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Why IETF changed and inverted OSPF Type-7 LSA VS Type-5 LSA selection

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meddane
Frequent Contributor

Why IETF changed and inverted OSPF Type-7 LSA VS Type-5 LSA election In RFC 3101 compared to OLD RFC 1587?

Many people learns that the Type-7 LSA and Type-5 election (ON Versus OE routes) depends on RFC 3101 for NSSA published in 2003 and RFC 1587 for NSSA published in 1994?

Per RFC 1587 for NSSA says that if a router receive a Type-7 LSA and Type-5 for the same destination the cost to the forwarding address or ASBR are equal, the Type-5 LSA is always preferred.

RFC 1587, section 3.5 Calculating Type-7 AS External Routes:

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.

Per RFC 3101, the order is inverted, in others words, if a router receive a Type-7 LSA and Type-5 for the same destination the cost to the forwarding address or ASBR are equal, the Type-7 LSA is always preferred.

RFC 3101, Section: 2.5 Calculating Type-7 AS External Routes says:

If the current LSA is functionally the same as an

installed LSA (i.e., same destination, cost and non-zero

forwarding address) then apply the following priorities in

deciding which LSA is preferred:

1. A Type-7 LSA with the P-bit set.

2. A Type-5 LSA.

3. The LSA with the higher router ID.

As you notice RFC 3101 inverted the LSA election. While RFC 1587 prefers the Type-5 LSA, RFC 3101 elects the Type-7 LSA.

Many people asks the question: Why IETF changed and inverted the election by publishing RFC 3101 in 2003 to replace RFC 1587 published in 1994?

Let's demystify it.RFC 2328 for OSPFv2 published in 1998, indicates the following rules which paths are preferred when multiple intra-AS paths are available to ASBRs or forwarding addresses:

1-Intra-area paths using nonbackbone areas are always the most preferred.
2-The other paths, intra area backbone paths and interarea paths, are of equal preference.

This means that an external route with an ASBR reachable through a non-backbone area such as an NSSA area is always preferred, in other words a Type-7 LSA learned through a non-backbone area is always preferred than a Type-5 LSA learned from a backbone area (intra area route) or another area (inter-area route) regardless the cost, this rule makes the RFC 1587 published earlier in 1994 obsolete, where in a special case, when a router receives both Type-7 LSA and Type-5 LSA for the same destination with equal cost to the Forward Address and/OR ASBR, the Type-5 LSA is always preferred.

This why when you enable RFC 1587 you should ensure that RFC 1583 is enable, because unlike with RFC 2328, the selection of external route is based solely on the cost.

To avoid the compatiblity issues between RFC 1587 and RFC 2328, RFC 3101 has been published in 2003 to adjust and to invert the Type-7/Type-5 preference "when a router receives both Type-7 LSA and Type-5 LSA for the same destination with equal cost to the Forward Address and/OR ASBR, the Type-5 LSA is always preferred" and to be compatible with both RFC 1583 and RFC 2328, which avoids undesirable behaviors regarding Path Selection decision caused by RFC compatibility between RFC 1587 and 2328.

And This is why on Cisco Routers RFC 3101 and RFC 1583 is enabled by default. So that when you disable RFC 1583 (which means you enable RFC 2328), there is no compatibility issues between RFC 3101 and RFC 2328.

OSPF with RFC is a BIG Topic to understand OSPF behavior per RFC.