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question about cdp

Unanswered Question
Nov 19th, 2013
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How CDP gets the information about other devices?

What are PDU's of CDP called as ? (For example STP PDU's are called as BPDU's - Bridge Protocol data units)

In what type of situations we take the help of CDP ?


Thanks in advance.


Regards,

Chandu

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Giuseppe Larosa Tue, 11/19/2013 - 07:19
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Hello Chandra,

CDP is a Cisco proprietary protocol that allows for neighbor discovery on each single link = (betweeen directly connected Cisco devices).

CDP is a an OSI L2 signalling protocol that uses special format CDP frames.

CDP frames are exchanged periodically (every 60 seconds).

The CDP frame has a special multicast address that helps other Cisco devices running CDP to process a received CDP frame.


Each device puts in the CDP frame information about its capabilities (example Transparent Bridge, Router and so on), the hostname, the network operating system type and version, and a management IP address. The source interface name also appears.


This information can be seen with


show cdp neigh detail


CDP is used for troubleshooting in a campus network to move from a LAN switch to another one, for example when searching where a specific user MAC address is connected to.


To be noted CDP is used in Cisco VOIP solutions to communicate the voice vlan from a Cisco LAN switch to a directly connected Cisco IP phone.


LLDP Link local discovery protocol is a standard based protocol that performs the same functions of CDP.


CDP frames have no special name.  The CDP frames have LLC/SNAP encapsulation with a Cisco OUI and a reserved destination MAC address.


For a more detailed description see the following link


http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a00801b49a4.shtml#cg27



Edit:

note that a non Cisco L2 device treats CDP frames as user traffic and it propagates them, so there are special cases where a Cisco device can have more then one CDP neighbor on a single port if a third party device is in the middle.


Hope to help

Giuseppe

chandra_rc16 Tue, 11/19/2013 - 21:48
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The CDP frame has a special multicast address that helps other Cisco devices running CDP to process a received CDP frame.

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What is the address ?


And also i read the below information from the cisco site about cdp. But need more clarity on this.

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Each CDP enabled device advertises at least one address at which the device can receive messages and sends periodic advertisements (messages) to the well-known multicast address 01:00:0C:CC:CC:CC. Devices discover each other by listening at that address. They also listen to messages to learn when interfaces on other devices are up or go down.

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Q1. What is the addresse that a CDP enabled device advertises at which the device can receive messages ?


Q2. Devices discover each other by listening at that address

      Again which address, is it the multicast address or the address that the device advertises ?

Q3. If device A is receiving information about device B on its Fa0/1 port. And if that port goes down will CDP erase all the CDP information learned on that port ? (Given if both the devices are connected with only single link.)


Thanks in advance.


Regards,

Chandu

Giuseppe Larosa Wed, 11/20/2013 - 01:06
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Hello Chandu,


Q1)

the well-known multicast address 01:00:0C:CC:CC:CC. this is the multicast address used as destination of CDP frames.

The source MAC address is less important here but it is that of the physical port.


Q2) by listening to the multicast address


Q3)   there is an hold time that is  three times the hello interval (60 seconds) if within 3*60 seconds no new CDP info is received from the other device the CDP entry will be deleted from the list of valid CDP neighbors. So even if the port stays up/up within 180 seconds the local device detects the neighbor failure. If the port goes down I would expect the CDP entry to be purged within few seconds.


Hope to help

Giuseppe

chandra_rc16 Wed, 11/20/2013 - 02:48
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If the port goes down I would expect the CDP entry to be purged within few seconds.

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No.Even if the port goes down/link is broken, the switches are holding the information for 180 seconds (3 adverstisments).

Just checked in PT.


Regards,

Chandu

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