From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Mon Oct 17 2005 - 19:55:19 GMT-3
I think I may be missing something here but ...
If an OSPF router advertises it's own reachability (LSA-1) as maxed ...
max-metric ... then what other OSPF router will prefer this router?
As each router constructs a SPF tree, each router sees the network from
their own perspective (they are the base of the tree) and this router
will not look appealing, especially the closer you get to it.
When this router's networks are advertised across the OSPF network then
just about any other path will be good. The closer the routers are to
this router, the worse it looks, again this is because SPF is built on
local perspective.
Now if this is the only way to reach a destination ... then maxing a
metric will not help.
Please correct me if I am not in tune with what you are trying to do /
achieve. Kindest regards
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Paresh Khatri
Sent: Monday, October 17, 2005 6:39 PM
To: kevin gannon; Ccie Lab (E-mail)
Subject: RE: ospf max-mteric router
Hi Kevin,
According to RFC2328, the metric value in type-1 (router-LSAs) LSAs is
16-bits, while the metric value in type-3, type-4 and type-5 LSAs is
24bits. Therefore, the maximum metric value for type-1 LSAs is 65535
while that for the other LSA types is 16777215.
The metric that you specify with the 'ip ospf cost' command is that for
a router link, which appears in the router-LSAs and therefore it is a
maximum of 65535. The metric indicated in your output is for an
inter-area route, which can be up to 24 bits.
There is a good reason why the metric for the inter-area routes is 24
bits - when you add up 16-bit metric values, it is quite possible that
you will get a value greater than 16 bits. Since the metric for
inter-area routes is a summation of the metrics of individual links that
make up the path, we need a metric greater than 16-bits. The OSPF
developers chose to use 24-bits.
Hope that helps,
Paresh Khatri
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
kevin gannon
Sent: Tuesday, 18 October 2005 03:16 AM
To: Ccie Lab (E-mail)
Subject: ospf max-mteric router
Just looking at this command again and you would think that it sets the
maximum possible metric for the routes. It states it sets them to 65535.
This is not the max metric as you can see from below:
SW2(config-if)#do sh ip rou 2.2.2.2
Routing entry for 2.2.2.2/32
Known via "ospf 1", distance 110, metric 131071, type inter area
Last update from 14.14.14.1 on FastEthernet0/14, 00:00:02 ago
Routing Descriptor Blocks:
* 14.14.14.1, from 123.123.123.2, 00:00:02 ago, via FastEthernet0/14
Route metric is 131071, traffic share count is 1
SW2(config-if)#
The metric is 131071. Any ideas why this might be the case, I know its
highly unlikey to get a cost higher that
65535 in real life. Its just someone in Cisco thought up this value
maybe its a safety mechanism just in case the router might be able to
route packets ?
Regards
Kevin
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