Table of Contents
Cisco Interfaces In Use
Jan 2024
Introduction
A really useful task to perform when looking at remote systems is to see what interfaces on a Cisco Switch are in use (as in connected).
Interface Status
Lets look at how to see what interfaces we are using: sh interfaces status This has to be performed from the top level, same part of the Cisco menu you would do sh vlan br for instance.
sh interfaces status Port Name Status Vlan Duplex Speed Type Gi0/1 connected 2 a-full a-1000 10/100/1000BaseTX Gi0/2 notconnect 2 auto auto 10/100/1000BaseTX Gi0/3 notconnect 2 auto auto 10/100/1000BaseTX Gi0/4 notconnect 2 auto auto 10/100/1000BaseTX Gi0/5 connected 2 a-full a-1000 10/100/1000BaseTX Gi0/6 connected 2 a-full a-1000 10/100/1000BaseTX Gi0/7 connected 2 a-full a-1000 10/100/1000BaseTX Gi0/8 connected 2 a-full a-1000 10/100/1000BaseTX Gi0/9 notconnect 2 auto auto 10/100/1000BaseTX Gi0/10 notconnect 2 auto auto 10/100/1000BaseTX Gi0/11 notconnect 2 auto auto 10/100/1000BaseTX Gi0/12 notconnect 2 auto auto 10/100/1000BaseTX Gi0/13 notconnect 2 auto auto 10/100/1000BaseTX Gi0/14 notconnect 2 auto auto 10/100/1000BaseTX Gi0/15 notconnect 2 auto auto 10/100/1000BaseTX Gi0/16 notconnect 2 auto auto 10/100/1000BaseTX Gi0/17 notconnect 2 auto auto 10/100/1000BaseTX Gi0/18 notconnect 2 auto auto 10/100/1000BaseTX Gi0/19 notconnect 2 auto auto 10/100/1000BaseTX Gi0/20 notconnect 2 auto auto 10/100/1000BaseTX Gi0/21 notconnect 2 auto auto 10/100/1000BaseTX Gi0/22 notconnect 2 auto auto 10/100/1000BaseTX Gi0/23 notconnect 2 auto auto 10/100/1000BaseTX Gi0/24 notconnect 2 auto auto 10/100/1000BaseTX Gi0/25 notconnect 2 auto auto 10/100/1000BaseTX Gi0/26 notconnect 2 auto auto 10/100/1000BaseTX Gi0/27 notconnect 2 auto auto 10/100/1000BaseTX Gi0/28 notconnect 2 auto auto 10/100/1000BaseTX Gi0/29 notconnect 2 auto auto 10/100/1000BaseTX Gi0/30 notconnect 2 auto auto 10/100/1000BaseTX Gi0/31 notconnect 2 auto auto 10/100/1000BaseTX Gi0/32 notconnect 2 auto auto 10/100/1000BaseTX Gi0/33 connected 3 a-full a-1000 10/100/1000BaseTX Gi0/34 connected 3 a-full a-1000 10/100/1000BaseTX Gi0/35 notconnect 3 auto auto 10/100/1000BaseTX Gi0/36 notconnect 3 auto auto 10/100/1000BaseTX Gi0/37 notconnect 3 auto auto 10/100/1000BaseTX Gi0/38 notconnect 3 auto auto 10/100/1000BaseTX Gi0/39 notconnect 3 auto auto 10/100/1000BaseTX Gi0/40 notconnect 3 auto auto 10/100/1000BaseTX Gi0/41 notconnect 3 auto auto 10/100/1000BaseTX Gi0/42 notconnect 3 auto auto 10/100/1000BaseTX Gi0/43 notconnect 3 auto auto 10/100/1000BaseTX Gi0/44 notconnect 3 auto auto 10/100/1000BaseTX Gi0/45 connected 1 a-full a-1000 10/100/1000BaseTX Gi0/46 connected 1 a-full a-1000 10/100/1000BaseTX Gi0/47 notconnect 1 auto auto Not Present Gi0/48 notconnect 1 auto auto Not Present
In the above example above we can see that there are 9 interfaces in use, and we can see the following information for each interface:
Port Name Status Vlan Duplex Speed Type Gi0/1 connected 2 a-full a-1000 10/100/1000BaseTX
The following is what each field means:
Port - This is the interface number on the switch Name - This only shows if any names have been assigned, generally they are not. Status - Connected. VLAN - What VLAN the interfaces is currently in. DUPLEX - a-full (automatic-full) SPEED - a-1000 (auto 1000) TYPE - 10/100/1000BaseTX (this interface supports 10Mb/s / 100Mbs and 1GBs)
Show the Mac Address Table
We can use the following to show what MAC Addresses are present on each interface. If you have an interface that is connected to another switch then you will see all MAC addresses from that switch interface.
sh mac address-table Vlan Mac Address Type Ports ---- ----------- -------- ----- All 0100.0ccc.cccc STATIC CPU All 0100.0ccc.cccd STATIC CPU All 0180.c200.0000 STATIC CPU All 0180.c200.0001 STATIC CPU All 0180.c200.0002 STATIC CPU All 0180.c200.0003 STATIC CPU All 0180.c200.0004 STATIC CPU All 0180.c200.0005 STATIC CPU All 0180.c200.0006 STATIC CPU All 0180.c200.0007 STATIC CPU All 0180.c200.0008 STATIC CPU All 0180.c200.0009 STATIC CPU All 0180.c200.000a STATIC CPU All 0180.c200.000b STATIC CPU All 0180.c200.000c STATIC CPU All 0180.c200.000d STATIC CPU All 0180.c200.000e STATIC CPU All 0180.c200.000f STATIC CPU All 0180.c200.0010 STATIC CPU All ffff.ffff.ffff STATIC CPU 2 0025.b4bc.2901 DYNAMIC Gi0/1 2 74fe.4857.6d4a DYNAMIC Gi0/7 2 74fe.4857.72a5 DYNAMIC Gi0/8 2 a4bf.0169.b7af DYNAMIC Gi0/5 2 a4bf.016a.d0eb DYNAMIC Gi0/6 3 74fe.4857.6d4c DYNAMIC Gi0/33 3 74fe.4857.72a7 DYNAMIC Gi0/34 1 74fe.4828.799d DYNAMIC Gi0/45 1 74fe.4828.c136 DYNAMIC Gi0/45 1 9418.8266.b82c DYNAMIC Gi0/45 1 9418.8266.b82d DYNAMIC Gi0/45 1 a4bf.0169.b7ad DYNAMIC Gi0/45 1 a4bf.016a.d0e9 DYNAMIC Gi0/45
I want to look at this specific interface:
Vlan Mac Address Type Ports 2 a4bf.0169.b7af DYNAMIC Gi0/5
It can be seen that this is interface 5 on our Cisco, that is in VLAN2 and there is a device connected to it with the MAC Address a4-bf-01-69-b7-af.
I have logged in to the server that I know is connected to this interface (the servers eno3 is connected) and if I run the command (Linux):
ip a show eno3
5: eno3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master data-out state UP group default qlen 1000
link/ether a4:bf:01:69:b7:af brd ff:ff:ff:ff:ff:ff
altname enp61s0f2
We can see that the link/ether address is listed as: a4:bf:01:69:b7:af
Look for a specific MAC Address
Because there could be many MAC addresses present on an interface when you use the command sh mac address-table we can use a simple filter to only show what we require: (remember, our last search revealed that the MAC Address on Gi05 ended in 'af'
sh mac address-table | incl af Now the only result we get back is this: 2 a4bf.0169.b7af DYNAMIC Gi0/5
We may not know what MAC we are looking for, but if we do this is very useful.