thanks for the response - I'm not sure that I explained myself clearly,
should have done a diagram
|------------| ether1
| |
fe| |fe
R1 R2
ge| |fe
| |
|-----------| ether2
|
|fe
R3
in view of the above, R3 has attached FE route to each of R1 and R2 which
will each announce ether1 with equal metrics, based on the equal cost
upstream. does R3 have any way of knowing that R1 is GE attached to ether1,
and that it (R3) should prefer the route via R1?
what I am going after is whether, when one might have different bandwidths
(10/100/1000 for ethernet), a router would be able to discern from the
ourting information, which router to prefer in the case of there being
parallel paths to a network, in the case of those routers having
different-speed attachments to an interrim, attached, broadcast network.
hope I've made sense this time ;-)
regards
Andy
----- Original Message -----
From: "EA Louie"
To: "Andy Harding" ;
Sent: Sunday, May 06, 2001 7:01 PM
Subject: Re: Route metrics on broadcast networks [7:3308]
> see these pages:
>
> http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/routing.htm
> http://www.cisco.com/warp/public/103/5.html (this one is very technical,
but
> addresses metric calculation in IGRP)
> http://www.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2907.htm (general
> presentation of route metrics)
>
> ----- Original Message -----
> From: "Andy Harding"
> To:
> Sent: Saturday, May 05, 2001 7:28 AM
> Subject: Route metrics on broadcast networks [7:3308]
>
>
> > bit of a teaser I have been thinking about for a while, and haven't
really
> > been able to get clear in my mind:
> >
> > how do routing protocols calculate metrics on broadcast networks where
the
> > metric may be different between different neighbors?
>
> In each routing protocol is an inherent metric value called administrative
> distance. That's the primary routing path determination for IGPs. Other
> metrics are local and determined by the interface characteristics.
>
> Let's use the case of IGRP. (RIP is hop-count sensitive and uses that as
> its routing metric).
>
> All the router knows is the properties of its directly connected
interfaces,
> so it uses parameters like interface bandwidth, and delay, and the its
> administrative distance to calculate the route metrics, including the ones
> it learns. It then sends the routes it has for its directly-connected
> routes and its learned routes to another router. That other router does
the
> same thing with the learned routes - that is, calculates learned route
> metrics based on the ingress interface bandwidth and delay parameters and
> the administrative distance of the IGRP itself.
>
> >
> > As an example, say you have a core router with a GE downlink into an
> ethernet
> > switch, and you have you distribution switches attached with FE. Do the
> > distribution-level routers know to prefer the core router's uplink (all
> other
> > things being equal)? and if so, how?
>
> Well, directly connected routes are best, regardless of what your other
> routes may be, so if the Distribution router and Core router are in
> parallel, the Distribution router would prefer his own route. Otherwise,
> the routers that are FE connected will have higher metrics than the GigE
> route. From an uplink perspective, let's say you have a distribution
router
> on the switch, and two paths out: via GigE core router and via FastE
access
> router. The distribution router will accept routes from both core and
> access routers. Distribution router will see them as the same route with
> exactly the same metric unless the metrics have been artificially altered
in
> one or the other router. Distribution router doesn't know how the core
and
> access routers are connected, and can't make a routing decision based on
> their interface bandwidths.
>
> Let's take another case, where the distribution router has two interface
> paths - one Gigabit to the core, one Fast to the access router. Let's
also
> say that the core and access routers are parallel - that is, have the same
> destination networks in its routing table. In the distribution routing
> table, it will have a smaller metric to the core router, and therefore
will
> prefer that path. However, the routes from the access router will also be
> there, so if the route to the Core router is lost, the backup will be to
the
> access router.
>
> To summarize, the distribution router really has no knowledge of the
uplink
> bandwidth of it's neighbors, so it's no wonder that this has caused you
> sleepless nights.
>
> EIGRP handles metrics a bit differently. See
> http://www.cisco.com/warp/public/103/eigrp-toc.html
> and read the section on EIGRP metrics
>
> OSPF also handles metrics a little differently - see
> http://www.cisco.com/warp/public/104/9.html
> and read the answer to the question "How does OSPF calculate its metric or
> cost?"
>
> >
> > many thanks
> >
> > Andy
> > FAQ, list archives, and subscription info:
> http://www.groupstudy.com/list/cisco.html
> > Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
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