CDP Problem

Answered Question
Feb 17th, 2008

dear all,

i know that cdp just shows only the directly connected but i have the case in which the show cdp neighbor command show devices that are not connected directly but through the 3com switch

so what is the problem??

I have this problem too.
0 votes
Correct Answer by Richard Burts about 6 years 2 months ago

Sadam

It is due to the difference in processing between Cisco switches and switches from other vendors. CDP uses a special multicast MAC address as its destination layer 2 address. When you send a CDP frame out an interface that is connected to a Cisco switch then the Cisco switch recognizes the destination MAC address as one that it processes and so it processes instead of forwarding the frame. The result is that when connected to Cisco switches you only see directly connected devices.

However when the switch is from another vendor (such as 3com) then it does not recognize the multicast MAC address as one that it processes and it forwards the CDP frame and you get responses from Cisco devices that are more than one hop.

HTH

Rick

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Correct Answer
Richard Burts Sun, 02/17/2008 - 09:32

Sadam

It is due to the difference in processing between Cisco switches and switches from other vendors. CDP uses a special multicast MAC address as its destination layer 2 address. When you send a CDP frame out an interface that is connected to a Cisco switch then the Cisco switch recognizes the destination MAC address as one that it processes and so it processes instead of forwarding the frame. The result is that when connected to Cisco switches you only see directly connected devices.

However when the switch is from another vendor (such as 3com) then it does not recognize the multicast MAC address as one that it processes and it forwards the CDP frame and you get responses from Cisco devices that are more than one hop.

HTH

Rick

miheg Sun, 02/17/2008 - 22:39

Indeed, and if you are lucky you get then CDP's from multiple devices. That way you may realize that those machines couldn't be connect to that same port.

I had a nortel passport in between some cisco's on what I believed (because it appeared) to be a point to point link, with a duplex problem as well.

When I fixed the duplex problem I lost contact to the remote machine and someone had to go on site to reset the switch to restore the config.

It would be nice if ciscoworks was able to "notice" the other mac addresses, STP host and other stuff in between.

Cheers,

Michel

Richard Burts Mon, 02/18/2008 - 10:00

Sadam

I am glad that my answer was helpful. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read a response that resolved the question.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick

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Posted February 17, 2008 at 2:44 AM
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