Subject: RE: default router selection based on network topology

From: Richard Draves <[email-masked]>
Date: Wed, 22 Nov 2000 09:46:30 -0800

> It seems that the subject of routers giving hints to hosts as to best
> routes off a network is tied in very closely with the issue of source
> address selection in a multihomed environment.
>
> Do you intend expanding the scope of the document to suggest what happens
> (for example) if the RA preference and src/dst selection algorithm happen
> to give conflicting suggestions for source address?  

Hmm. I don't see the connection. The RA preferences are for router
selection, not source address selection. Are you suggesting that the two
should be connected somehow?

> 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.

Rich
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From: Tim Chown <[email-masked]>
Date: Wed, 22 Nov 2000 23:03:11 +0000 (GMT)

> should be connected somehow?

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).

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.

> 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?

Tim