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

[Akhmedd Aly]

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