Subject: RE: ndproxy loop prevention

From: "Dave Thaler" <[email-masked]>
Date: Tue, 15 Mar 2005 15:21:55 -0800

This question was asked during the meeting, and yes we did think through
the details.

Consider the case where you only use the P-bit scheme.
However, on each segment, there could be standard L2 bridges
which don't use NDproxy.  Those L2 bridges are responsible for
ensuring that that segment is loop-free.

To P-bit NDproxies, NDproxies which run STP are the same in this
respect as normal L2 bridges.  

As a result, the two schemes co-exist fine.

-Dave

> -----Original Message-----
> From: Erik Nordmark [mailto:[email-masked]]
> Sent: Tuesday, March 15, 2005 11:22 AM
> To: Dave Thaler
> Cc: IPv6
> Subject: ndproxy loop prevention
> 
> 
> I didn't make the IPv6 meeting last week so I looked over the slides.
> 
> I'm a bit concerned about the complexity of having both the 'P' bit
and
> the optional STP. What happens if you have some proxies which run STP,
> and others that use the P-bit?
> Seems a bit complex to figure out how this can work.
> 
> Have you worked out the details?
> 
>     Erik

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---

From: Erik Nordmark <[email-masked]>
Date: Tue, 15 Mar 2005 20:52:48 -0800

> respect as normal L2 bridges.  

This means that the STP-running sub-cloud need to be P-bit transparent, 
right?

Thus if there is an upstream which sets the P-bit, then any downstreams 
from the sub-cloud should receive the P-bit.
To do that the ndproxy nodes which run STP need to behave quite 
differently than then ones that don't run STP. What are the exact rules 
for them?
Are they
  - ignore any P-bit
  - make sure any RAs that are received with the P-bit set are proxied 
(transmitted) with the P-bit

    Erik

---

From: Bob Hinden <[email-masked]>
Date: Wed, 16 Mar 2005 09:13:52 -0800

Erik,

>This means that the STP-running sub-cloud need to be P-bit transparent, right?

Yes, that is my understanding.  I think the draft is consistent with 
this.  That is, a ND proxy implements either the P-bit (choice a) or the 
STP (choice b) modes, but not both.  In STP mode the P-bit (or any of the 
other bits in the RA) isn't touched.

> (transmitted) with the P-bit

Right, and I think that is the defined behavior of a proxy implementing STP 
(i.e., they pass the P-bit setting through unchanged).

I think it would be good if another paragraph was added to Section 4.1.4.3 
"ICMPv6 Router Advertisements" that clarifies this behavior.  Also, 
something added to Section 6. "Loop Prevention" that explains the case of a 
site with a mixture of proxies.

Thanks,
Bob