Subject: [rbridge] IS-IS pseudonodes, nicknames, and rooted trees

From: Radia.Perlman at sun.com (Radia Perlman)
Date: Sun, 07 Jan 2007 17:01:19 -0800

In IS-IS, a shared link is treated as a "pseudonode" -- as if it were 
another router. The purpose of
this is because the routing algorithm scales as the number of links, so 
a link with n router neighbors
becomes n+1 nodes with n links rather than fully connected, order of n^2 
links.

The Designated Router gives the pseudonode a name (in IS-IS, a 7-byte ID 
consisting of
a 6-byte EUI owned by the DR, plus an extra byte to differentiate among 
links that that router
is DR for), and generates an LSP on behalf of the pseudonode.

Anyway, the question is -- should we allow/require pseudonodes to be 
assigned RBridge nicknames?
There are two reasons for obtaining an RBridge nickname

1) to be specified as the egress RBridge of a data packet -- I don't 
think this will ever be relevant
to pseudonodes
2) to be specified as the root of a tree

My inclination is to say pseudonodes cannot be egress RBridges, or 
specified as tree roots, and
therefore pseudonodes will not be given nicknames.

Comments?

Radia

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From: eric.gray at ericsson.com (Eric Gray (LO/EUS))
Date: Mon, 8 Jan 2007 15:14:16 -0600

Radia,

	As I understand it, IS-IS creates a pseudo-node as an
avatar of the designated router.  You probably would not see
a local forwarding entry from the pseudo-node to the DR, and
then from there onto whatever link the DR will forward the
packet.

	Is there any reason in L2 forwarding why we cannot use
the same identifier for the DR(Bridge) in lieu of any sort of
distinguished pseudo-node identifier?

	If we can't use the DR(Bridge) identifier/nickname as
a "pseduo-node identifier", I don't think we can say that a 
"pseudo-node" would not be either an ingress.  An important 
reason why we need to determine a DR(Bridge) is to handle the 
potential for ingress/egress on a shared link.

	Unless you're saying that the pseudo-node would be the
first next-hop for an ingress or last hop before an egress...

--
Eric

> -----Original Message-----
> From: rbridge-bounces at postel.org 
> [mailto:rbridge-bounces at postel.org] On Behalf Of Radia Perlman
> Sent: Sunday, January 07, 2007 8:01 PM
> To: rbridge at postel.org
> Subject: [rbridge] IS-IS pseudonodes, nicknames, and rooted trees
> 
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---

From: Radia.Perlman at sun.com (Radia Perlman)
Date: Mon, 08 Jan 2007 21:26:25 -0800

Pseudonodes aren't quite avatars for the DR,
but I think in the case of RBridges, it might as well be.
IS-IS does treat the LAN as if it is a node distinct from the DR, and 
the DR issues an
LSP on behalf of the pseudonode, as well as one on behalf of itself, and 
there is a zero cost
link between the pseudonode and each of the routers on the link (with 
nonzero cost from a node
to the pseudonode, and zero cost the other way, or vice versa). 
Traditionally, endnode information for the link
would go into the pseudonode's LSP rather than the DR's.

For RBridges, I think the answer to my question is that it does not 
matter -- I *think* the same
tree would be calculated whether you chose as root the pseudonode or the 
DR. So it's probably better
to say that pseudonodes do not request nicknames, and cannot be 
specified as the root of trees,
and would never appear as the ingress or egress RBridge. I don't think 
it matters whether the
endnode information appears to be advertised by the DR or the 
pseudonode. I think we can let the IS-IS
people decide that detail.

Radia

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From: mshand at cisco.com (mike shand)
Date: Tue, 09 Jan 2007 11:36:47 +0000

At 05:26 09/01/2007, Radia Perlman wrote:
>DR.

I don't think that is true. Since the cost from the DR to the 
pseudonode is non-zero (although the cost from the pseudonode to the 
DR is zero), I think you could end up with a different tree depending 
on which is root. Of course in normal IS-IS you would never make the 
pseudonode the root, so the issue doesn't arise.

But I don't think it matters, since I agree with what you say below.

         Mike

>So it's probably better

---

From: riw at cisco.com (Russ White)
Date: Tue, 09 Jan 2007 09:42:15 -0500

> people decide that detail.

I would agree with this. The main issue we might have is how to handle
the pnode advertisement by the DIS. I think this is okay, the
connections are all still there:

A---A'---B---C

If A' is the pnode, and A is the DIS, then A will be advertising:

o An LSP with it's own information, including its nickname and ID
o An LSP with the information form A', which would not include
 a nickname

When C calculates its SPT, I assume that it will build the the link from
B--A', and A'--A, but without A' having a nickname, will this work? I
think it will, just wanted to think it through to make certain. The next
hop towards some destination, for B, could be A, but the local tree
shows this as through A', which should resolve in the forwarding table
down to the next hop A, just as it works today.

:-)

Russ