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CDP

Description
Packet format
History
Copyright and License
Disclaimer

Description

Cisco Discovery Protocol exchanges informations between and about Cisco devices in the network (routers, switches and IP phones).

Packet format

     ------------------------------------------------------------
    | Version |      TTL     |   Checksum   |        Data        |
    | 1 byte  |    2 bytes   |   2 bytes    |      variable      |
     ------------------------------------------------------------

where:

Version is the version of CDP packet;
TTL is the time-to-live of data contained in the packet, in second;
Checksum of the CDP packet;
Data is a list of type-length-value fields.

The format of a type-length-value field is:

     ------------------------------------------------
    |     Type    |    Length   |       Value        |
    |   2 bytes   |   2 bytes   |      variable      |
     ------------------------------------------------

where:

Type of contained information;
Length is the total length in bytes, including the length of Type and Length field components;
Value is the value associated with the specified Type component.

Common Type values are:

1 Device-ID: value is the hostname of the device or it's serial number;
2 Address: the Layer 3 address of the interface from which the CDP was originated;
3 Port-ID: value is the port from which the CDP packet was sent;
4 Capabilities: value represents the capabilities of the device;
5 Version: value represents the software version;
6 Platform: value represents the hardware platform.

There are more types, actually I have identified 21.

Format of Value when Type is Address is:

     -----------------------------------------------------
    | Protocol | Protocol | Protocol | Address | Address  |
    |   Type   |  Length  |          | Length  |          |
    |  1 byte  |  1 byte  |  1 byte  | 2 bytes | variable | 
     -----------------------------------------------------

Format of Value when Type is Capabilities is:

0x01 - Router;
0x02 - Transparent Bridge;
0x04 - Source Route Bridge;
0x08 - Layer 2 Switch;
0x10 - Layer 3 Switch;
0x20 - IGMP Snooping;
0x40 - Repeater.

This document is far from beeing complete but I hope it is making some light about the format of CDP packets.

History

20 December 2006, First version.

Copyright and License

Disclaimer

No liability for the contents of this document can be accepted. Use the concepts, examples and information at your own risk. There may be errors and inaccuracies that could be damaging to your system. Proceed with caution, the author do not take any responsibility.

All copyrights are held by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark. Naming of particular products or brands should not be seen as endorsements.

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Last page modification is 20 December 2006