Devil's Advocate - Segment Routing, Why?

Mark Tinka mark.tinka at seacom.mu
Sun Jun 21 16:00:12 UTC 2020


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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