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If you have a big network with multiple Access Switches connecting to the core switches or routers then tracing a device like a PC or a laptop for troubleshooting or security purposes is one of those tasks that you often end up doing. This is not a difficult task but can certainly be time consuming.

Lets start with an IP address on hand. If you have an IP address on hand quickly ping and check if the device is pingable. If yes, then simply logon to one of your core switches or routers and do a simple sh ip arp

Core1# sh ip arp 192.168.1.15

Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.1.15           22   0000.1111.1111  ARPA   Vlan1

From the above you know the MAC Address of for the device:

IP Address : 192.168.1.15
MAC Address : 0000.1111.1111

Now, do a show mac-address command on the core switch or router. This will show the interface to which it is connected or through which it is learned.


Core1# sh mac-address-table address 0000.1111.1111

Legend: * – primary entry
age – seconds since last seen
n/a – not available

 vlan   mac address     type    learn     age              ports
——+—————-+——–+—–+———-+————————–
Supervisor:
*   1  0000.1111.1111   dynamic  Yes         10   Te1/1

This indicates that the device is either connected to the port or though another switch which is connected to the interface. Looking at this, it is very likely that this is a uplink (TenGigabit Ethernet link) to another Distribution or Access switch.

Sometimes, the output might show as follows [note the Po1]


Legend: * – primary entry
age – seconds since last seen
n/a – not available

 vlan   mac address     type    learn     age              ports
——+—————-+——–+—–+———-+————————–
Supervisor:
*   1  0000.1111.1111   dynamic  Yes         10   Po1

This indicates that there is a etherchannelis being setup. So do a “show etherchannel” command to find the phsycial ports that are paired.


Core1# show etherchannel summary
Flags:  D – down        P – bundled in port-channel
I – stand-alone s – suspended
H – Hot-standby (LACP only)
R – Layer3      S – Layer2
U – in use      f – failed to allocate aggregator

 M – not in use, minimum links not met
u – unsuitable for bundling
w – waiting to be aggregated
Number of channel-groups in use: 6
Number of aggregators:           6

Group  Port-channel  Protocol    Ports
——+————-+———–+———————————————–
1      Po1(SU)          -        Te1/1(P)   Te2/1(P)

This shows the ports Te1/1 or Te2/1 as a source through which the address is learnt.

Now, do a “show cdp neighbors” to show the directly connected devices.


Core1# sh cdp neighbors

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
Access1          Ten 1/1            129         R S I     WS-C6509  Ten 1/1

That tells you, it is the Access switch 1 that is connected to Te1/1 and not the device itself.

Now, log onto the Access switch and do a “show mac-adddress-table” command for the MAC address and that should show the interface to which it is connected

[NOTE: unless it is a distribution switch to again there are a bunch of Access switches connected in which case, you need to go through the whole procedure as above again]


Access1# show mac-address-table 0000.1111.1111 

 vlan   mac address     type    learn     age              ports
——+—————-+——–+—–+———-+————————–
Supervisor:
*   1  0000.1111.1111   dynamic  Yes         10   Gi1/24

As you can see which port the device is connected and on which switch.

Now do a “show interface” command to show the port details.


Access1>sh int gigabitEthernet 1/24

GigabitEthernet1/24 is up, line protocol is up (connected)
Hardware is C6k 1000Mb 802.3, address is xxxx.xxxx.xxxx (bia xxxx.xxxx.xxxx)
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s
…..
…
..
.

There you go you found the device switchport that you tried to trace!!!

source: here