[frr] 2 MPLS Questions

Thomas Morin thomas.morin at orange.com
Tue Mar 21 05:47:30 EDT 2017


Hi Jeff,

2017-03-20, Jeff Tantsura:
> I think it is a requirement to have something efficient to trigger a 
> lookup in any {routing table, vrf interface, netns}.
>>
>> I hadn't tried (because no need). I thought we might achieve 
>> something like that by forwarding the packet on 'lo', or on a vrf 
>> interface, or on a veth device: wouldn't this kind of next hop 
>> specification trigger a re-enter of the packet in the IP stack after 
>> the pop operation ?
> [jeff] would’t this be a tad inefficient? :)

Well, I was not implying that the above would be efficient, and I would 
actually have the same concern as you have.
But to be honest I also lack hard facts to back this concern: e.g. I 
don't know whether going through a vrf interface is a small or high cost.

Best,

-Thomas




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