ARP detstination mac-address 0000.0000.0000

Question

Internet Router--->3550 Switch-->Nortel Contivity
63.169.164.134-->63.169.164.140--> 63.169.140.136
All the devices are having public IP addresses and are in Vlan 100.
Sometimes the Internet Router is not able to ping or connect to the Contivity where as the switch is able to access both the devices (Internet Rtr & Contivity).
While checking on the switch & Internet rtr i capture the following logs.
Switch#sh log
Mar 25 11:17:17.990 EDT: IP ARP: creating incomplete entry for IP address: 63.169.164.136 interface Vlan100
Mar 25 11:17:17.990 EDT: IP ARP: sent req src 63.169.164.140 0012.800b.a780,dst 63.169.164.136 0000.0000.0000 Vlan100
Internet RTR#sh log
Mar 25 11:17:17 EDT: IP ARP: rcvd req src 63.169.164.140 0012.800b.a780, dst 63.169.164.136 GigabitEthernet0/1
Mar 25 11:20:54 EDT: IP ARP: rcvd req src 63.169.164.138 0018.b964.66fc, dst 63.169.164.145 GigabitEthernet0/1
We have done a static ARPA entry in internet rtr for Contivity but still the issue remains same. The moment the issue persists again i tried to clear the arp on switch but it didn't make router to get reply from Contivity when i did the same clear arp-cache on internet rtr, it started getting communicate with Contivity.
I am not able to find the solution of this issue and the reason for that, now every time i have to do clear ip arp-cache on rtr whenever the issue comes down.
Also i want to understand the situation when a dest mac-address can be 0000.0000.0000.
Any help on this will be appreciated.
Thanks.

Answer 1

Bhupesh
If a router receives a packet to forward to a destination address which is on a local LAN but the destination IP address does not appear in the arp table then the router creates an incomplete entry in the ARP table (it is incomplete because the router does not have the destination MAC address and is attempting to learn it). The router creates the incomplete entry in the ARP table and sends an ARP request. If the router receives an ARP response then it puts the destination MAC address into the ARP table and the entry is now complete. If no ARP response is received the router will purge the incomplete entry. Note that the router can not forward the IP packet that caused the incomplete entry and the router will drop that IP packet.
In considering the problem with contivity I had been assuming that the problem was on rtr. But it occurs to me that it is quite possible (and even likely given the fact that you mention which is that in the problem the switch can still ping contivity ) that the problem is on contivity. I wonder if for some reason contivity gets an incorrect MAC for rtr? I suspect that clear arp on rtr fixes the problem because as it clears the arp table I believe that rtr will send a gratuitous arp which refreshes the ARP entry in contivity. In the time of the problem can you check the table in contivity?
HTH
Rick