[c-nsp] IOS-XR BGP RR MCID (Multiple Cluster ID)

[Job Snijders]

On Sun, Mar 11, 2018 at 01:57:01PM +0200, Mark Tinka wrote:
> On 11/Mar/18 12:50, Job Snijders wrote:
> > Have you considered the downsides of sharing a Cluster-ID across
> > multiple boxes,
> 
> IIRC, the biggest issue with this was if the RR was in-path (as it
> used to be back in the days - and in some networks today - when core
> routers doubled as RR's), and there was no physical link between
> clients and all RR's.
> 
> In our network, RR's in the same PoP sharing a single Cluster-ID is
> fine because every client has a redundant physical link toward each
> RR, and each client has an iBGP session with all RR's. The benefit of
> using the same Cluster-ID for the RR's in the same PoP is much reduced
> overhead per RR device, as each RR only needs to hold one copy of
> NLRI.

Folks - i'm gonna cut short here: by sharing the cluster-id across
multiple devices, you lose in topology flexibility, robustness, and
simplicity.

Cluster-ID sharing _only_ exists to save some RAM. Of course most
optimisations have some kind of hidden price. In the case of MCID the
cost is that you must manoeuvre within very specific RR topology
contraints.

Perhaps a terrible analogy: Before crossing a street, you're supposed to
look both left and right for any oncoming traffic. With MCID the idea is
that you cross the street together with a friend, and you assume your
friend will be looking left, and he'll assume that you'll be looking
right. Anyone can see that it is far more robust and simpler if each
just looks out for themselves.

Just set the cluster-id to your router's IPv4 loopback IP address and
you'll be far less likely to get hit by a bus.

Kind regards,

Job