Devil's Advocate - Segment Routing, Why?

Sun Jun 21 17:34:03 UTC 2020

> The LFIB in each node need only be as large as the number of LDP-enabled
routers in the network.

That is true for P routers ... not so much for PEs.

Please observe that label space in each PE router is divided for IGP and
BGP as well as other label hungy services ... there are many consumers of
local label block.

So it is always the case that LFIB table (max 2^20 entries - 1M) on PEs is
much larger then LFIB on P nodes.

Thx,
R.

On Sun, Jun 21, 2020 at 6:01 PM Mark Tinka <mark.tinka at seacom.mu> wrote:

>
>
> On 21/Jun/20 15:48, Robert Raszuk wrote:
>
>
>
> Actually when IGP changes LSPs are not recomputed with LDP or SR-MPLS
> (when used without TE :).
>
> "LSP" term is perhaps what drives your confusion --- in LDP MPLS there is
> no "Path" - in spite of the acronym (Labeled Switch *Path*). Labels are
> locally significant and swapped at each LSR - resulting essentially with a
> bunch of one hop crossconnects.
>
> In other words MPLS LDP strictly follows IGP SPT at each LSR hop.
>
>
> Yep, which is what I tried to explain as well. With LDP, MPLS-enabled
> hosts simply push, swap and pop. There is not concept of an "end-to-end
> LSP" as such. We just use the term "LSP" to define an FEC. But really, each
> node in the FEC's path is making its own push, swap and pop decisions.
>
> The LFIB in each node need only be as large as the number of LDP-enabled
> routers in the network. You can get scenarios where FEC's are also created
> for infrastructure links, but if you employ filtering to save on FIB slots,
> you really just need to allocate labels to Loopback addresses only.
>
> Mark.
>
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