[j-nsp] ISIS - Shortest Path First Algorithm [JNCIS-M Guide page 192]

[Akhmedd Aly]

P.S.

is it mistake about IS-IS default policy - that IS-IS rejected by default
learned routes from direct protocol?
Check this link
http://img843.imageshack.us/img843/3264/isisdefaultpolicy.png

I think its must be accepted, and also in the next term we have the same
reject action...

--
AA


2009/12/4 Akhmedd Aly <akhmed.aly at gmail.com>

> Hi,
>
> can someone explain this difference between this 1-12 steps order in case
> of internal/external metric
>
> Each IS-IS router translates the information in the database into usable
> routes by implementing the
> Shortest Path First (SPF) algorithm. This computation is performed
> separately within each IS-IS
> level, and the results are compiled together and presented to the routing
> table on the router. The
> algorithm locates the metrically shortest path to each unique destination
> in the network. On occasion,
> the result of the calculation encounters multiple paths to the same
> destination learned through
> different means. To decide which path to use, the protocol has some
> tie-breaking rules to follow. The
> order of precedence for using a route is:
>  1. Level 1 intra-area routes with an internal metric
>  2. Level 1 external routes with an internal metric
>  3. Level 2 intra-area routes with an internal metric
>  4. Level 2 external routes with an internal metric
>  5. Inter-area routes (Level 1 to Level 2) with an internal metric
>  6. Inter-area external routes (Level 1 to Level 2) with an internal metric
>  7. Inter-area routes (Level 2 to Level 1) with an internal metric
>  8. Inter-area external routes (Level 2 to Level 1) with an internal metric
>  9. Level 1 external routes with an external metric
> 10. Level 2 external routes with an external metric
> 11. Inter-area external routes (Level 1 to Level 2) with an external metric
> 12. Inter-area external routes (Level 2 to Level 1) with an external metric
>
>
> Is it just meaning that ISIS protocol has two different preferences for
> each level:
>  IS-IS Level 1 Internal Intermediate System to Intermediate System Level 1
> internal routes 15
>  IS-IS Level 2 Internal Intermediate System to Intermediate System Level 2
> internal routes 18
>  IS-IS Level 1 External Intermediate System to Intermediate System Level 1
> external routes 160
>  IS-IS Level 2 External Intermediate System to Intermediate System Level 2
> external routes 165
>
>
> TIA!
>
> --
> AA
>