Just curious as to what people are using for metrics in their IGP and what their reasons are; bandwidth? geographical distance? latency? etc...
Thanks
-Tom
Think most ISPs use actual fiber miles (with an arithmetic factor to get to
a certain range of course) as the means for the value of IGP metrics... of
course there are always some "twinking" done regularly to give higher
priorities to the higher bandwidth, link condition etc.
Sush
Since both isis and ospf support a large range of metrics nowadays, the
actual mileage itself is an option for the metric too. For example,
before isis wide metrics, a route with fiber mileage of 1000 might be
given an isis metric of 16(using the method Sush mentioned to get the
metric within the isis metric range of 0-63, so 1000/64 = 15.625, round
up to 16), but now with wide metrics, the actual mileage can be used.
Using the wide metrics also helps reduce/eliminate the equal cost paths
that used to crop up in large networks with a limited metric range.
and actual mileage of 1000On Fri, 19 Jul 2002, Sush Bhattarai wrote:
Date: Fri, 19 Jul 2002 14:25:16 -0400
From: Sush Bhattarai <netnews@sush.org>
To: nanog@merit.edu, Tom Holbrook <tomhol@corp.earthlink.net>
Subject: Re: IGP metrics on WAN linksThink most ISPs use actual fiber miles (with an arithmetic factor to get to
a certain range of course) as the means for the value of IGP metrics... of
course there are always some "twinking" done regularly to give higher
priorities to the higher bandwidth, link condition etc.Sush
From: "Tom Holbrook" <tomhol@corp.earthlink.net>
To: <nanog@merit.edu>
Sent: Friday, July 19, 2002 12:27 PM
Subject: IGP metrics on WAN links>
> Just curious as to what people are using for metrics in their IGP and what
> their reasons are; bandwidth? geographical distance? latency? etc...
>
> Thanks
> -Tom
>
> __________________
> Tom Holbrook
> Sr. Network Engineer
> Atlanta
> Earthlink
>
-sean
Spoon!
That seems unlikely to me. Do you really want your intra-AS traffic
to always follow a short OC3 in preference to a long OC48?
Joe
I think you missed part of his comment:
" of course there are always some "twinking" done regularly to give higher
priorities to the higher bandwidth, link condition etc"
so fiber mileage is just the base, with modifications to make it work
correctly, based on bandwidth, etc.
Yeah, my (limited) experience is the opposite. At the previous large
operator at which I had enable, the IGP metrics were chosen primarily
according to circuit size, and were subsequently tweaked for other
issues (such as circuit latency, or the requirement to balance cross-
US traffic across non-parallel circuits).
In my experience, congestion is a much more effecive killer of service
than latency due to optical distance. Hence attracting traffic to
circuits where there is more likely to be headroom seems a more
reasonable first-order approach for choosing metrics.
That experience is all in networks where intra-AS traffic engineering
was done at the IP layer, however; in networks where there is a lower
layer of soft traffic engineering maybe other approaches would be more
appropriate.
Joe
I would expect that approach wouldn't scale as well as the network
grows in complexity, since it will be much more sensitive to unexpected
traffic flow changes due to addition of new circuits or routers.
Definitely, congestion is an issue. If you've got backbone congestion,
you better be tweaking your traffic as best you can in any case. But the
network should behave in an intuitive manner with minimum latency
(shortest distance) as much as possible.
Warren Van Camp.
I suspect the approach you take depends on how your network looks. If you
have many pipes of a variety of sizes, doing IGP metrics based on pipe size
makes a good deal of sense, then adding twists for things like ckt latency.
However, folks with uniform sized networks, and uniform traffic between
coasts probably tend to set IGP metrics for latency, with pipe size being
the exception that they bias for afterwards.
The latter is probably more prevelent in an established network, the former
in a network undergoing a large fiber build.
- Dan