[frr] 2 MPLS Questions

Jeff Tantsura jefftant at gmail.com
Sat Mar 18 06:27:02 EDT 2017

That's only needed with label per VRF mode, and if I recall correctly in currently supported/discussed label per prefix mode where POP is followed by IP lookup in VRF context. IMHO - would be really bad (why repeat bad early IOS choices?)
The most optimal case for FRR would be label per NH allocation, where label lookup should yield fully resolved adj, withno need for additional lookup.
It would also support the case with >1 NH's in a VRF (multihoming).

There are some corner cases, like EIBGP LB + BGP FRR, however we are rather far away from that point in life...


> On Mar 18, 2017, at 00:27, 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.
>>> 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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