[c-nsp] debug mpls packet

roy bandwidth.user at gmail.com
Tue Nov 17 22:38:48 EST 2009


Oliver Boehmer (oboehmer) wrote:
>  
>>>>>> Does anyone know what the middle number represents in a "debug
> mpls
>>>>>> packet" ( eg: {7963 6 254} )?
>>>>>> I can't find this information anywhere.
>>>>>>
>>>>>> 7693 = Label
>>>>>> 6 = ???
>>>>>> 254 = I presume is the TTL
>>>>>>
>>>>>> What does the 6 represent??
>>>>> it's the EXP value. you're right about the last being the TTL.
>>>>>
>>>>> 	oli
>>>> Could it be the 3-bit EXP and 1-bit Bottom of Stack Flag combined?
>>> Hmm, why do you think so? Looking at the code, it only prints the 3
> exp.
>>> bits.
>> Cisco must have combined RFC3032 [2.1. Encoding the Label Stack] into
>> one value.
> 
> still not sure what you refer to, and why you think the debug discussed
> shows the 4-bit Exp+S value rather than the 3-bit Exp only? 

If I may, MPLS Fundamentals refers to the stack on Fig 2-1 as 
Label/EXP/BoS/TTL. It then breaks this on Example 3-8 with {label EXP 
TTL}. All things held constant; label at 20, TTL at 8, then EXP must be 3+1.

Roy


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