Devil's Advocate - Segment Routing, Why?

Baldur Norddahl baldur.norddahl at
Sun Jun 21 12:58:02 UTC 2020

On Sun, Jun 21, 2020 at 1:30 PM Mark Tinka <mark.tinka at> wrote:

> On 21/Jun/20 12:45, Baldur Norddahl wrote:
> Yes I once made a plan to have one VRF per transit provider plus a peering
> VRF. That way our BGP customers could have a session with each of those
> VRFs to allow them full control of the route mix. I would of course also
> need a Internet VRF for our own needs.
> But the reality of that would be too many copies of the DFZ in the routing
> tables. Although not necessary in the FIB as each of the transit VRFs could
> just have a default route installed.
> We just opted for BGP communities :-).
Not really the same. Lets say the best path is through transit 1 but the
customer thinks transit 1 sucks balls and wants his egress traffic to go
through your transit 2. Only the VRF approach lets every BGP customer, even
single homed ones, make his own choices about upstream traffic.

You would be more like a transit broker than a traditional ISP with a
routing mix. Your service is to buy one place, but get the exact same
product as you would have if you bought from top X transits in your area.
Delivered as X distinct BGP sessions to give you total freedom to send
traffic via any of the transit providers.

This is also the reason you do not actually need any routes in the FIB for
each of those transit VRFs. Just a default route because all traffic will
unconditionally go to said transit provider. The customer routes would
still be there of course.


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