[j-nsp] Longest Match for LDP (RFC5283)

[Krzysztof Szarkowicz]

James,

As mentioned in my earlier mail, you can use it even with DU. If ABR has
10000 /32 LDP FECs, you can configure LDP export policy on ABR to send only
subset (e. g. 20 /32 FECs) to access.

Saying that, typical deployment is with DoD, since typically access PEs
(and not ABRs) have better knowledge which loop backs are needed. So,
basically access PEs send the info to ABR, which loop backs are needed, and
which loop backs are not needed via LDP DoD machinery.

Sent from handheld device. Sorry for typos.

On Mon, Jul 30, 2018, 11:15 James Bensley <jwbensley at gmail.com> wrote:

> Hi Krasimir, Krzysztof,
>
> On 24 July 2018 at 17:25, Krasimir Avramski <krasi at smartcom.bg> wrote:
> > It is used in Access Nodes(default route to AGN) with
> > LDP-DOD(Downstream-on-Demand) Seamless MPLS architectures - RFC7032
> > A sample with LDP->BGP-LU redistribution on AGN is here.
>
> Thanks Krasimir. Sorry for the delay, I read
> https://tools.ietf.org/html/rfc7032,
> https://tools.ietf.org/html/rfc5283 and
> https://tools.ietf.org/html/draft-ietf-mpls-seamless-mpls-07 before
> responding.
>
> On 25 July 2018 at 09:14, Krzysztof Szarkowicz <kszarkowicz at gmail.com>
> wrote:
> > The purpose of “Longest Match for LDP” is to be able to distribute /32
> LDP
> > FECs, if corresponding /32 routes are not available in IGP.
> > So, on ABR you inject e.g. default route into access IGP domain. ABR has
> /32
> > LDP FECs, and advertises this /32 FECs in LDP (but not in IGP) downstream
> > into access domain. In access domain, LDP readvertises hop-by-hop these
> /32
> > LDP FECs, assigning the labels.
> >
> > It is typically used with LDP DoD. On the other hand, however, nothing
> > prevents you from having LDP policy on ABR to inject into access domain
> only
> > specific /32 LDP FECs.
>
> Thanks Krzysztof, that was my understanding from the Juniper link I
> provided and the RFC, but it's still nice to have my understanding
> clarified by someone else.
>
> After reading the above RFCs I see that the specific use case for this
> feature is when using LDP in Downstream on Demand mode, although that
> isn't actually called out in RFC5283 anywhere or the Juniper
> documentation. I was thinking in DU mode in my head :)
>
> In DU mode, an agg node will advertise all labels to the access node.
> If the access node has say 10.0.0.0/22 summary route (an example range
> loopback IPs are assigned from) and RFC5283 enabled, and the agg node
> advertises 1024 /32 IPv4 FEC labels (one for each loopback assuming
> 1000 PEs exist) the access node will keep all 1000 labels even if it
> only needs a few of them, matching them against the summary route.
> This is the default LDP DU behaviour unless we create horrible per-LDP
> neighbour policies on the agg node that only allow the labels for the
> exact loopbacks that access node needs to reach. So relaxing the LDP
> exact match rules is kind of useless for LDP DU. In LDP DoD mode, the
> access nodes only request the label mappings for the labels they need,
> so no need for per-LDP neighbour policies, but we would still need
> per-LDP neighbour IP routing policies to only advertise the /32
> loopback IPs that neighbor needs in the IGP, unless we use RFC5283 and
> advertise a summary route (or install a static summary route).
>
> Cheers,
> James.
>