[frr] 2 MPLS Questions

Roopa Prabhu roopa at cumulusnetworks.com
Sat Mar 18 01:27:56 EDT 2017

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

> 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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