MAC Address not in ARP Table 1

[viadisky]

Hi,

How come I can see a MAC Address in the forwarding table but not in the ARP Table? Is it because the node connected to my network device isn't sending any data at all (although it is still receiving broadcast or multicast traffic).

Is there any command which I can use to make the connected node to send data (except for ping) back to my network device (router or layer 3 switch) so that I can see its MAC Address in the ARP Cache?

Many thanks!!!

 

[KiscoKid]

There's a couple of things that puzzle me about your post.

I'm not aware of MAC addresses being present in the routing table or route cache. Did you mean something other than these when you referred to the 'forwarding table'

If the node in question is directly-connected (i.e. it resides on a subnet directly-attached to the router) and you don't see an ARP entry for it, I would summise this is because it's not routing traffic to/via your router. This node will only ARP to the router if it needs to communicate offnet, i.e. to a subnet it is not a member of (assuming this router is the default gateway for the node of course).

There's only a limited number of ways that may generate an ARP entry for the node in question. They are by ping, traceroute and telnet.

Remember that if this node is not a member of a directly-connected subnet that the router belongs to, the ARP entry you will see will not be the node itself but the MAC address of a router that knows how to get to this node.

I hope this helps.

 

[viadisky]

Hi KiskoKid,

Thanks for the information you provided.

The forwarding table I mentioned in my previous post is like the bridging table or MAC Address Table in Cisco (sorry I used the Nortel term "forwarding") ...

There is a possibility that the node/server directly connected is only talking to the nodes on the same subnet that's why it isn't in the ARP cache, but majority of the nodes connected to the Cisco 6500 we have are our main servers (so for sure users are connecting to them unless this is a test server

Then the only way for the node to generate ARP cache is to do ping sweep on the range of IP Address in the router/switch (6500).Traceroute and telnet are not possible since I only have the MAC Address info of the node.

Best regards.