[rbridge] Can we ever have pt-to-pt links?

[Eric Gray]

Radia,

The usual approach to specifying the (optimized) behavior you 
describe would be to say something like:

"An implementation MAY be aware that a specific link is point-to-
 point, in which case <the DRB election process> MAY be omitted."

However, it is worth realizing that this relies on a common view
at both ends of the point-to-point link.  What happens if one end
tries to perform a DRB election and the other does not?

It then becomes necessary to specify the behavior in this case.

I suspect that - if one has valid reasons to think that a link is 
point-to-point - one might configure both ends to assume this is 
the case.  In the event that any RBridge on the link tries to do
DRB election, then this MUST become the common mode.

Using that approach, the only failure mode where RBridges will
become confused is if three (or more) RBridges are 1) connected
to the same shared media and 2) configured to assume P2P.

Clever implementations MAY deal with that scenario in some robust
way - but we should not need to specify how that may be done (it
is always possible to misconfigure yourself into a hole, so there
is little point in trying to prevent that).

--
Eric Gray
Principal Engineer
Ericsson  

> -----Original Message-----
> From: rbridge-bounces at postel.org 
> [mailto:rbridge-bounces at postel.org] On Behalf Of Radia Perlman
> Sent: Tuesday, November 06, 2007 1:11 AM
> To: Developing a hybrid router/bridge.
> Subject: [rbridge] Can we ever have pt-to-pt links?
> 
> Suppose the topology consisted only of RBridges with pt-to-pt links 
> either to each other or
> to endnodes, e.g., no bridges, no CSMA/CD. As specified,  
> we'd wind up 
> creating
> a pseudonode for each link. Not a total disaster, but it does seem 
> suboptimal, especially
> if we created a pseudonode for every port that contains an endnode.
> 
> I think we discussed this and said that there was no way of 
> knowing for 
> sure that a port
> was a pt-to-pt link.
> 
> Do we want to try to optimize this case, for instance by:
> a) allowing a port to be configured as "pt-to-pt", and 
> freaking out (as 
> in disabling
> the port and declaring an error) if an RBridge sees multiple nodes on 
> that port, or sees
> a BPDU?
> b) assuming if there is only one RBridge neighbors on a port 
> that it is 
> a pt-to-pt
> link and there does not need to be a pseudonode? That makes 
> me nervous 
> because if
> a third RBridge comes and goes, then things could get messy.
> c) if there are zero RBridge neighbors on a port (just an endnode or
> set of endnodes), do not create a pseudonode
> for it
> d) something else?
> 
> Or is it fine as is (if R1 and R2 are connected with a 
> pt-to-pt link, R1 
> will get elected DRB,
> name the link R1.x, and issue two LSPs -- one for R1, one for 
> R1.x, with 
> R1.x claiming
> connectivity to R1 and R2). Basically, there will be a pseudonode for 
> every RBridge-RBridge
> link. I hope we don't have to create a pseudonode for every port to 
> every endnode.
> 
> Radia
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