Re: OSPF external type-2 route load balance

[Gary Duncanson <gary.duncanson@xxxxxxxxxxxxxx>]

Thanks for that Scott.

Looks like this demonstrates this rather well, at least how I read it.

> From OSPF Design Guide on CCO.

sic 'External routes fall under two categories, external type 1 and external 
type 2. The difference between the two is in the way the cost (metric) of 
the route is being calculated. The cost of a type 2 route is always the 
external cost, irrespective of the interior cost to reach that route.'

Rgds
Gary
----- Original Message ----- 
From: "Scott Smith" <hioctane@xxxxxxxxx>
To: "Sadiq Yakasai" <sadiqtanko@xxxxxxxxx>
Cc: "Joseph Brunner" <joe@xxxxxxxxxxxxxxxxxxx>; "Herbert Maosa" 
<asawilunda@xxxxxxxxxxxxxx>; "ISolveSystems" <support@xxxxxxxxxxxxxxxxx>; 
"Cisco certification" <ccielab@xxxxxxxxxxxxxx>
Sent: Friday, August 31, 2007 7:49 PM
Subject: Re: OSPF external type-2 route load balance


> Bingo!
>
> A1 learns 100.100.100.0 from both 10.43.1.61 & 10.43.1.73.
>
> Before - BW 1536 to both ASBRs
>
> A1(config-router)#do sh ip route ospf
>     100.0.0.0/24 is subnetted, 1 subnets
> O E2    100.100.100.0 [110/20] via 10.43.1.61, 00:00:36, Serial1/0.1
>                                 [110/20] via 10.43.1.73, 00:00:36, 
> Serial1/0.2
>
>
> After - Dropped BW to 512 on s1/0.1
>
> A1(config-subif)#do sh ip route ospf
>     100.0.0.0/24 is subnetted, 1 subnets
> O E2    100.100.100.0 [110/20] via 10.43.1.73, 00:00:06, Serial1/0.2
>
>
> Then changed BW back to 1536 on s1/0.1 (now both are equal again)
>
> A1(config-subif)#do sh ip route ospf
>     100.0.0.0/24 is subnetted, 1 subnets
> O E2    100.100.100.0 [110/20] via 10.43.1.73, 00:00:02, Serial1/0.2
>                                 [110/20] via 10.43.1.61, 00:00:02, 
> Serial1/0.1
>
> The metric is a constant 20, however, for the externals OSPF also uses
> the cost to the ASBR.
>
> --
> Scott
> CCIE #17040 (R&S)
>
>
> On 8/31/07, Sadiq Yakasai <sadiqtanko@xxxxxxxxx> wrote:
> > Can you show the Type 4 LSA database as well?
> >
> > I think the cost to the ASBR has a role to play in selecting 2 Type 5
> > LSA's reporting the same prefix.
> >
> > I am not quite sure about this though.
> >
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