CDP

[muraripadhan]

Dears

When i check on switch 

Switch#show cdp neighbor

At that time it shows a lot of neighbor on one port

Like on F0/24

It shows a lot of neighbor

Why this is shows lot of neighbor

How i will find out which is actual neighbor of that switch

[Julio E. Moisa]

Hi

Try to disconnect and connect again the cable, what is connected to that port, could be it a hub?

[muraripadhan]

no sir i checked in my L3 and L2 switch so sir please sugest me and give me some

command to rectify the problem

[Julio E. Moisa]

Try to clear the cdp table or counters (it should no affect but take your considerations)

clear cdp counters
clear cdp table

[Richard Burts]

Can you tell us what is physically connected to that port?

HTH

Rick

[Dennis Mink]

Shut the port, physically disconnect it and stick a single Cisco device it it for testing like a phone or switch. Clear CDP table and unshut the port see what you get then.

Please rate if helpful

[moidinmkm]

me too have the same problem.. is there any chance it has tagged with vlan and 2 or more router interfaces come under same vlan tagging

[kirkchris01]

I have a similar problem. Our end user switches (3850) are connected to a Brocade BigIron which acts as the distribution switch for the entire campus. There are two layer two connected switches on that Brocade that are showing up when I check cdp neighbor. I can't get cdp and fdp to work with the Cisco switches. I tried clearing the cdp table and counters. They keep coming back. 

[Alex Pfeil]

Have you tried running LDP?

[Alex Pfeil]

I meant LLDP. You might have better luck with LLDP since it is an industry standard.

[Richard Burts]

The problem here is actually an expected behavior. The key to the issue is that the Cisco switch is connected to a non Cisco switch. To understand what is happening we need to start by understanding that the CDP packet is sent with a destination mac address that is a multicast address specific to Cisco. This is what is happening: cisco switch sends CDP, Brocade receives the frame with multicast mac destination and correctly forwards a copy to all the other ports in the vlan. The CDP packet is now received by multiple devices. When a Cisco switch receives the CDP frame it recognizes the destination mac address and processes CDP rather than forwarding as the Brocade did. So multiple CDP responses are sent to the original Cisco switch and it shows multiple neighbors on one interface. 

Other than seeing multiple neighbors where you expect to see one neighbor, is there any other problem here?

HTH

Rick

[kirkchris01]

No, I don't think it's much of a problem. Our IAM used to be a Network guy. Based on his experience prior to the v12 IOS, he said to disable cdp globally and enable it just on the trunked interface. The switches will not let me do that. Cdp has to be enabled globally. One side of this is to quickly see who a switch is connected to. I label my ports, so I can just look at the config. Another factor is to be STIG compliant. STIG reading seems to depend on ones interpretation. A third factor for me is I try to minimize whatever unnecessary traffic that I can, including broadcast traffic, because I am struggling to learn to become a better network technician. What settings and tweaks can I do to make the network run better but still be DISA STIG compliant?

The Brocades will be gone by summer as we are getting our new equipment soon. I should have all 3850 switches at that point.

[Richard Burts]

I am surprised at his advice about disabling cdp globally and then enabling it on specific interfaces. My experience is that when you disable something globally then it will not accept attempts to enable it on specific interfaces. If you want to be selective about what interfaces cdp runs on then I believe that you must enable it globally and then disable it on specific interfaces where you do not want it.

You make a comment about wanting to eliminate unnecessary traffic especially broadcast. I would repeat my point about cdp being multicast. My experience is that cdp traffic is an extremely small amount of traffic in most networks if you look at the bandwidth that it consumes. You need to make a judgement call about whether the advantages of cdp will classify it as necessary or as not necessary in your network.

HTH

Rick