From: William Nellis (nellis_iv@yahoo.com)
Date: Wed Jul 11 2007 - 14:03:11 ART
Yes, with the lack of presence of a forwarding address the router will determine best path based on cost to ASBR, in the case of forwarding address it will use best cost to the forwarding address.
Reason for this is obvious in a ring or full mesh topology when the cost doesnt change for E2 routes. I could have ECMP going East and West on a ring topology when the site is one hop to the west. It used to trip me out in a previous engagement until I grasped this concept... (How do the routers KNOW the best path if cost doesn't change?)
So, with E1 routes, it will increment cost at each hop and with E2 routes it will not, but will still choose optimal path TO the ASBR advertising the route...
And forwarding addresses, which only come into play in certain scenarios' (broadcast link, next hop in broadcast link and neighbor on that link), open a whole other can of worms.
-------------------------------------------------------
r/s
William Nellis IV
nellis_iv@yahoo.com
----- Original Message ----
From: Antonio Soares <amsoares@netcabo.pt>
To: Sergey <public@svlp.net>; Cisco certification <ccielab@groupstudy.com>
Sent: Wednesday, July 11, 2007 5:08:14 AM
Subject: RE: About E2 routes
The router will decide taking into account the forward-metric to each ASBR:
+++++++++++++++++++++++
R3#sh ip route 12.12.12.0
Routing entry for 12.12.12.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
781
Last update from 23.23.23.2 on Serial1/3, 00:00:13 ago
Routing Descriptor Blocks:
* 13.13.13.1, from 1.1.1.1, 00:00:13 ago, via Serial1/2
Route metric is 20, traffic share count is 1
23.23.23.2, from 2.2.2.2, 00:00:13 ago, via Serial1/3
Route metric is 20, traffic share count is 1
R3#
+++++++++++++++++++++++
R3#
R3#sh ip ospf int s1/2 | inc Cost
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 781
R3#
R3#
R3#sh ip ospf int s1/3 | inc Cost
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 781
R3#
R3#
+++++++++++++++++++++++
R3(config)#int s1/3
R3(config-if)#ip ospf cost ?
<1-65535> Cost
R3(config-if)#ip ospf cost 1
R3(config-if)#do sh ip route 12.12.12.0
Routing entry for 12.12.12.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
1
Last update from 23.23.23.2 on Serial1/3, 00:00:07 ago
Routing Descriptor Blocks:
* 23.23.23.2, from 2.2.2.2, 00:00:07 ago, via Serial1/3
Route metric is 20, traffic share count is 1
R3(config-if)#
R3(config-if)#
R3(config-if)#do sh ip ospf int s1/2 | inc Cost
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 781
R3(config-if)#
R3(config-if)#
R3(config-if)#do sh ip ospf int s1/3 | inc Cost
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 1
R3(config-if)#
R3(config-if)#
+++++++++++++++++++++++
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Sergey
Sent: quarta-feira, 11 de Julho de 2007 10:01
To: 'Cisco certification'
Subject: About E2 routes
I my lab I've got next topology
R1-----R4-----R2
R1 & R2 both announce E2 route 67.0.0.0/24, but R1 use PPP multilink to R4
and R2 use HDLC serial interface to R4
OSPF Network Design Solutions says about E2
"E2 routesE2 routes are the default external routes for OSPF. They do not
add the internal OSPF metrics; they use the remote AS only, regardless of
where they are in the AS. For example, if a packet is destined for another
AS, E2 routes add only the metrics from the destination AS associated with
reaching the destination."
So I expect to see in routing table both routes (one from R1, another from
R2), in real life I see only
67.0.0.0/24 is subnetted, 1 subnets
O E2 67.0.0.0 [110/20] via 192.168.100.1, 00:00:15, Multilink10
if I shutdown int Multilink10 another route appear in routing table with the
same metric
67.0.0.0/24 is subnetted, 1 subnets
O E2 67.0.0.0 [110/20] via 192.168.200.1, 00:00:15, Serial1/1
after some investigation I've found that ASBR has different cost
R4#sh ip ospf border-routers
i 192.168.200.1 [64] via 192.168.200.1, Serial1/1, ASBR, Area 10, SPF 48 i
192.168.10.100 [1] via 192.168.100.1, Multilink10, ASBR, Area 10, SPF 48
I've changed ospf cost on serial interface so
R4#sh ip ospf border-routers
i 192.168.200.1 [1] via 192.168.200.1, Serial1/1, ASBR, Area 10, SPF 52 i
192.168.10.100 [1] via 192.168.100.1, Multilink10, ASBR, Area 10, SPF 52
And only after I've changed ospf cost both routes appear in routing table
67.0.0.0/24 is subnetted, 1 subnets
O E2 67.0.0.0 [110/20] via 192.168.200.1, 00:00:15, Serial1/1
[110/20] via 192.168.100.1, 00:00:15, Multilink10
My question is, why cisco take in account cost to ASBR for E2 routes?
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