[rbridge] Proposed resolution of DRB election/MTU testing
Les Ginsberg (ginsberg)
ginsberg at cisco.com
Fri May 1 12:15:37 PDT 2009
> -----Original Message-----
> From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org]
> Behalf Of Radia Perlman
> Sent: Friday, May 01, 2009 11:44 AM
> To: rbridge at postel.org
> Subject: Re: [rbridge] Proposed resolution of DRB election/MTU testing
> Re: Mike Shand asking if we really need to say "I think the DRB's ID
> priority is x,y", given that
> the IS-IS Hello contains the LAN ID.
> You really need priority too, otherwise, if R3 hears that R2 thinks
> R1.17 is the LAN ID (aka "pseudonode ID"),
> R3 has no way of whether R1 should be higher priority or not.
R3 would know this from the hellos it receives from R1.17.
Or are you suggesting that R3 should take R2's word for it and override
it's own internal logic as to who should be DRB even if it had not heard
(I certainly hope not...)
> And I'm not sure we need to have anyone other than the DRB put in the
> LAN ID into
> their TRILL-Hello. What purpose does it serve?
It serves as confirmation that all ISs have converged on their selection
of the DIS/DRB and that each IS has correctly obtained the pseudo-node
ID as selected/transmitted by the winner of the election. This is useful
in debugging operational problems.
> And as for whether R1's address is embedded in the LAN ID:
> I'd always intended that the LAN ID (the "pseudonode ID") could be any
> byte quantity that is guaranteed to
> be campus-wide unique. A good way of choosing the LAN ID, if you are
> "R1", is to make the LAN ID
> "R1" followed by a byte to distinguish between up to 255 other links
> might be DR on. But
> really, it just needs to be a unique 7 byte quantity (with 7th byte
> nonzero). Somehow it seemed to morph over the years
> into the top 6 bytes MUST be the system ID of the DR, right? Is there
> anything that depends on those
> 6 bytes being the system ID of the DR, or is it just folklore that it
> has to be the system ID of the DR?
The system ID is required to have exactly those properties i.e. it is
required to be unique area wide at Level 1 and unique domain wide at
Level 2. This is obviously necessary so as not to confuse the LSPs
generated by two different systems.
> mike shand wrote:
> > On 01/05/2009 Radia Perlman wrote:
> >> **********
> >> Now a refinement that would be useful to discuss:
> >> While we're on the subject, one extra field that could make things
> >> even safer would
> >> be a TLV for "this is the (ID, priority) of who I think is DRB".
> > The LAN ID field in the IS-IS hello already tells you who the sender
> > thinks is the DIS, but it doesn't include the priority. Instead it
> > the ID assigned by the DIS. Why would you want the priority?
> > Mike
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> rbridge at postel.org
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