VTP MD5 digest length

[amharsaputra]

Hi,

Here is an image of VTP summary advertisement packet format taken from

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml#vtp_msg

We can see the MD5 digest field is 16 bytes.

Now here is a sample output of "show vtp status"

Notice the MD5 digest (highlighted). I know "0x" is just a prefix for hexadecimal numbers. One hexadecimal character equals to 4 bits, two hexadecimal characters equal to 8 bits (1 byte). There is only 16 hexadecimal characters there, so just 8 bytes in total is shown in the output. But the Cisco documentation states the MD5 digest is 16 bytes long. So why is there only 8 bytes shown in MD5 digest in "show vtp status"?

Regards,

Putra

[cadet alain]

Hi,

can you put wireshark into action and see how it is encoded in the advertisement ?

Regards.

Alain

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[Arumugam Muthaiah · Accepted solution]

Hi Amhar,

Alain said right option. I would like add some more detail,

• Message Digest 5 (MD5) carries the VTP password, if MD5 is configured and used to authenticate the validation of a VTP update.
  
• VTP takes the VTP domain name into account when calculating the VTP MD5 hash
  
• The MD5 hash of a null (default) password. If you debug, you see like below data,
  

Log:

03:51:51: VTP LOG RUNTIME: Transmit vtp summary, domain CCIE, rev 11, followers 0, tlv blk size 8 (inc #tlv field),
   MD5 digest calculated = E1 92 86 29 E9 A5 D0 CC 6F 89 32 34 21 A0 C4 6D

Regards,

Aru

*** Please if this post is useful ***

[amharsaputra]

Hi Alain and Aru,

Thank you for your responses.

I put the Wireshark into action and here is the result:

The switch that I sniffed:

The result of wireshark sniff:

So apparently Cisco only show the first 8 bytes (out of 16 bytes) of MD5 digest from "show vtp status". But the actual length is really 16 bytes as shown in the Wireshark.

Furthermore, I just found out that on newer Catalyst IOS, "show vtp status" will show the whole 16 bytes of MD5 digest. Here is a sample:

Thank you Alain for leading me to use Wireshark.

Thank you Aru for further details and sample debug output.

Regards,

Putra