[j-nsp] Spine & leaf

Thomas Bellman bellman at nsc.liu.se
Wed Jun 27 11:40:16 EDT 2018

On 2018-06-26 21:38, David Sinn wrote:

> OSPF scales well to many multiples of 1000's of devices.

Is that true even for Clos (spine & leaf) networks, and in a single area?

My understanding, solely based on what others have told me, is that
the flooding of LSAs in a Clos network can start to overwhelm routers
already at a few hundred devices, as each time e.g. a spine sends out
an LSA, all of the other spines will hear that from each of the leaves,
all more or less simultaneously.  And since the OSPF protocol limits
lifetimes of LSAs to 1 hour, you will get a constant stream of updates.

(And if you have multiple VRFs, the load might be multiplied by some
factor, depending on how many devices those VRFs exist on.)

The topologies of classical core-distribution-access networks would
not suffer as bad, nor would most ISP networks.

My own experience is only with pretty small networks (we currently have
2 spines and 11 leaves in our area, the rest of the university have a
couple dozen routers, and the OSPF database in our area contains ~1600
LSAs).  Thus, I can only repeat what others have told me, but I'm
curious to hear real-world experience from people running larger
OSPF networks.

Thomas Bellman,  National Supercomputer Centre,  Linköping Univ., Sweden
"We don't understand the software, and sometimes we don't understand
 the hardware, but we can *see* the blinking lights!"

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