compiled Wed 24-Jan-07 20:17 by ccai
Image text-base: 0x60008AF4, data-base: 0x61A62000
ROM: System Bootstrap, Version 12.2(8r)B, RELEASE SOFTWARE (fc1)
BOOTLDR: 7200 Software (C7200-KBOOT-M), Version 12.2(15)B, EARLY DEPLOYMENT RELEASE SOFTWARE (fc1)
Thank you in advance.