Find different switches from cdp output

Question

I have three cisco switches, all with CDP enabled. From the console of the "core" switch I give the command "show cdp neighbors" and the output is:

    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
    Switch     Gig 5/14        154           S I      WS-C2950-2  Fas 0/9
    Switch     Gig 5/11        154           S I      WS-C2950-2  Fas 0/6

So I try to see if the two "Switch" entries indicate different switches or two paths to to same switch, and I try the command "show cdp entry Switch". This is what I get:

    -------------------------
    Device ID: Switch
    Entry address(es): 
    Platform: cisco WS-C2950-24,  Capabilities: Switch IGMP 
    Interface: GigabitEthernet5/14,  Port ID (outgoing port): FastEthernet0/9
    Holdtime : 149 sec

    Version :
    Cisco Internetwork Operating System Software 
    IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(11)EA1, RELEASE SOFTWARE (fc1)
    Copyright (c) 1986-2002 by cisco Systems, Inc.
    Compiled Wed 28-Aug-02 10:25 by antonino

    advertisement version: 2
    Protocol Hello:  OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000FFFFFFFF010221FF000000000000000AF4CF2E80FF0000
    VTP Management Domain: 'ypex'
    Duplex: full

    -------------------------
    Device ID: Switch
    Entry address(es): 
    Platform: cisco WS-C2950-24,  Capabilities: Switch IGMP 
    Interface: GigabitEthernet5/11,  Port ID (outgoing port): FastEthernet0/6
    Holdtime : 147 sec

    Version :
    Cisco Internetwork Operating System Software 
    IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(11)EA1, RELEASE SOFTWARE (fc1)
    Copyright (c) 1986-2002 by cisco Systems, Inc.
    Compiled Wed 28-Aug-02 10:25 by antonino

    advertisement version: 2
    Protocol Hello:  OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=00000000FFFFFFFF010221FF000000000000000AF4CF2E80FF0000
    VTP Management Domain: 'ypex'
    Duplex: full

So, the entries are the same for both switches, but the thing is, they are two different switches. And this is my problem: How can I find the true topology in such a situation?

Can you clarify what you're seeing there that makes you think that it's two different switches? — Shane Madden, Mar 29 '15 at 22:03
I see the three switches in front of me, so I know it's two different switches. — gmelis, Mar 30 '15 at 04:06
Is this a lab setup, then? In a normal network, the switches will have different hostnames and IP addresses configured, which will show in the CDP data. — Shane Madden, Mar 30 '15 at 07:14
Yes, it's lying right on my desk. I configured the "core" switch with an IP address and then just connected the other two switches.I expected to see some difference between them in the cdp details, like a hex code or something, but I didn't see anything. Thus the question, how can somebody tell what's going on in a remote network, i.e. if the switches have not been correctly configured. — gmelis, Mar 31 '15 at 07:05

score 1 · Accepted Answer · answered Mar 31 '15 at 17:36

Identically configured switches of identical models and software versions are identical in CDP; the advertisement doesn't include a MAC address or serial number that would differentiate the switches (though if desperate, you could capture the CDP packet - the source MAC in the ethernet header would give you that (though it may be spoofed, unsure of the exact behavior of the protocol).

But, that wouldn't be useful to you anyway - the purpose of CDP is for you to be able to track down the network topology without actually tracing cables; if your peer switches have no management IP addresses configured, then it's useless to be able to figure out how they're plugged in remotely, since you'll need to plug a serial cable into them anyway to manage them.

So, essentially: in the scenario you're describing, even with identifiers CDP is effectively useless, and once you've got your switches configured with an IP address it's no longer a concern.

Find different switches from cdp output

1 Answers1