[frr] 2 MPLS Questions

Vivek Venkatraman vivek at cumulusnetworks.com
Sat Mar 18 01:19:34 EDT 2017


This is correct. By definition, if a router is the penultimate hop, it
means the actual egress is downstream and has signaled (advertised) an
implicit-null label to this router. The router doing the PHP knows the next
hop to forward to (the egress) without doing any additional lookup.

This behavior should already be supported.

What is not supported (if I remember right) is the ability on the egress to
terminate a label and perform a (route) lookup.  That is needed to really
be able to support any L2/L3 VPN service properly.

On Thu, Mar 16, 2017 at 10:45 AM, Jeff Tantsura <jefftant at gmail.com> wrote:

> Donald,
>
> Wrt PHP, this is incorrect, PHP node MUST not perform IP lookup, or in
> fact any lookup after POP. In most cases (labeled services, L2/L3 VPN)
> there's another label(s) in the stack, looking it up would be fatal.
>
> Regards,
> Jeff
>
> > On Mar 15, 2017, at 08:05, Donald Sharp <sharpd at cumulusnetworks.com>
> wrote:
> >
> > David/Roopa -
> >
> > Olivier asked me about these two issues yesterday in the FRR Technical
> > Meeting.  I just wanted to make sure I didn't loose track of these
> > questions that he had:
> >
> > 1) More than 2 labels in the kernel at a time, when will this be
> > allowed in the kernel?
> >
> >   -> David is currently working on this issue.  When he is done it
> > will be upstreamed.  So soonish(tm).
> >
> > 2) PenUltimate Hop Popping:
> >
> > I know this issue is not trivial to solve. In fact, once the POP
> > instruction perform, the packet must re-enter in the IP packet
> > processing to determine what action must apply. A possible solution
> > would be to process this packet as a new incoming IP packet when
> > output interface is the loopback disregarding the IP address value.
> > But, this issue is less urgent than the first one. Our OSPF Segment
> > Routing implementation could announce if the router works in
> > PenUltimate Hop Poping mode or not. So, for the moment, the option is
> > force to yes.
> >
> > thanks!
> >
> > donald
> >
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>
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