The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Raghu Thirumalairajan wrote:
> 
> I am a bit confused; going through rfc3031 [MPLS-ARCH]:
> 
>    "If a particular LSR Rd is attached to a particular LSR Ru over two
>    point-to-point interfaces, then Rd may distribute to Ru a binding
>    of label L to FEC F1, as well as a binding of label L to FEC F2, F1
>    != F2, if and only if each binding is valid only for packets which
>    Ru sends to Rd over a particular one of the interfaces.  In all
>    other cases, Rd MUST NOT distribute to Ru bindings of the same
>    label value to two different FECs."
> 
> Consider the following figure:
>       +----+         +----+
>    ---| R1 |---------| R2 |---
>       +----+         +----+
> 
> R1 is connected to R2 over a single multi-access interface (like 100
> Mbps ethernet).
> Can R2 distribute L to two FEC requests for F1 & F2 (F1 != F2) from
> R1? Essentially, can more than one FEC point to an NHLFE if R1 & R2
> are connected over a single non-point-to-point interface?

No.  If two different FECs are given the same label, then they will be
forwarded identically - which is the wrong behavior if they really are
two different FECs.

> The statement "In all othercases, Rd MUST NOT distribute to Ru
> bindings of the same label value to two different FECs." seems
> confusing/contradicting to what's stated below...

Below what?  Do you mean my message that you quoted?

>> Note that per FEC does not necessarily mean per IP prefix.  Multiple
>> prefixes that all leave the MPLS cloud through a common router
>> interface and have identical traffic characteristics can be
>> considered a single FEC and can use a single LSP.

An IP prefix is not an FEC.  And an FEC is not an IP prefix.

An FEC is a collection of (one or more) host addresses and prefixes that
are all forwarded identically.

If multiple prefixes are all to be forwarded identically (same egress
router, same service level), then a router can place them all in the
same FEC.  Or it can choose not to.

A single label can not be assigned to two FECs, but there is nothing
stopping a router from placing multiple IP prefixes into a single FEC,
if they all leave the MPLS cloud via the same router and are to be
delivered with the same service level.

Please see the discussion on aggregation granularity (section 3.20 of
RFC 3031)

-- David