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Hi Group,
ASBR 1 | Area 1 | Area 0 | R ------------------------ ASBR 2 Area 3
Suppose that ASBR 1 and ASBR 2 are equi-distant from R.
Suppose that ASBR 1 and ASBR 2 advertise route to a destination N with same cost & type (say type 1).
As per RFC 2328, R chooses the intra-area path through Area 3 to ASBR2. (ASBR path preference rules – RFC 2328, Sec 16.4.1)
My questions:
1. If Area 3 is NSSA, should the ASBR path preference rules be applied while comparing type-5 and type-7 route paths?
2. RFC 1587 says “When a type-5 LSA and a type-7 LSA are found to have the same type and an equal distance, tie breaking rules are used”. Is the ASBR path preference check implicit in the above statement?
RFC 1587, section 3.5, step 5:
“Otherwise, compare the cost of this new AS external path to the ones present in the table. Note that type-5 and type-7 routes are directly comparable. Type-1 external paths are always shorter than Type-2 external paths. Type-1 external paths are compared by looking at the sum of the distance to the forwarding address/ASBR and the advertised Type-1 paths (X+Y). Type-2 external paths are compared by looking at the advertised Type-2 metrics, and then if necessary, the distance to the forwarding address/ASBR. 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. If the new path is shorter, it replaces the present paths in the routing table entry. If the new path is the same cost, it is added to the routing table entry's list of paths.”
Thanks, Anup |