Link check using CDP

[Andras Dosztal]

Hi,

I'm thinking of creating a feature request but want to hear your opinion first. This idea came up when I was configuring switches remotely, and a miscabling caused some issues.

My idea is to have a test feature of CDP that would work like this:

1. The interface is admin down.
2. After issuing a command (something like 'test cdp int gi1/1') the device brings up the interface but only allows CDP packets to pass.
3. The device sends and waits for CDP packets.
4. The interface goes back to shutdown after a timeout or receiving CDP on that interface.
5. The device prints out to CLI what it received but does not learn it.

A new capability code could be used (like 't' for testing) to mark this connection temporary, so the other end would also not learn anything about the new device.

What do you think?

[f-baker]

There are a lot of ways to accomplish that without a new feature. If it is a switch port, simply configure it as an access port with an undefined vlan. Routed ports even eaiser no ip address. Or is there some reason these techniques would not work.

[Andras Dosztal]

They wouldn't work if the cables are laid to the wrong switchports; and you can't check it from a remote location. I've seen a few incidents because of this. :)

[f-baker]

I dont understand the statement.  It seems that your feature would a) not pass any real traffic and b) automatically shut the port back down. While my suggestion would not accomdate b), when could you do your test cdp command but not do switchport access vlan 999 no shut, as your stated suggestion requires access to the device CLI remotely or not.

[Andras Dosztal]

Oh, I pushed the wrong button, see my answer above (with the drawing).

[Andras Dosztal]

Consider this scenario:

I've requested a link between Switch 1 and Switch 2. The subcontractor made a mistake and laid the cable between Switch 1 and Switch 22. I configure the switchports on Switch 1 and Switch 2, bring up the interfaces, then nothing happens.

Your solution wouldn't help in this case, because I don't know where the cable is connected. I can't set all unused ports on all switches to find it, and I don't want to risk creating a loop or other issues.