[c-nsp] OSPF+BGP and MPLS Q's

[Peter Rathlev]

On Mon, 2018-07-23 at 12:23 +0200, ringbit at mail.com wrote:
> Anyone else can give an opinion to those three questions?

Opinions are easy to give. :-) Authority is a different question
altogether. I spend my daytime in a place that started with just 6 PE
routers and has slowly grown to 51 over ~13-15 years. Still not a big
number, but I have been a part of it all the time and have learned some
things the hard way.

We're an closed ("enterprise") IS-IS-based MPLS L3VPN network so things
might not translate directly. We only have IGP links and loopbacks in
IS-IS, just above 200 routes in the global table currently. We around
7000 prefixes in BGP.

I'm also not sure I understand the first question. BFD is a way to
overcome certain failure scenarios if you need to use some kind of L2
transport between the routers. But the better way, since you ask, is to
have two or more physically direct and diverse L3 links between the
routers.

We have always been single area. Our small size considered this will
most likely never be a problem. If the size of your network is within
an order of a magnitude of ours I see no reason at all to introducing
the complexity of multi-area.

We started the network without route reflectors. When we grew to more
than 20 PE routers we configured three existing PEs as RRs. This was a
bit more complex than we wanted, so we eventually started using two
Cisco 2901 as RRs. They have been with us since and are not breaking a
sweat handling the ~7000 networks and ~17000 paths we have in BGP.

Having RRs really simplifies deploying new PEs. Big networks probably
have even the deployment of new PEs fully automated, but would then
choose RRs for scalability reasons.

If I were to start over I would implement RRs from the very start.

We use unique (type 1) RDs per PE and thus have all PEs see paths from
all other PEs, not just the "best" path, giving us quick failover.

-- 
Peter