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We have nexus 7000, there are multiple vlans

But I am unable to ping from 10.48.32.0/24 to 10.48.42.0/24

There is user 10.48.32.54 i am unable to trace its switch port,please advise.

Below is troubleshooting I have tried.

SW01# sh ip arp | in 10.48.32.54
10.48.32.54     00:09:11  8cdc.XXX.471c  Vlan32      

USIDCDISSW01# show  mac address-table | include 471c
* 32       8cdc.XXX.471c    dynamic   0          F    F  Po201

SW04# show  ip arp | in 471c

SW03# show  ip arp | in 471c
below is show output of below command where SW04 and SW03 are showing but I am unable to trace switch port of mac 8cdc.XXX.471c 
 cdp  neighbors interface Po201 detail ############################

USIDCDISSW01# show  cdp  neighbors interface Po201 detail

----------------------------------------

Device ID:SW03()

System Name: SW03



Interface address(es):

    IPv4 Address: 10.48.130.55

Platform: N5K-C5596UP, Capabilities: Switch IGMP Filtering Supports-STP-Dispute

Interface: Ethernet2/45, Port ID (outgoing port): Ethernet1/1

Holdtime: 122 sec
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2 Answers 2

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Unless the switch itself is generating packets, there will be nothing in the switch's ARP table. As a layer 2 device, it doesn't need to resolve IP addresses. A layer 3 switch only needs to resolve a host address if it is directly connected. Otherwise, it forwards the packet to the next hop router (which will be in the ARP table).

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Extending Ron's answer, here is an advice how to trace the user:

  1. Find out the user's MAC address. There are multiple ways to do that, but possibly you have an IP Address Management (IPAM) system or you can find it out using the IP address. On the switch or router being the layer 3 gateway of the user's VLAN, try to ping the end PC. It does not matter if it answers or not, the device should however ARP for the MAC address of the user's device, which you can find out using show ip arp | i <IP-Address>, which should give you the MAC address.
  2. The next step is to track the path of this MAC address using the layer 2 forwarding information of each switch. Use show mac address-table | i <MAC-Address> which will give you the entry in the MAC address table including the outgoing interface of this entry.
  3. Find out which switch is connected to this port for example using CDP.

Repeat 2 and 3 until you found the edge port, where the end user is connected to.

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  • Thanks Waza, Same step 2 and 3 I am trying , but through CDP I am getting two switches (SW03 and SW04) but in none of switches ARP has that MAC, so I am unable to trace the user 10.48.32.54
    – shailendra harinkhede
    Jul 19, 2016 at 14:18
  • Again, you won't find the IP address in the ARP table as the switch has no need to resolve the IP address. You only will find the IP address in the ARP cache of the device having the gateway address of the subnet. On the other devices you have to look at the MAC table, not the ARP table.
    – Daniel
    Jul 19, 2016 at 14:43

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