From: zzk (ccie_99@yahoo.com)
Date: Mon Nov 24 2003 - 09:04:57 GMT-3
Hi
I mean changing the cost of X interfaces on R1 or R2.
The R3 will learn the cost from them, which affect the
route selection.
regards
--- Brian McGahan <bmcgahan@internetworkexpert.com>
wrote:
> Cost of a directly connected link is determined by
> your local
> cost for that interface. Therefore in the previous
> example changing the
> cost on R1 or R2 would not affect R3.
>
>
> 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
> > zzk
> > Sent: Friday, November 21, 2003 11:29 PM
> > To: Brian McGahan; 'Howard C. Berkowitz';
> ccielab@groupstudy.com
> > Subject: RE: tune ospf cost in a broadcast media
> >
> > 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.
> > > >
> > > >
> > >
> >
>
This archive was generated by hypermail 2.1.4 : Fri Dec 12 2003 - 12:29:16 GMT-3