Questions

Cisco Switches/Stacks and Finding connected devices IP's

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Cisco Switches/Stacks and Finding connected devices IP's

BrianBlaze
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:
<ommited output>
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
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    robo_dev

    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

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    BrianBlaze

    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?

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    robo_dev

    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.

  • +
    0 Votes
    robo_dev

    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

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    0 Votes
    BrianBlaze

    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?

    +
    0 Votes
    robo_dev

    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.