[j-nsp] MPLS and QoS at penultimate hop ?

[Daniel Roesen]

On Tue, Feb 05, 2013 at 01:31:14PM +0400, Alexandre Snarskii wrote:
> PHP router: 
> 
> 604050             *[LDP/9] 4d 00:37:22, metric 1
>                     > to AA.BBB.233.13 via ae4.9, Pop      
> 604050(S=0)        *[LDP/9] 4d 00:37:22, metric 1
>                     > to AA.BBB.233.13 via ae4.9, Pop      
> 
> (BA-classification is most possibly done on ip-prec after Pop is done, 
> but that's the point where I'm not sure....

BA classification is done on EXP bits, if you have defined a MPLS
classifier (there is a default classifier with DPCs, but non for MPCs).

It's a common strategy to enforce UHP via explicit-null label
signalling, in order to pass L3VPN traffic with unmodified IPP/DSCP
end-to-end between CEs. The egress PE BA-classifies the L3VPN packet
according to EXP/TC, and and forwards the packet to the CE with the
explicit null label popped, without rewriting IPP/DSCP on egress.

> Classification on EXP may be more accurate: as there are no "payload
> type" field in MPLS header, this router just can't know that packet's
> content is IPv4).

MPCs (Trio) uses a heuristic to determine MPLS payload for
load-balancing purposes. If the first payload octet is 0x04, IPv4 is
assumed, for 0x06, IPv6 is assumed. Otherwise, it will be treated as
ethernet frame, IIRC.


Best regards,
Daniel