From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Fri Apr 18 2008 - 03:34:20 ART
Good explanation Tom!
RFC1587 says the following:
When a type-5 LSA and a type-7 LSA are found to have the
same type and an equal distance, the following priorities
apply (listed from highest to lowest) for breaking the tie.
a. Any type 5 LSA.
b. A type-7 LSA with the P-bit set and the forwarding
address non-zero.
c. Any other type-7 LSA.
Paul.
Thomas Fowles wrote:
> Suryakant,
>
> In Scenario 1, both the N2 and E2 routes presumably have the same
> characteristics (forwarding address of router C, cost, etc). All things
> being equal, an E2 is preferred over an N2 route. So router B installs the
> E2 advertised by router A into area 0.
>
> In Scenario 2, when router A strips out the forwarding address, router A's
> area 0 interface becomes the next hop for the E2 route. At this point
> (judging from your sesults) the N2 route with router C as the next hop
> becomes more attractive than router A's area 0 interface as next hop.
>
> Here are a couple of documentation links that might clarify things a bit,
> although admitedly this can be a tough one to wrap your head around.
>
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml
> http://www.cisco.com/en/US/docs/ios/12_4/ip_route/configuration/guide/hoadsup.html#wp1037600
>
> HTH
>
> -Tom
> CCIE#18762(R&S, SP in progress, humbled daily)
>
>
>
> On Thu, Apr 17, 2008 at 8:32 AM, Suryakant P <suryakant.pandian@gmail.com>
> wrote:
>
>
>> Hi All,
>>
>> Routers A ,B and C are connected to the ethernet LAN network.OSPF peering
>> is
>> established between them and the area is configured as the NSSA area.
>>
>> Router C acts as the ASBR and Routers A and B have connectivity
>> to Area 0 and hence act as ABRs.
>>
>> I have made the router A as the type 7 to type 5 converter(by increasing
>> the
>> router-id on the A).
>>
>> Now when C advertises a network(by redistributing into OSPF),both router A
>> and B receive the TYPE 7 LSA.
>>
>> A and B will have the TYPE 5 LSA generated by the router A.
>>
>> *Scenario 1 :* When the forwardaddress in TYPE 5 LSA is set to Router C
>> router id.
>> Router A installs N2 route in the routing table but router B installs E2
>> route in the routing table?Could you please explain the reason behind the
>> selection of E2 route on the router B.
>>
>> *Scenario 2 :* When the forwardaddress in TYPE 5 LSA is set to
>> 0.0.0.0(suppress-fa configured on the converter)
>> Both router A and router B install N2 route to the network
>> advertised.Could
>> you please explain the reason behind the change in route selection once
>> the
>> forward has been suppressed.
>>
>> Thanks
>> With regards
>> Suryakant
>>
>>
>> Pass the CCIE in six weeks, Guaranteed!
>> http://www.certscience.com/CCIE
>> _______________________________________________________________________
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>>
>
>
> Pass the CCIE in six weeks, Guaranteed!
> http://www.certscience.com/CCIE
> _______________________________________________________________________
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