[c-nsp] LDPv6 Census Check

adamv0025 at netconsultings.com adamv0025 at netconsultings.com
Mon Jun 15 09:24:20 UTC 2020

> David Sinn
> Sent: Friday, June 12, 2020 4:42 PM
> > On Jun 12, 2020, at 8:26 AM, Saku Ytti <saku at ytti.fi> wrote:
> >
> > On Fri, 12 Jun 2020 at 18:16, David Sinn <dsinn at dsinn.com> wrote:
> >
> The label stack question is about the comparisons between the two
> extremes of SR that you can be in. You either label your packet just for
> ultimate destination or you apply the stack of the points you want to pass
> through.
> In the former case you are, at the forwarding plane, equal to what you see
> with traditional MPLS today, with every node along the path needing to
> know how to reach the end-point. Yes, you have lowered label space from
> traditional MPLS, but that can be done with site-cast labels already. And,
> while the nodes don't have to actually swap labels, when you look at
> commodity implementations (across the last three generations since you
> want to do this with what is deployed, not wholesale replace the network)
> null swap still ends up eating a unique egress next-hop entry. So from a
> hardware perspective, you haven't improved anything. Your ECMP group
> count is high.
Yes this is where each node needs to have a label uniquely identifying every
LSP passing through it.
With IP header you don't need this, 
Consider this: 
PE1 to PE2 via 3 P-core nodes 
With ECMP in IP, then PE1 just needs single FEC the DST-IP of PE2, which
will be load-shared across all 3 paths. 
Using MPLS If you need to uniquely identify each path you need 3 FECs (3
LSPs one via each P core node), now imagine you have 100K possible paths
across the fabric
 -that's a lot of FECs on PE1 or any node in the fabric where each has to
have a unique label for every possible unique path via the core that the
particular node is part of.

> In the extreme latter case, you have to, on ingress, place the full stack
> every "site" you want to pass through. That has the benefits of "sites"
> need labels for their directly connected sites, so you have optimized the
> implications on commodity hardware. However, you now have a label stack
> that can be quite tall. At least if you want to walk the long-way around
> world, say due to failure. On top, that depth of label stack means devices
> the middle can't look at the original payload to make ECMP decisions. So
> can turn to entropy labels, but that sort of makes matters worse.
You can use hierarchy of tunnels to help with the deep label-stack
imposition problem.  


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