From: Peter van Oene (pvo@usermail.com)
Date: Wed Oct 08 2003 - 09:19:03 GMT-3
At 11:43 PM 10/6/2003, Marco P. Rodrigues wrote:
>.: -Implicit null lable-lable 3:By default LDP advertises lable 3 (i.e
>.:implicit null lable).This causes the previous router to do PHP.In some
>.:situatios like MPLS QoS (where QoS in IP HDR is different than the one
>.:in EXP field)-it's a problem .
>.:
>
>Besides the previous example can someone provide another practical
>reason why one should use an explicit-null as opposed to an implicit one?
The previous example is likely the one. The null label coming into the PE
box would carry a non unique label (0) and would have nothing interesting
in it beyond a, the TTL, and B, the EXP bit settings. If you have a need
to use these values directly on the PE, using explicit allows the PE to
actually see them. With implicit, the label is dropped before it ever gets
to the PE as you know.
Pete
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