Alex,

I am too late in replying to this letter.Have you forgetton it?
comments are made inside your text marking with >.

----- Original Message -----
From: "Alex Zinin" <azinincisco.com>
To: "Li Zhong Jie" <lzjcs.tsinghua.edu.cn>
Cc: <OSPFDISCUSS.MICROSOFT.COM>
Sent: Sunday, September 17, 2000 1:21 PM
Subject: Re: route table calculation
Li:

I will not comment on your sequence.
Here's what in fact happens.

1. Initial condition:

   o router-LSAs and network-LSAs have been flooded throughout
     associated areas.
     
   o ASE-LSAs have been flooded throughout the domain.

2. Route calculation:

   2.1 All routers calculate intra-area SPF for all attached
       areas that results in installation of routes (both network
       and router) in the routing tables, which triggers summary-LSA
       origination by ABRs (see 3).
       
   2.2 Inter-area route calculation. Since we have no summary-LSAs
       yet, it does not give us anything.

   2.3 ASE route calculation. Only routers in the same area as
       the ASBRs succeed in this process, since routers in other
       areas do not have type-4 summary-LSAs to see where the ASBRs
       are

3. ABR duty:

   3.1 All ABRs originate summary-LSAs based on the routes installed
       in 2.1, and flood them into attached areas. This causes
       route calculation (see4)


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)

> 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? 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?

> 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?
5. ABR duty

   5.1 Based on backbone inter-area routes, ABRs originate type-3 and
       type-4 summary-LSAs and flood them into attached non-bb areas.
       This triggers another round of route calculation in routers
       internal to those areas (see 6).

       
6. Route calculation

   6.1 Routers in non-bb areas calculate inter-area and ASE routes
       based on information distributed in 5.1
       

Hope this helps.
       
--
Alex Zinin

********************************************************************
* Zhongjie Li *
* Ph.D candidate, Department of Computer Science & Technology *
* Tsinghua University, Beijing, 100084, P.R.China *
* Tel: +86+10-62788109 Fax: +86+10-62788109 *
* Email: lzjcsnet1.cs.tsinghua.edu.cn *
********************************************************************

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