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Find a device in the network`Z`ZBOOKMOBI8ޢx\MOBI FC @EXTHdCollini%Miscellanous ,Troubleshootingg

Always start at the Layer 3 boundary of the subnet. Do a show arp to find the MAC address. CORE1#sh arp | inc 10.142.3.2 Internet 10.142.3.207 11 0001.e67a.dc20 ARPA Vlan423 Internet 10.142.3.206 10 0001.e697.cbe6 ARPA Vlan423 Internet 10.142.3.254 - …

j'Sun, 26 Aug 2012 22:41:00 +0000pKhttp://www.packetpros.com/2012/08/find-a-device-in-the-network.htmlePacketprosf Copyright packetpros.comFind a device in the network

Always start at the Layer 3 boundary of the subnet. Do a show arp to find the MAC address.

CORE1#sh arp | inc 10.142.3.2
Internet 10.142.3.207 11 0001.e67a.dc20 ARPA Vlan423
Internet 10.142.3.206 10 0001.e697.cbe6 ARPA Vlan423
Internet 10.142.3.254 - 0000.0c07.ac01 ARPA Vlan423
Internet 10.142.3.253 - 000d.662f.2980 ARPA Vlan43
Internet 10.142.3.252 0 000d.662f.2140 ARPA Vlan423
Internet 10.142.3.250 0 00c0.f044.5b72 ARPA Vlan423

If you do not see the device in the ARP Table (like above) you may need to ping it.

CORE1#ping 10.142.3.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.142.3.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

CORE1#sh arp | inc 10.142.3.2
Internet 10.142.3.207 11 0001.e67a.dc20 ARPA Vlan423
Internet 10.142.3.206 10 0001.e697.cbe6 ARPA Vlan423
Internet 10.142.3.254 - 0000.0c07.ac01 ARPA Vlan423
Internet 10.142.3.253 - 000d.662f.2980 ARPA Vlan423
Internet 10.142.3.252 0 000d.662f.2140 ARPA Vlan423
Internet 10.142.3.250 0 00c0.f044.5b72 ARPA Vlan423
Internet 10.142.3.2 0 000c.5502.1e62 ARPA Vlan423
CORE1#

Now we see the device and its MAC address. Now we need to find out where the core switch learned the address from.

CORE1#sh mac-address-table address 000c.5502.1e62
Legend: * - primary entry
age - seconds since last seen
n/a - not available

vlan mac address type learn age ports
------+----------------+--------+-----+----------+--------------------------
Supervisor:
* 423 000c.5502.1e62 dynamic Yes 80 Gi13/13

We can see that the switch learned it via VLAN 423 through interface Gi13/13. Next we need to see what Gi13/13 is connected to.

CORE1#sh cdp neighbors gi13/13
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID Local Intrfce Holdtme Capability Platform Port ID
DISTR-1 Gig 13/13 150 R S I WS-C4506 Gig 2/5
CORE1#

You can see from CDP that Gi13/13 is connected to switch DISTR-1. Next we telnet into that switch and look for the MAC address again.

DISTR-1#sh mac-address address 000c.5502.1e62
Unicast Entries
vlan mac address type protocols port
-------+---------------+--------+---------------------+--------------------
423 000c.5502.1e62 dynamic ip GigabitEthernet4/1

DISTR-1#

Again the switch learned the MAC on VLAN 423 via Gi4/1. We again look to see what is connected to interface Gi4/1.

DISTR-1#sh cdp neighbors gi4/1
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID Local Intrfce Holdtme Capability Platform Port ID
ACCESS-SW23.wpsic.com
Gig 4/1 136 S I Cisco WS-CGig 0/2
DISTR-1#

We see that the MAC address was learned from switch ACCESS-SW23. Telnet into that switch and look for the MAC again.

ACCESS-SW23#sh mac-address-table address 000c.5502.1e62

Mac Address Table
-------------------------------------------

Vlan Mac Address Type Ports
---- ----------- -------- -----
423 000c.5502.1e62 DYNAMIC Fa0/27
Total Mac Addresses for this criterion: 1
ACCESS-SW23#

By looking under the Ports header we see that the access layer switch learned the MAC address from switch port Fa0/27. That’s the switch and switch port the device is connected too, ACCESS-SW23 on port fa0/27!

FLIS00 00 00 0800 4100 0000 00 00 00ff ff ff ff00 0100 0300 00 00 0300 00 00 01ff ff ff ffFCIS00 00 00 1400 00 00 1000 00 00 0100 00 00 0000 00 0f 1100 00 00 0000 00 00 2000 00 00 0800 0100 0100 00 00 003918400778