determine ports used

Unanswered Question
Apr 2nd, 2008

Hi.

I have a novice question regarding router ports. The scenario is that I have a new router and after connecting the cables to other devices, i log in to its console port. Since it is a new router, there are no configurations setup on its ports. Is there a way to determine at this point which ports are currently physically connected? I am asking this question because if I have several routers physically interconnected, and I want to know how to find which port is actually physically connected granting that i am not sure whether i have connected the e0 port or the e1 port of a certain router prior to configuring its IP address.

Thanks in advance to anyone who could help.

I have this problem too.
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Hi Marcus

On a NEW router all ports are administratively shut down and in this state I don't think there is much you can do.

However I believe that if you open the ports then you can run the command "show cdp neighbors" on the routers you will see which ports are connected to which neighboring routers/switches.

I have confirmed this using dynamips as I am in work and have no access to my home lab. I configured a 3 router lab as follows

R2 s1/0 => R1 s1/0

R2 s1/1 -> R3 s1/0

All interfaces up though there is no IP configuration on any interface.

Router1

R1_7200#sh cdp neigh

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID

R2_7200 Ser 1/0 178 R 7206VXR Ser 1/0

R1_7200#

Router2

R2_7200#sh cdp neigh

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID

R1_7200 Ser 1/0 42 R 7206VXR Ser 1/0

R3_7200 Ser 1/1 9 R 7206VXR Ser 1/0

R2_7200#

Router3

R3_7200#sh cdp nei

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID

R2_7200 Ser 1/0 159 R 7206VXR Ser 1/1

R3_7200#

Another command which will show you what ports are up but not what they are connected to is "show interface | include line"

Example output

R2_7200#sh int | inc line

FastEthernet0/0 is administratively down, line protocol is down

Serial1/0 is up, line protocol is up

Serial1/1 is up, line protocol is up

Serial1/2 is administratively down, line protocol is down

Serial1/3 is administratively down, line protocol is down

Serial1/4 is administratively down, line protocol is down

Serial1/5 is administratively down, line protocol is down

Serial1/6 is administratively down, line protocol is down

Serial1/7 is administratively down, line protocol is down

R2_7200#

Serial1/0 and Serial1/1 above show UP UP because keepalives in set to 10. With "no keepalives set this would show UP DOWN.

The portion that states "Serial 1/0 is UP" is refering to a physical connection. If there was no physical connection then the output would show "Serial 1/0 is down, line protocol is down".

HTH

Best Regards,

Michael

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