[frr] 2 MPLS Questions

[Marc Sune]

Hi all,

Kind of an aside question about MPLS implementation in FRR;

On Mon, Mar 20, 2017 at 5:38 PM, Thomas Morin <thomas.morin at orange.com>
wrote:

> Hi everyone,
>
> 2017-03-18, Vincent Jardin:
>
> +Thomas to be on track.
>
>
> Vincent, you'll tell if what is below helped or not :)
>
> Le 18 mars 2017 06:19:47 Vivek Venkatraman <vivek at cumulusnetworks.com>
> <vivek at cumulusnetworks.com> a écrit :
>
>> 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.
>>
>
> (Note that with BGP/MPLS VPNs this is the typical behavior, but it is not
> a mandatory behavior: the egress router may have advertise a real label
> (i.e. not implicit null) in which case the penultimate router will swap the
> topmost label of the stack, not seeing/touching the vpn label. This is also
> a behavior that relates to the use of MPLS for transit, but with
> MPLS-over-GRE or MPLS-over-UDP, MPLS can be used with IP transit, in which
> case this behavior is not used).
>

In regular MPLS processing (no SR), does FRR currently support advertising
any label other than implicit NULL for MPLS traffic termination(itself)?
And for L2VPNs?

thanks
Marc


>
>
>
>> This behavior should already be supported.
>>
>
> Yes, I can confirm that forwarding via a neighbor on an interface based on
> the incoming MPLS label is supported.
> This is what we use in bagpipe IP VPN 'linux' driver [1].
>
>
>> 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.
>>
>
> 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 ?
>
> -Thomas
>
> [1] http://git.openstack.org/cgit/openstack/networking-bagpipe/t
> ree/networking_bagpipe/bagpipe_bgp/vpn/ipvpn/mpls_linux_dataplane.py#n194
>
>
>
>
>
>> 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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