Ah shucks!
This is one of those "thanks to groupstudy" moments. There was a very long
discussion of the intricacies of managing and configuring cost in OSPF a
ways back. Priscilla and Howard and a bunch of folks chimed in. I of course
read everything, and apparently absorbed a lot of it. Learned about the two
byte "cost" field in the ospf packet. So you can't have a cost of less than
one because the field is an integer field with a defined value of 1-65xxx
Anyway, one of the daughter threads was what happens when you manually
adjust the cost to take into account DS3, for example?
Well, using the ospf auto-cost reference-bandwidth command, one can adjust
for high speed links. The entire reference copied from the 12.x command
reference follows. But the idea is that one may now configure the ospf
process to take into account bandwidths greater than the default 100K
ospf auto-cost reference-bandwidth
To control how OSPF calculates default metrics for the interface, use the
ospf auto-cost router configuration command. To assign cost based only on
the interface type, use the no form of this command.
ospf auto-cost reference-bandwidth ref-bw
no ospf auto-cost reference-bandwidth
Syntax Description
reference-bandwidth ref-bw Rate in megabits per second (bandwidth). The
range is 1 to 4294967; the default is 100.
Defaults
100 Mbits
Command Modes
Router configuration
Command History
Release Modification
11.2 This command was introduced.
Usage Guidelines
In Cisco IOS Release 10.3 and later, by default OSPF will calculate the OSPF
metric for an interface according to the bandwidth of the interface. For
example, a 64K link will get a metric of 1562, while a T1 link will have a
metric of 64.
The OSPF metric is calculated as ref-bw divided by bandwidth, with ref-bw
equal to 108 by default, and bandwidth determined by the bandwidth command.
The calculation gives FDDI a metric of 1.
If you have multiple links with high bandwidth (such as FDDI or ATM), you
might want to use a larger number to differentiate the cost on those links.
The value set by the ip ospf cost command overrides the cost resulting from
the ospf auto-cost command.
Examples
The following example changes the cost of the FDDI link to 10, while the
gigabit Ethernet link remains at a cost of 1. Thus, the link costs are
differentiated.
router ospf 1
ospf auto-cost reference-bandwidth 1000
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Cthulu, CCIE Candidate
Sent: Thursday, June 15, 2000 10:29 PM
To: [EMAIL PROTECTED]
Subject: re: OSPF correction
I may have to abdicate the throne:)
Chuck Larrieu privately pointed out an error in my post:
The correct formula for OSPF's calculating cost is 100 Mbps/bandwidth.
Thanks Chuck!! I am also glad that I did not "James Cameron" on this
one!!
Charles
-----Original Message-----
From: [EMAIL PROTECTED] ("M Z") [mailto:[EMAIL PROTECTED]]
Posted At: Friday, June 16, 2000 1:33 AM
Posted To: cisco
Conversation: OSPF !!!
Subject: re: OSPF !!!
You are the OSPF king, charles :)
Load balancing with ospf happens automatically, assuming the two
interfaces
have the same cost.
The fact that they are load balancing per packet or per destination
depends
on the switching mechanism on the router.
Last note, if you want a router not to be a DR , set the priority to
zero, I
believe the default is one, the decision if both routers have the same
priority will go the router with the highest IP (router ID) address,
this is
another good reason to use loopbacks with OSPF.
>From: "Cthulu, CCIE Candidate" <[EMAIL PROTECTED]>
>Reply-To: "Cthulu, CCIE Candidate" <[EMAIL PROTECTED]>
>To: [EMAIL PROTECTED]
>Subject: re: OSPF !!!
>Date: Thu, 15 Jun 2000 16:30:16 +0200
>
>Elias,
>
>I am going to display my awesome lack of OSPF knowledge in public, so
>bear with me:)
>
> The ip ospf cost and ip ospf priority are interface specific
commands:
>unless something drastic has happened at Cisco, you can not enter them
>under the ospf process configuration mode as implied here.
>
>Priority only affects DR elections: it has no impact on route
>selection.
>
>Cost is a trickier one, but generally speaking, it does affect route
>selection. If you have two routes to the same destination, you can set
>the cost of one higher than the other, and the lower cost one will be
>preferred (assuming it is up and operational, of course).
>
>OSPF calculates the cost of each interface using the formula of
>interface bandwidth/100Mbps; the resulting number is the cost of the
>interface by default.
>
>
>You could also indirectly set the interface cost by setting the
>bandwidth of the interface higher or lower using the the bandwdith
>command: OSPF will then calculate based on that.
>
>I am not going to get into load balancing between interfaces to the
same
>destination because, frankly, it's beyond the scope of my brain. I hope
>the previous info helps even a little bit.
>
>Flames to: [EMAIL PROTECTED]
>
>Charles
>
>
>
>
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
>Posted At: Wednesday, June 14, 2000 5:59 PM
>Posted To: cisco
>Conversation: OSPF !!!
>Subject: OSPF !!!
>
>
>Hello everyone!
>
>I have a bit of confusion with how to route selection occurs in OSPF by
>icluding the following commands Please shade some light on this
>
>Ok here comes
>
>does cost and priority affects route selection ??
>
>1. router ospf 3
> network 158.13.0.0 0.0.255.255 area 0
> default-information originate
> default-metric 1
> distance 85
> ip ospf cost 10
> ip ospf priority 4
>
>
>2 router ospf 3
> network 158.13.0.0 0.0.255.255 area 0
> default-information originate
> default-metric 1
> distance 85
> ip ospf cost 20
> ip ospf priority 5
>
>Thanks,
>Elias
>
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