From: Scott Smith (hioctane@gmail.com)
Date: Fri Aug 31 2007 - 18:23:32 ART
We're specifically talking about load balancing type2 externals here.
Type 1 Example
Before - equal BW on both paths
A1(config-subif)#do sh ip route ospf
100.0.0.0/24 is subnetted, 1 subnets
O E1 100.100.100.0 [110/85] via 10.43.1.73, 00:00:02, Serial1/0.2
[110/85] via 10.43.1.61, 00:00:02, Serial1/0.1
**************************************
After - changed BW on s1/0.1 to 512
A1(config-subif)#do sh ip route ospf
100.0.0.0/24 is subnetted, 1 subnets
O E1 100.100.100.0 [110/85] via 10.43.1.73, 00:00:01, Serial1/0.2
**************************************
Here I've shutdown s1/0.2 so we can see the route from 10.43.1.61 is
there but the metric is higher because I changed the BW to 512 and the
type1 metric includes the cost along the path.
A1(config-subif)#do sh ip route ospf
100.0.0.0/24 is subnetted, 1 subnets
O E1 100.100.100.0 [110/215] via 10.43.1.61, 00:00:15, Serial1/0.1
**************************************
Notice with the type1 the metric changes so it is the deciding factor.
We know the cost to the ASBR is not equal because if it were the
metric would be equal.
-- Scott CCIE #17040 (R&S)On 8/31/07, Herbert Maosa <asawilunda@googlemail.com> wrote: > Thrown a little offset here. I thought the whole point of type-2 external > route was that the cost to ASBR was irrelevant ? If type-2 is looking at the > cost to the ASBR, then what is the difference with type-1 ? > > > > On 8/31/07, Scott Smith <hioctane@gmail.com> wrote: > > > > While that is true that type1 metric changes along the path and the > > type2 is constant this isn't the point of the show output. > > > > What I've pointed out is there is another metric in play with > > externals aside from the metric of the route. The router also > > considers the cost to the ASBR when deciding if externals are truly > > equal. For two externals to be considered equal everything must be > > equal. Both the metric of the route as seen via "sh ip route" and the > > metric to the ASBR seen via "sh ip ospf border-routers" must be equal. > > > > -- > > Scott > > CCIE #17040 (R&S) > > > > > > On 8/31/07, Gary Duncanson <gary.duncanson@googlemail.com > wrote: > > > 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@gmail.com> > > > To: "Sadiq Yakasai" < sadiqtanko@gmail.com> > > > Cc: "Joseph Brunner" <joe@affirmedsystems.com>; "Herbert Maosa" > > > < asawilunda@googlemail.com>; "ISolveSystems" > <support@isolvesystems.com>; > > > "Cisco certification" < ccielab@groupstudy.com> > > > 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@gmail.com> 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. > > > >> > > > >> > _______________________________________________________________________ > > > >> Subscription information may be found at: > > > >> http://www.groupstudy.com/list/CCIELab.html > > > > > > > > > _______________________________________________________________________ > > > > Subscription information may be found at: > > > > http://www.groupstudy.com/list/CCIELab.html > > > > > _______________________________________________________________________ > > Subscription information may be found at: > > http://www.groupstudy.com/list/CCIELab.html > > > > > > -- > Kindest regards, > hm
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