From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Sat Apr 20 2002 - 11:35:39 GMT-3
At 6:52 AM -0700 4/20/02, Brian Dennis wrote:
>OSPF will not send traffic from area 0 across area 1 to get to a
>network in area 0 no matter how much better the cost is. An "O"
>route will always be preferred over a "IA" route. You need to look
>into using a virtual link or maybe a GRE tunnel between R2 and R3.
>
>Brian Dennis, CCIE #2210 (R&S/ISP Dial)
Brian is absolutely correct, but I thought I might add why OSPF has
this rule, which I've never seen in a Cisco course, and I'm not even
sure is explicit in the standard. It is discussed in John Moy's book,
"OSPF: Anatomy of an Internet Routing Protocol," and one of those
"trade secrets" that OSPF developers know but rarely bother to tell
OSPF users. Understanding the rule may help with some similar things
in BGP and ISIS, and even with redistribution in general.
The reason that OSPF always prefers intra-area to inter-area,
inter-area to external*, is preventing loops from something similar
to incorrect redistribution. Think of it as a matter of trust.
All the routers in an area have the same database and KNOW that a
route is loop-free. Since inter-area routes started out in an area
that followed this rule and passed in a controlled way through the
backbone, they are almost as trusted...but less so, because some
weird backbone error conceivably could have caused a route generated
in the local area to go into the backbone and get readvertised back
into the local area. Externals can't really be trusted at all.
After all, they could have resulted from the local system
redistributing into another routing protocol at ASBR1, and the local
system relearning them at ASBR2.
The same sort of thinking goes into the strict information hiding
from the ISIS backbone into level 1 areas, and the restrictions about
not readvertising a route learned from iBGP.
* External 1 vs. external 2 are differentiated for reasons of routing
policy, not loop prevention.
>
>---------- Original Message ----------------------------------
>From: dwhitley@dynis.com
>Reply-To: dwhitley@dynis.com
>Date: Fri, 19 Apr 2002 09:07:07 -0400
>
>>Set the bandwidth on the serial links to reflect the real speeds, ospf will
>>then choose the correct lowest cost link.
>>
>>
>>-----Original Message-----
>>From: Joe Jia [mailto:ellenjjl@rogers.com]
>>Sent: Thursday, April 18, 2002 10:56 PM
>>To: ccielab@groupstudy.com
>>Subject: How to change route from O to IA in OSPF?
>>
>>
>>Hi,
>>how to change the path at R2 to R1's ethernet0 from R2's serial0 to its
>>ethernet 0?
>>I have the case as following: R1,R2,R3 hub-spoke topology. R1 has ethernet0
>>and searial 0 at area 0
>>R2 has 64k serial0 and 10M ethernet interfaces, s0 in area0, ethernet0 in
>>area1
>>R3 has T1 serial0 and 10M ethernet interfaces, so in area 0 , ethernet 0
>>with
>>R2's e0 in area 1
>>
>>I have the question, how can change R2's route to R1's ethernet0 from R2's
>>serial0 to Ethernet0 because this way has a low cost?
>>
>>O , IA, E1, E2?
>>
>> 128.10.1.0/24
>> ______(e0)
>> |
>> (area0)
>> R1
>> |
>> (s0) 172.16.1.5/24
>> (area0) / \ (area0)
>> 64k / \ T1
>> / \
>> (s0) (s0)
>> 172.16.1.6/24 | | 172.16.1.1/24
>> R2 R3
>> | (e0) | (e0)
> > (area1)|________| (area1)
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