Subject: RE: default router selection based on network topology

From: Richard Draves <[email-masked]>
Date: Wed, 22 Nov 2000 17:00:06 -0800

> Would you always pick a longest match for source address over the RA
> that had the best preference?  
> 
> For example in the selection draft you say
> 
> "For example, suppose a node has interfaces on two different links, 
>  with both links having a working default router. Both of the 
>  interfaces have preferred global addresses. When sending to a global 
>  destination address, if there's no routing reason to prefer one 
>  interface over the other, then an implementation MAY preferentially 
>  choose the outgoing interface that will allow it to use the source 
>  address that shares a longer common prefix with the destination."
> 
> The RAs now convey "routing reasons", which affect the 
> selection (in that
> without them, another choice might be made).

I'm still not understanding your scenario. The paragraph you qoute above is
a suggestion of how source address selection concerns *might* be used to
influence the choice of router. (I don't think it's a great idea but I
wanted to discuss it in the draft.) Whereas I think you're asking about the
other direction - how will the routing preferences influence source address
selection. The answer is simple: the routing preferences influence the
choice of sending interface, and that strongly influences source address
selection.

> Of course, this may be exactly want you want, but I suspect the common
> "multihomed" scenario at present is sites with a 2001: link 
> and a 3ffe:
> link, one of which is probably much better than the other, even though
> both may route happily anywhere.

OK. Suppose a host has two interfaces. On Interface 1, it receives RAs
advertising a 2001 prefix. Interface 1 is better for reaching 2001
destinations. On Interface 2, it receives RAs advertising a 3ffe prefix.
Interface 2 is better for reaching 3ffe destinations. The host has a default
route on interface 1 and a default route on interface 2. Now the host is
sending to a 2001 destination.

This is a situation where the paragraph you qouted applies. An
implementation might choose the default router on interface 1 (the "right"
answer), by looking at its autoconfigured addressses on the two interfaces
and realizing that it has a "better" source address on interface 1.

Now, suppose the router on interface 1 advertises a more specific route for
2001::/16 and the router on interface 2 advertises a more specific route for
3ffe::/16. Now the routing table will select the "right" interface - it
doesn't need any help from source address selection.

I think this is a better approach. Which is why I've implemented it and not
implemented the quoted paragraph.

> > > I'm also interested in just how the routers will be 
> configured to make
> > > the advertisements, as it would seem this would impact the router
> > > renumbering process?
> > 
> > My first thought is that this should not impact router 
> renumbering. Router
> > renumbering is about making configuration changes uniformly 
> across a set of
> > routers, while the advertisement of default router preferences &
> > more-specific routes is very topology dependent. I think 
> it's something that
> > an administrator would need to selectively configure on 
> relatively few
> > routers, not configure uniformly across a site.
> 
> The question is how deeply the "topologically dependent" 
> changes are made 
> by the administrator through the network.  Having manually configured 
> preferences in the config file only adds to the list of 
> things that (might) 
> need changing if the network is renumbered, or a network 
> you're giving a 
> preference to is renumbered?  I appreciate there are other 
> issues such as 
> IOS access lists and of course IPs embedded in host configs.  
> How realistic
> a goal is (relatively) transparent renumbering?

Consider an enterprise with 100 routers. I certainly do not imagine an
administrator will be going into each of those routers' config and
individually tweaking advertised router preferences and routes.

Maybe an administrator might have a policy like here's a specific /16 or /48
route that I want to advertise in RAs, and it would be nice to update this
prefix across a bunch of routers. I imagine an extension of router
renumbering could handle this.

Rich
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---

From: Robert Elz <[email-masked]>
Date: Fri, 24 Nov 2000 18:29:40 +0700

Date:        Wed, 22 Nov 2000 17:00:06 -0800
    From:        Richard Draves <[email-masked]>
    Message-ID:  <[email-masked]>

  | > The RAs now convey "routing reasons", which affect the 
  | > selection (in that
  | > without them, another choice might be made).
  | 
  | I'm still not understanding your scenario.

I think what Tim is getting at is that different source addresses
may be chosen with the router preferences than without.

I don't think this is an issue at all - the routing table has always
influenced source address selection, all the draft in question does
is alter the way the routing table is built.

On the other hand, I don't think the effects of more config in
routers is something to be swept under the table - that, in co-ordination
with router renumbering does need some work.   It may be as simple
as noting that when router renumbering causes a prefix to be altered,
and that prefix is in the router preference list, then it should be
altered there as well.   But I haven't thought that through.

kre