The MPLS WG Archive

[Giles Heron <giles@packetexchange.net>]

Toby Smith wrote:
> 
> Giles Heron <giles@packetexchange.net> writes:
> 
> > > ...which I think is the correct approach, and I think that's best
> > > served by simply maintaining the received mapping with the mismatched
> > > MTU and reconfiguring one of the boxes based on the information that
> > > the user can see in the sent & received label mappings show commands,
> > > instead of having an LSR send a release as soon as it receives a
> > > mapping with an MTU it doesn't expect, and then having to figure out a
> > > way to get the released mappings back again later.
> >
> > but if you retain the received mapping then presumably the circuit will
> > start "working"?
> 
> No, not at all.  Luca is very sage, and accounts for this in the trans
> draft:
> 
>      - Interface MTU
> 
>        A 2 octet value indicating the MTU in octets. This is the Maximum
>        Transmission Unit, excluding encapsulation overhead, of the
>        egress packet interface that will be transmitting the
>        decapsulated PDU that is received from the MPLS network. This
>        parameter is applicable only to VC types 1, 2, 4, 5, 6, and 7,
>        and is REQUIRED for these VC types. If this parameter does not
>        match in both directions of a specific VC, that VC MUST NOT be
>        enabled.
> 
> So, we can happily retain the mismatched label mapping without fear of
> any forwarding taking place, and still have the information available
> for the user to perform diagnosis.  As a benefit, when the problem's
> resolved, one of the LSRs will issue a withdraw/mapping to convey the
> modified MTU, and things will start working.

ah.  My bad.  I misread "MUST NOT be enabled" as "MUST be released".

Giles
-- 
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Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
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