From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Tue Jul 23 2002 - 18:57:52 GMT-3
At 2:13 PM -0700 7/23/02, Shuyi Li wrote:
>The BW is used to communicate the bandwidth value of an interface to
>the higher-level protocols, and being also used among protocols
>advertisements.
>
>In the OSPF, briefly speaking, it's not very dynamic or accurate.
>Since it's using the BW concept, which may not be the current
>available resource,
It doesn't have to use BW...interface cost could just as well be set
to delay or monetary cost.
And it's consciously designed NOT to be dynamic, to avoid
oscillation. Basing local metrics on dynamic link bandwidth also has
proven to be a bad idea, as in IGRP.
There have been proposals for multihop utilization metrics in ISIS
and OSPF (the Optimized Multipath -- OMP -- work by Curtis
Villamizor), but the general thinking in traffic engineering is to
decrement the interface bandwidth by reservations, route MPLS tunnels
with QoS through them, and then determine best effort route cost from
interface bandwidth minus reserved bandwidth.
>
>
>At 12:01 PM 7/21/2002 -0400, Peter van Oene wrote:
>>The OSPF metric is dimensionless. That is to say its value is not
>>designed to relate to any physical property. Most implementations,
>>including Cisco, will automatically derive a value based upon the
>>configured bandwidth of a link. However, this is the absolute
>>configured bandwidth of the link, not the current bandwidth
>>available. In other words, OSPF does not use a real time,
>>utilization based metric. Doing so would most likely yield a lot
>>of SPF churn as traffic moved around and produce a reasonably
>>unstable network.
>>
>>Pete
>>
>>
>>At 11:20 AM 7/17/2002 -0700, Shuyi Li wrote:
>>>Jason,
>>>
>>>Regarding the OSPF metric, I have a question for you that, the
>>>cost of links is considered to be the current available BW, or
>>>just the total BW even it's being occupied, say 50%. Please advise.
>>>
>>>thanks in advance.
>>>/shuyi
>>>
>>>
>>>At 05:22 PM 7/17/2002 +1000, Jason Sinclair wrote:
>>>>Tom,
>>>>
>>>>In OSPF the metric is the cost. What I mean here is best clarified as
>>>>follows:
>>>>
>>>>1. In RIP the metric as we know is hop count.
>>>>2. EIGRP/IGRP use a composite metric based on things such as bandwidth,
>>>>delay, etc
>>>>3. In OSPF the metric is based on the cost of links. The lower the cost the
>>>>more preferred the path
>>>>
>>>>Hope this makes sense.
>>>>
>>>>Regards,
>>>>
>>>>
>>>>-----Original Message-----
>>>>From: Tom Young [mailto:gitsyoung@yahoo.co.jp]
>>>>Sent: Wednesday, 17 July 2002 16:35
>>>>To: ccielab@groupstudy.com
>>>>Subject: OSPF 's cost and metric
>>>>
>>>>Hi, group.
>>>>
>>>> The OSPF's "cost" and "metric" parameters made me
>>>>confused. Who can clear it for me?
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