[frr] 2 MPLS Questions

Renato Westphal renato at opensourcerouting.org
Sat Mar 18 09:23:59 EDT 2017

On Sat, Mar 18, 2017 at 9:45 AM, Renato Westphal
<renato at opensourcerouting.org> wrote:
> On Sat, Mar 18, 2017 at 2:27 AM, Roopa Prabhu <roopa at cumulusnetworks.com> wrote:
>> On 3/17/17, 10:19 PM, Vivek Venkatraman wrote:
>>> 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.
>> yes, correct. terminating a label and performing a route lookup is not supported
>> today.
> The exception being the ipv4/ipv6 explicit null labels. Please take a
> look: https://github.com/torvalds/linux/blob/master/net/mpls/af_mpls.c#L1872

Correction.. the explicit-null labels don't seem to work on Linux.

Using ldpd, if I configure the "label local advertise explicit-null"
command in all nodes of a network, all LDP sessions flap every 3
minutes (the default holdtime).

I can't even ping a directly attached Linux LSR if I push an
explicit-null label. It's like the kernel just drops all incoming
packets that have an explicit-null label.

I'm using an relatively old kernel (v4.4.0) so maybe this was fixed
already, will check later.

Renato Westphal

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