RE: tune ospf cost in a broadcast media posted 11/23/2003
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.
Brian McGahan, CCIE #8593
Internetwork Expert, Inc.
Toll Free: 877-224-8987
Direct: 708-362-1418 (Outside the US and Canada)
> -----Original Message-----
> From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf
> Sent: Friday, November 21, 2003 11:29 PM
> To: Brian McGahan; 'Howard C. Berkowitz'; ccielab@xxxxxxxxxxxxxx
> 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...
> --- Brian McGahan <bmcgahan@xxxxxxxxxxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxxxxxxxxx
> > 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@xxxxxxxxxxxxxx
> > [mailto:nobody@xxxxxxxxxxxxxx] On Behalf
> > Of
> > > Howard C. Berkowitz
> > > Sent: Friday, November 21, 2003 10:33 AM
> > > To: ccielab@xxxxxxxxxxxxxx
> > > 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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