Li,

 In this particular case you don't need the BB routers to know
 which path exactly (RT1-RT2-RT4-N1 or RT1-RT3-RT5-N1) is the
 best, since they will forward traffic to RT1 and RT1 will make
 the decision based on it's own routing table.

 In a situation where there are two ABRs, only one has a VL,
 but the best path goes through the other, the second ABR would
 have to be configured with a VL for the BB routers to see
 the path.

--
Alex Zinin
Tuesday, May 15, 2001, 8:03:56 PM, Li Zhong Jie wrote:
> Alex,
> Sorry I have forgetton to refer to the figures discussed.
> figure6 refers to RFC2328:Figure 6
> N1 where you cann't understand refers to N1 in RFC2328:Figure 17.
> Would you explain my question then?
> Li
>> Li,
>>
>> See inline, pls.
>>
>>
>> > 4. Route calculation.
>>
>> >    4.1 Routers in all areas have seen new type-3 and type-4 LSAs,
>> >        so they redo inter-area and ASE route calculation
>>
>> >    4.2 When ABRs calculate inter-area routes above, they consider only
>> >        BB summaries. If any new route has been installed, this
>> >        triggers another round of summary-LSA origination (see 5)
>> >>here should add?
>> >>  4.3 when an ABR connects to a transit area,it should consider also
>> >>      transit area summaries. If any new route has been installed,this
>> >>      triggers another round of summary-LSA origination (see 5)
>>
>> yes, we may have to announce updated backbone routes to non-backbone
>> areas.
>>
>> >> my question occurs here:
>>
>> >> in 4.3, if the router's inter area routes are updated,should the resulting
>> >> routing table entry be advertised to BB area?
>>
>> No. 16.3 leaves updated routes associated with the BB and we don't
>> announce routes back to the areas they belong to.
>>
>> >>  If not,other routers in BB area
>> >> would not gain from the shortcut via transit area. e.g.in Figure 6,RT7 finds
>> >> its way to N9-N11 via RT7->N6->RT10-->N9-N11 with total cost 15,how RT5 knows
>> >> this shortcut to N9-N11 and selects RT7 instead of RT6 as next hop?
>>
>> You'd need to have a VL to RT7.
>>
>>
>> >> if the router's intra area routes are updated(suppose Network N1),how the resulting
>> >> routing table entry is known to BB area's other routers so that they can update
>> >> their route table entry describing N1?
>>
>> Couldn't decipher this... What is the ABR and where are the VLs here?
>>
>> Alex.
>>
>>
>>

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