Networks·41,393 discussions

Cisco Switches/Stacks and Finding connected devices IP’s

Locked

Hello beautiful people!

I have a question that I am not sure where to ask so I will ask you fine people! When I am on my Catalyst 3750 Stacked (x2). When I do a show arp I only see Vlan1 when I have Vlan100 set up also… If I do show mac address-table it shows me:

100 0000.5e00.010a DYNAMIC Po1
100 001a.8c18.328c DYNAMIC Po3
100 0022.83f6.dff0 DYNAMIC Po1
100 0023.9cd5.e7f0 DYNAMIC Po1

I know I have a few interfaces set up as port-channels and obviously these 4 connect to VLAN100 so why can’t I see it in my arp table? How would you go about figuring out the IP’s of what are connected to those ports… I hope someone may be able to help me!

Thanks

Topic

Clarifications

In reply to Cisco Switches/Stacks and Finding connected devices IP’s

Clarifications

you can only see layer three…

In reply to Cisco Switches/Stacks and Finding connected devices IP’s

You will only see arp entries if the switch has a layer 3 interface for that vlan. Vlan1 has an ip that is why you see arp entires. For the other vlans you will only see layer2 , mac addresses

Ping the broadcast address of the local LAN and then check the arp cache. this will show you IP.

For example:

3560# ping 192.168.0.255

3560# show arp

Reponse To Answer

In reply to you can only see layer three…

I did ping the broadcast address before and it didn’t help me at all although I am curious if I ping the broadcast address will it only get responses from directly connected devices or all over the network?

Sorta tricky question to answer…it depends

In reply to Cisco Switches/Stacks and Finding connected devices IP’s

Of course the ICMP protocol (Ping) is only going to propagate within a particular layer-3 subnet, so you would have to do the ping while connected to a particular VLAN to see what’s in that VLAN. VLANs break up broadcast domains, so protocols like ICMP, ARP, and DHCP will, of course, only play within the walls of each VLAN.

Solarwinds makes a free IP address tracker which discovers and tracks IP addresses. There’s also a product with a free trial called Orion IP Address Manager that will do this.

[Author]

Replies