The MPLS WG Archive

[Eric Rosen <erosen@cisco.com>]

Mark> Usually, LSRs only receive packets with labels that the LSR has itself
Mark> distributed.  Is that true for  these labels?  Presumably not or there
Mark> would have been no need to reserve specific values for them. 

Reserved  values were  used because  they  are easy  to look  up; it's  much
quicker to determine that the label is  one of a small set of constants than
it is to  look up a label in  your 20-bit lookup table.  When  a packet with
explicit null is received, the explicit  null needs to be detected, and then
the packet's IP  address needs to be looked up.  By  using a reserved value,
the time  to look  up the  label plus the  IP address  becomes approximately
equal to the time  to look up the IP address.  Without  a reserved value, it
would be approximately double that.

Of course,  once a reserved value is  used, it does become  possible for the
penultimate node  to put  on explicit  null even when  it hasn't  been asked
for.  By "be liberal in what you accept", this should be handled properly at
the egress.