Subject: Re: [rbridge] maximumPathSplits

From: mike shand <[email-masked]>
Date: Fri, 01 May 2009 09:43:51 +0100

Radia Perlman wrote:

I can't think of any reason that IS-IS would preclude splitting on
zillions of paths, and it wouldn't
affect any packet formats. It's just localized behavior, certainly as
long as the paths are all
equal cost paths.

So it seems like all that's necessary is not to explicitly say that
there is a limit to the number of choices
that an RBridge can choose.

And perhaps tweak the MIB not to explicitly preclude it as well.

I can't imagine why that limit was ever put in...I'm curious if anyone
remembers what the reasoning was.
I don't remember it being there in DECnet.

Its not actually defined in the body of the spec. The 32 limit only
comes from the ASN1 definition of the parameter used in the GDMO (ISO
management speak) description of the attribute, and I suspect that was
simply inherited from the DECnet management interface where we did
restrict it to 32 (because that was all our implementation could handle
at the time). In fact most implementations of IS-IS that I know about
have had a limit <= than that.

Presumably the MIB reflects this too.

But I see no reason, from a protocol perspective, why it couldn't be
any value that the implementation could handle.

The only time where it would be important to to enforce an upper bound
would be if you were using some more exotic path computation algorithm
which made assumptions about what the maximum number of path splits
could be in ANY router.

Mike

Radia

Ian Cox wrote:

A lot of requests today are based around getting larger etherchannels
between switches to get more cross sectional bandwidth. If you use
etherchannels and TRILL ECMP together you have a way to build larger
connectivity between nodes.

Example

Max ECMP = 16 paths, etherchannel = 16 members, 256 total links

Question comes down to how much cross sectional bandwidth is enough?

Ian

Jeff Pickering wrote:

Both 10589 and RFC4444 specify a max for maximumPathsSplits of 32. In a
datacenter environment, I
am hearing requests for values much larger than this. Is there any point
in trying to modify this? We are
already doing non-standard 10589 stuff anyway. Just looking for feedback.

Jeff

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