The MPLS WG Archive

[Olle Pers <Olle.K.Pers@telia.se>]

*  
*  Let us assume that the middle router does aggregation !
*  
*  Now the packet enters the middle router labelled 
*  and he cannot have a direct outgoing 
*  labelled path as he has not got labels 
*  for the summary but only for the 
*  specific destinations for forwarding the packet.
*  
That is a different problem. With basic LDP, all routers 
should have the same IGP routing table. 


*  Therefore whenever he receives a labelled packet 
*  and does not have any outgoing label for that he 
*  has to remove the label and do a normal layer-3
*  lookup in his routing table.
*  
Yes that would be a third option. (And if this layer-3 
lookup identifies an outgoing interface, should the 
packet be sent out on that interface, or should it be 
dropped anyway?)    


But back to the original question: 

Section 3.22 of RFC 3031 applies when the incoming label does not 
map to an NHLFE. This is not necessarily the same as having no 
outgoing label.  

In David's example, the incoming label would map to an NHLFE containing 
the next hop "router C" and the operation "pop the label stack". 
When B later receives the label mapping from C, that NHLFE will 
be changed to contain a "replace label" operation. 


Olle