OSPF inter-area learned routes [7:22268]
Okay, I've been beating my head over this one for a week, and finally give
up:
I'm trying to understand the logic behind OSPF's and Cisco's routing
selection behavior in this instance:
I have 6 routers, segmented into 3 areas - areas 0, 1, and 2.
Routers A1 and A2, which sit in area 1, have fast ethernet interfaces on the
same network (we'll call this "network A"), have formed adjacencies with
eachother across this network, and seem to be exchanging routes properly.
Routers B1 and B2 are in areas 0, 1, and 2. Again, each router has a fast
ethernet interface on the same network ("network B"), which is in area 0.
B1 has a T3 link to A1, and this numbered link is in area 1. B2 has a T1
link to A2, and this numbered link is in area 1.
Routers C1 and C2 sit in area 2, and have fast ethernet interfaces on the
same network ("network C"). B1 has a T1 link to C1, and B2 has a T3 link to
C2.
Pretty much everything else is default, including link metrics, summary
metrics, etc. No summarization is taking place.
Now, I'm confused, because if I do "show ip route" on router A1 or A2 for
network C, the selected inter-area route for network C points across the T1
link to B2, as opposed to the lower-cost path across the T3 to B1 (then
across the FE link to B2, and across the T3 link to C2)! If I do a "show ip
ospf database summary" for network C, I do see that I've received summary
routes for network C from both B1 and B2, and that B1 has a lower metric,
but it seems that the A routers select the path across to B2 anyway.
I assume that this has something to do with the fact that the core routers
are both backbone routers AND ABRs for the other areas, and that LSAs for
area 1 don't pass across the fast ethernet interface (instead, I expect to
see summary-LSAs for the area 1 passing across area 0, then being
re-summarized as available paths back to area 1), but I suspect that there's
just something fundamental about OSPF that I don't understand. Can anybody
explain to me why the better-metric path isn't being selected in this case?
Thanks,
Ian Schorr
CCNP
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Re: [OSPF inter-area learned routes [7:22268]
Are you sure that the B1 Network Summary LSA had a lower cost then the B2
Summary LSA? According to how I understood your topology, B1 should
advertise
a cost of 65 to reach network C and B2 should advertise a cost of 2. So they
should both use the T1 link to B2 to exit the area. The reason for this is
that because routers B1 and B2 both have intra-area paths to reach network C
they will always select them, even if a lower cost inter-area route through
the backbone exists. For this reason, B1 will select the higher cost Area 2
intra-area link to C1 over the T1 rather then use the lower cost inter-area
link through the Fast-Ethernet interface to B2 and then on to C2 via the T3.
"Ian Schorr" wrote:
> Okay, I've been beating my head over this one for a week, and finally give
> up:
>
> I'm trying to understand the logic behind OSPF's and Cisco's routing
> selection behavior in this instance:
>
> I have 6 routers, segmented into 3 areas - areas 0, 1, and 2.
>
> Routers A1 and A2, which sit in area 1, have fast ethernet interfaces on
the
> same network (we'll call this "network A"), have formed adjacencies with
> eachother across this network, and seem to be exchanging routes properly.
>
> Routers B1 and B2 are in areas 0, 1, and 2. Again, each router has a fast
> ethernet interface on the same network ("network B"), which is in area 0.
> B1 has a T3 link to A1, and this numbered link is in area 1. B2 has a T1
> link to A2, and this numbered link is in area 1.
>
> Routers C1 and C2 sit in area 2, and have fast ethernet interfaces on the
> same network ("network C"). B1 has a T1 link to C1, and B2 has a T3 link
to
> C2.
>
> Pretty much everything else is default, including link metrics, summary
> metrics, etc. No summarization is taking place.
>
> Now, I'm confused, because if I do "show ip route" on router A1 or A2 for
> network C, the selected inter-area route for network C points across the T1
> link to B2, as opposed to the lower-cost path across the T3 to B1 (then
> across the FE link to B2, and across the T3 link to C2)! If I do a "show
ip
> ospf database summary" for network C, I do see that I've received summary
> routes for network C from both B1 and B2, and that B1 has a lower metric,
> but it seems that the A routers select the path across to B2 anyway.
>
> I assume that this has something to do with the fact that the core routers
> are both backbone routers AND ABRs for the other areas, and that LSAs for
> area 1 don't pass across the fast ethernet interface (instead, I expect to
> see summary-LSAs for the area 1 passing across area 0, then being
> re-summarized as available paths back to area 1), but I suspect that
there's
> just something fundamental about OSPF that I don't understand. Can anybody
> explain to me why the better-metric path isn't being selected in this case?
>
> Thanks,
> Ian Schorr
> CCNP
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Re: OSPF inter-area learned routes [7:22268]
OSPF (Open Shortest Path First) calculates the shortest path to area 0
first, then the shortest path to the next area boundary via intra-area
routes. The best illustration of this was in the Networkers 2000 CCIE Power
Session presentation
http://www.ieng.com/networkers/nw00/pres/3304/3304_c1_sec2.pdf
p 16
hth...
-e-
- Original Message -
From: "Ian Schorr"
To:
Sent: Friday, October 05, 2001 1:49 PM
Subject: OSPF inter-area learned routes [7:22268]
> Okay, I've been beating my head over this one for a week, and finally give
> up:
>
> I'm trying to understand the logic behind OSPF's and Cisco's routing
> selection behavior in this instance:
>
> I have 6 routers, segmented into 3 areas - areas 0, 1, and 2.
>
> Routers A1 and A2, which sit in area 1, have fast ethernet interfaces on
the
> same network (we'll call this "network A"), have formed adjacencies with
> eachother across this network, and seem to be exchanging routes properly.
>
> Routers B1 and B2 are in areas 0, 1, and 2. Again, each router has a fast
> ethernet interface on the same network ("network B"), which is in area 0.
> B1 has a T3 link to A1, and this numbered link is in area 1. B2 has a T1
> link to A2, and this numbered link is in area 1.
>
> Routers C1 and C2 sit in area 2, and have fast ethernet interfaces on the
> same network ("network C"). B1 has a T1 link to C1, and B2 has a T3 link
to
> C2.
>
> Pretty much everything else is default, including link metrics, summary
> metrics, etc. No summarization is taking place.
>
> Now, I'm confused, because if I do "show ip route" on router A1 or A2 for
> network C, the selected inter-area route for network C points across the
T1
> link to B2, as opposed to the lower-cost path across the T3 to B1 (then
> across the FE link to B2, and across the T3 link to C2)! If I do a "show
ip
> ospf database summary" for network C, I do see that I've received summary
> routes for network C from both B1 and B2, and that B1 has a lower metric,
> but it seems that the A routers select the path across to B2 anyway.
>
> I assume that this has something to do with the fact that the core routers
> are both backbone routers AND ABRs for the other areas, and that LSAs for
> area 1 don't pass across the fast ethernet interface (instead, I expect to
> see summary-LSAs for the area 1 passing across area 0, then being
> re-summarized as available paths back to area 1), but I suspect that
there's
> just something fundamental about OSPF that I don't understand. Can
anybody
> explain to me why the better-metric path isn't being selected in this
case?
>
> Thanks,
> Ian Schorr
> CCNP
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Re: OSPF inter-area learned routes [7:22268]
I sketched your network and it made me wonder if the path you think is
"shortest" really is shortest.
To get to the C routers in Area 2, the Area 1 A routers have four possible
paths:
1. A1---T3---B1---B2---T3---C2
2. A1---T3---B1---T1---C1
3. A2---T1---B2---T3---C2
4. A2---T1---B2---B1---T1---C1
Which ones looks shortest?
I think you were concerned that the A routers were using the T1 link
instead of the T3 link, but that's OK as long as they go out the T3 from
B2, which I think you said that they do. In other words, if they use route
3 instead of route 4 in my list above, there's no problem??
Priscilla
At 04:49 PM 10/5/01, Ian Schorr wrote:
>Okay, I've been beating my head over this one for a week, and finally give
>up:
>
>I'm trying to understand the logic behind OSPF's and Cisco's routing
>selection behavior in this instance:
>
>I have 6 routers, segmented into 3 areas - areas 0, 1, and 2.
>
>Routers A1 and A2, which sit in area 1, have fast ethernet interfaces on the
>same network (we'll call this "network A"), have formed adjacencies with
>eachother across this network, and seem to be exchanging routes properly.
>
>Routers B1 and B2 are in areas 0, 1, and 2. Again, each router has a fast
>ethernet interface on the same network ("network B"), which is in area 0.
>B1 has a T3 link to A1, and this numbered link is in area 1. B2 has a T1
>link to A2, and this numbered link is in area 1.
>
>Routers C1 and C2 sit in area 2, and have fast ethernet interfaces on the
>same network ("network C"). B1 has a T1 link to C1, and B2 has a T3 link to
>C2.
>
>Pretty much everything else is default, including link metrics, summary
>metrics, etc. No summarization is taking place.
>
>Now, I'm confused, because if I do "show ip route" on router A1 or A2 for
>network C, the selected inter-area route for network C points across the T1
>link to B2, as opposed to the lower-cost path across the T3 to B1 (then
>across the FE link to B2, and across the T3 link to C2)! If I do a "show ip
>ospf database summary" for network C, I do see that I've received summary
>routes for network C from both B1 and B2, and that B1 has a lower metric,
>but it seems that the A routers select the path across to B2 anyway.
>
>I assume that this has something to do with the fact that the core routers
>are both backbone routers AND ABRs for the other areas, and that LSAs for
>area 1 don't pass across the fast ethernet interface (instead, I expect to
>see summary-LSAs for the area 1 passing across area 0, then being
>re-summarized as available paths back to area 1), but I suspect that there's
>just something fundamental about OSPF that I don't understand. Can anybody
>explain to me why the better-metric path isn't being selected in this case?
>
>Thanks,
>Ian Schorr
>CCNP
Priscilla Oppenheimer
http://www.priscilla.com
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Re: [Re: [OSPF inter-area learned routes [7:22268]
Sorry, I meant to say that I would have expected B2's cost to be 3 not 2. Curtis Call wrote: > Are you sure that the B1 Network Summary LSA had a lower cost then the B2 > Summary LSA? According to how I understood your topology, B1 should advertise > a cost of 65 to reach network C and B2 should advertise a cost of 2. So they > should both use the T1 link to B2 to exit the area. The reason for this is > that because routers B1 and B2 both have intra-area paths to reach network C > they will always select them, even if a lower cost inter-area route through > the backbone exists. For this reason, B1 will select the higher cost Area 2 > intra-area link to C1 over the T1 rather then use the lower cost inter-area > link through the Fast-Ethernet interface to B2 and then on to C2 via the T3. > -- [EMAIL PROTECTED] OSPF Practice Exam www.boson.com\tests\Advanced.htm Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=22278&t=22268 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
