[j-nsp] Spine & leaf

David Sinn dsinn at dsinn.com
Mon Jun 25 14:19:02 EDT 2018

At most networks scale you won't notice the difference, but OSPF will also converge faster then BGP at very large scale.  Adding on top the costs of re-using AS's in a eBGP world, verses mutual-RR with iBGP, having a good summarization plan with OSPF is a bit more trivial and retains a overall net smaller configuration on-box even if you are generating it programatically.  The concerns about chattiness is also overblown as even Quagga can keep up with massive Leaf/Spine deployments on really small CPU's in a only OSPF world.

I would caution that the auto-discovery can also have a downside as it more readily opens you up to mis-cabling, which can be fairly negative in a Leaf/Spine topology.  It's one of the reasons Cumulus came up with PTM so that you can deploy a described version of your topology and have the device alert/react when the actual version is different.  Some embodiment of that is useful, but need not be on-box.


> On Jun 25, 2018, at 10:37 AM, Scott Whyte <swhyte at gmail.com> wrote:
> In balance then, we have better filtering versus less config, which has already been noted can (must) be completely automated.  Where one's shop is on the NetDevOps curve probably has a lot of impact on the decision, which is unfortunate.
> On 6/25/18 10:29 AM, Thomas Bellman wrote:
>> On 2018-06-25 18:22, Scott Whyte wrote:
>>> BGP, as you say, provides excellent filtering capabilities.  What
>>> does OSPF/ISIS bring to the table?
>> Automatic discovery of peers, and thus less unique configuration.  You
>> don't need to configure each peer individually, just the interface.  If
>> you do unnumbered links, you don't even need to allocate link networks
>> for your routing links, giving even less unique configuration.  Just
>>   set interfaces xe-0/0/17.1 family inet unnumbered-address lo0.1
>>   set interfaces xe-0/0/17.1 family inet6
>>   set protocols ospf area A.B.C.D interface xe-0/0/17.1 interface-type p2p
>>   set protocols ospf3 area A.B.C.D interface xe-0/0/17.1 interface-type p2p
>> and you're done.  The nice thing is that the only unique piece of
>> configuration is the interface name.
>> Doing unnumbered links for BGP seems to at least be more complicated,
>> but Cumulus Linux is supposed to have support for it, making it as easy
>> to configure as OSPF.
>> (https://blog.ipspace.net/2015/02/bgp-configuration-made-simple-with.html;
>> I've never used Cumulus, just read about it.)
>> 	/Bellman
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