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

[Radia Perlman]

Hmm. I don't know what LLDP is, but here is a proposal.

Observation: It's actually OK to consider a link with n RBridges to be 
n*2 pt-to-pt
links as long as n isn't very large.

So here's a proposal:

The Designated RBridge, R1, gets to decide if a particular link is to 
have a pseudonode
or not. R1 specifies either a pseudonode name in its Hello, or some 
value (like 0) that
says "Hey, I don't want to bother with a pseudonode, so just report all 
your RBridge
neighbors as pt-to-pt links".

So if there is a link with R1, R2, R3, and R4, then R1 gets to decide 
whether to
a) create pseudonode R1.x, and R1, R2, R3, and R4 all claim just the 
neighbor R1.x,
and R1.x claims neighbors R1, R2, R3, and R4, or
b) don't create a pseudonode, so R1 claims neighbors R2, R3, and R4...R2 
claims
neighbors R1, R3, and R4, etc.

The only thing special we were doing with the pseudonode was reporting 
the bridge root.
So that means that R1, the DRB, would report the root bridge ID in its 
LSP if
R1 has decided not to create a pseudonode.

We could declare some threshold, like 4 RBridges, and if R1 is DRB and 
there are
every 3 RBridge neighbors on a port, then R1 declares a pseudonode, and 
perhaps
leaves the pseudonode even if one of the neighbors goes down. Maybe once 
a port
is a pseudonode, it stays a pseudonode.

That way R1 would definitely not create pseudonodes for ports with just 
endnodes, or
for ports that are just pt-to-pt links.

The default would be "no pseudonode", but once a port had 3 RBridge 
neighbors, R1 decides
that port should have a pseudonode.

Radia



Anoop Ghanwani wrote:
>  
>
>   
>> -----Original Message-----
>> From: rbridge-bounces at postel.org 
>> [mailto:rbridge-bounces at postel.org] On Behalf Of Radia Perlman
>> Sent: Monday, November 05, 2007 10:11 PM
>> 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.
>>     
>
> If you were using LLDP, you could be certain that the link
> is a point to point link if the system information in LLDP
> matches what you receive in the IS-IS hello.  Or you we could
> identify RBridges as a type of device in LLDP itself.
>  
>   
>> Do we want to try to optimize this case, for instance by:
>>     
>
> I think it would be useful to optimize.
>
>   
>> 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?
>>     
>
> Is a combination of (a) and (b) possible?  A node can
> either use LLDP to discover it is point to point (and
> that won't change as long as the link stays up), or 
> it can be manually configured (in which case if a third
> adjacency is discovered, then you switch to broadcast-media
> mode and once in broadcast-media mode you never switch
> back).
>
>   
>> 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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