From: Gary Duncanson (gary.duncanson@googlemail.com)
Date: Wed Jul 11 2007 - 15:24:38 ART
William,
In a simple way then in this situation is metric-type 1 E1 the way to go to
make sure you get to the ASBR the 'best' way. I recall reading that E1
increments cost at each hop(or is that link? for hops I read 'RIP'). Sounds
pretty useful if you are (in terms of links) close to the ASBR but
nevertheless learning several paths to get to it from different routers
within a large area.
Regards
Gary
----- Original Message -----
From: "William Nellis" <nellis_iv@yahoo.com>
To: "Antonio Soares" <amsoares@netcabo.pt>
Cc: <ccielab@groupstudy.com>
Sent: Wednesday, July 11, 2007 6:03 PM
Subject: Re: About E2 routes
> 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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