Subject: [mpls] Problem about the collision between multicast and unicast label

From: 黄郑 <[email-masked]>
Date: Wed, 07 Jun 2006 18:32:27 +0800

In multicast, all downstream can be allocated the same label value in order to avoiding branch LSR traffic replication on a LAN for point-to-multipoint (P2MP)LSPs. 
   The series documents of mpls upstream label are already made for wg docs. 
   Through upstream label assignment or downstream label assignment (based on upstream label suggestion),  all next-hop downstream routers will receive and process the packet with the same label.

   But if unicast label is allocated before multicast label, it is possible that the same label will be allocated on the same LSR for unicast LSP and multicast LSP on the same link. 
   The unicast LSP and the multicast LSP will use the same incoming label on the same interface. 

For example:
 
                                D 
                               / 
                              /   
                         A---B---C---E 
                              \      
                               \     
                                F 

   There already exists one unicast LSP (A-B-C-E). And the incoming label on LSR C is equal to K.
   There also exists one multicast tree (A-B, B-D, B-F). And the incoming label on LSR D and LSR F is equal to K. LSR D and LSR F are the next hop downstream router of LSR B, and their incoming labels must  be same. 
   Subsequently, LSR C wants to join the multicast tree. LSR C is also the next hop downstream router of LSR B. The incoming multicast label on LSR C must be equal to K, just like LSR D and LSR F on the same multicast tree.
   So there will appear the same incoming labels on LSR C if we don’t distinguish between multicast and unicast.

----------------------------------------------------------------------------------------------------------------
   In order to distinguish between multicast and unicast LSPs with the same label value, the labels can be differentiated by the use of different Ethertypes that indicate whether the labeled packet that is carried in a layer 2 frame is for unicast or multicast transfer. 
   But if LSR does not support these procedures, it can't discern the labeled packet and must ignore it. 
   On the other hand, the process of layer 2 frame is complex.

_______________________________________________
mpls mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/mpls