no FAT label , load balancing in MPLS core

[sarahr202]

Hi Everybody

First of all ,  a special thank to Nagendra Kumar for introducing me to " FAT" label in MPLS :)

What if the payload is EoMPLS and there is no " FAT" label  to guide our router to select a particular link to forward labelled packet ?

We already know a LSR , upon receiving mpls labelled packets, first  check  the first  " nibble" after mpls labels ( which is IP ver field) to find the type of payload  , if it is 4 ( which ip ver 4 ) or 6 ( which means ipv6) our router gladly performs src ip dst ip load balancing.

If LSR sees something else i.e not 4 or 6, LSR then checks the last label ( fat label) to choose a link to forward packet. However, it is not clear how will LSR react if it does not find " FAT" label .

Have a great weekend.

[Nagendra Kumar Nainar · Accepted solution]

Hi Sarah,

The default behavior is to use the bottom label (VC label) as key input for hashing algorithm. So without FAT label, traffic for different VC may flow over different ECMP path (due to different VC label). But all traffic to one VC will end up flowing over the same egress interface. 

But with FAT label, Ingress can perform a DPI to get src/dst address and impose the FAT label as bottom one. SO traffic to same VC can still flow over different egress path due to different FAT label.

Thanks,

Nagendra