show arp on a router

F Martinez · ‎08-11-2016

Hello,

Could you tell me what it means when you have two routers connected to each. And the interfaces on both sides are configured with IP Addresses but you dont see the MAC address of each interface after issuing the show arp command?

Thanks in advance!

Peter Paluch · ‎08-11-2016

Hello,

Lack of ARP entries is not always an indication of a problem. Yes, it can mean that one device was unable to find out the MAC address of the other device, but it can also mean that it never asked for it in the first place.

A couple of questions:

Are you using Ethernet ports to connect the two routers together, or is it a different interface type?
Does the show ip int brief command say that the interfaces on both routers are "up" in the "Status" and "Protocol" columns?
Are the IP addresses of these interfaces in the same IP network according to their netmask?
Can you ping one router from the other?

Best regards,
Peter

F Martinez · ‎08-11-2016

Im sorry. I thought it was two routers on both sides. But it's only one router that's connected to a DSL modem.
- The router is using its interface fastethernet to connect to the modem
- Yes the show ip int brief command say that the interfaces are "up" "up"

Actually, I was trying to turn up the Internet connection of the router. And like I said above, the interface fastethernet of the router is connected to the DSL modem. We found out that the modem was configured with DHCP, not static IPs. Anyway, we resolved the issue by asking the ISP to configure their modem with static IPs. And they me a different IP that I configured on the interface fastethernet. Now, everything is working as expected.

So looks like that's the cause of the issue which is, I was not able to see the arp of the DSL modem after issuing the show arp command.

Peter Paluch · ‎08-13-2016

Hello,

Oh, I see - very well, then. Glad you got your issue resolved, and thanks for letting us know.

Best regards,
Peter