From: zzk (ccie_99@yahoo.com)
Date: Sat Nov 22 2003 - 02:28:35 GMT-3
Thanks. Thats a good solution.
In the drawing, can we change the ip cost on the r1 &
r2's X interfaces instead? thus they can affect r3's
route selection.
In a broadcast media, if change one router's cost,
does this violate the OSPF principle? I feel ospf is a
link state, so every router should have the same cost
on all interfaces...
regards
--- Brian McGahan <bmcgahan@internetworkexpert.com>
wrote:
> Cisco's OSPF implementation (not sure about the
> standard) has a
> special network type to deal with this problem.
> This is this network
> type 'point-to-multipoint non-broadcast' Typically
> it is used in a NBMA
> scenario when you have one logical network that has
> VCs of varying
> speeds, it can be applied to this problem as well.
>
> Suppose we have the following:
>
> _X_
> R1 R2
> _|____|_
> |
> R3
>
> R1, R2, and R3 share a broadcast Ethernet segment.
> R1 and R3
> connect to the segment with FastEthernet interface,
> while R2 uses a
> regular 10Mbps Ethernet interface. R1 and R2 are
> attached to
> destination "X". From the perspective of R3, the
> cost to destination X
> is equal through R1 and R2 (cost of 1 for FastE on
> R3). However, since
> R2 only has 10Mb in actuality it's not. Changing
> the cost of R3's
> interface isn't going to affect anything because as
> you mentioned
> before, it connects to the same segment. With
> network type P2M
> non-broadcast, you can specify the cost on a per
> neighbor basis:
>
> R3:
> interface Ethernet0/0
> ip ospf network point-to-multipoint non-broadcast
> !
> router ospf 1
> neighbor 10.0.0.1 cost 1
> neighbor 10.0.0.2 cost 10
>
> This way R3 knows that prefixes learned from R2
> should be offset
> with a cost of 10 instead of the interface cost of
> 1.
>
>
> HTH,
>
> Brian McGahan, CCIE #8593
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Direct: 708-362-1418 (Outside the US and Canada)
>
>
> > -----Original Message-----
> > From: nobody@groupstudy.com
> [mailto:nobody@groupstudy.com] On Behalf
> Of
> > Howard C. Berkowitz
> > Sent: Friday, November 21, 2003 10:33 AM
> > To: ccielab@groupstudy.com
> > Subject: Re: tune ospf cost in a broadcast media
> >
> > At 7:41 AM -0800 11/21/03, zzk wrote:
> > >Hi
> > >I understand in a point to point link, it is
> better to
> > >change 'ip ospf cost' to the same value on both
> sides.
> > >However if in a broadcast media (e.g. multiple
> routers
> > >in a backbone vlan), if we change 'ip ospf cost'
> on
> > >one router interface only, what will be the
> impact?
> > >I feel ospf is a link state protocol, and all
> routers
> >
> > Setting different costs, as opposed to different
> timer values, won't
> > break anything. I agree uniformity is better for
> troubleshooting, but
> > there very well may be reasons to have different
> costs.
> >
> > Now, for your more specific example, with a
> different interface cost
> > on one router connected to a multiaccess link,
> does the router have
> > additional links that it can use to reach the same
> destination? In
> > that case, having a higher cost on the
> less-preferred link may be
> > perfectly valid means of expressing an intra-area
> routing policy.
> >
> >
>
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