RFC 1583 compatibility

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Question: Does RFC 1583 compatibility have impact on route selection? Is RFC 1583 compatibility enabled by default?

Answer: The rfc1583 compatible command is used to enable the routing selection rule compatible with RFC 1583. After the undo rfc1583 compatible command is executed:
If two routes, one in the backbone area and the other in a non-backbone area, direct to the ASBR, the route in the non-backbone area is preferentially selected.
If all routes to the ASBR are in non-backbone areas, the route with the lowest cost is preferentially selected. If the routes have the same cost value, the route with the largest area ID has the highest precedence.
If the routes to the ASBR are in both backbone area and non-backbone areas, the route with the lowest cost is preferentially selected. If the routes have the same cost value, the route with the largest area ID is preferentially selected. The area ID of the backbone area is 0, so that routes in the backbone area will not be selected.
On an S series switch supporting OSPF, RFC 1583 compatibility is enabled by default. If multiple routes exist, the switch selects the route with the lowest cost. If the routes have the same cost value, the switch selects the route with the largest area ID.

Other related questions:
What does RFC 1583 compliance mean
The rfc1583 compatible command is used to enable a device to support the route selection rules defined in RFC 1583. After the undo rfc1583 compatible command is run: If there are two paths from a device to an ASBR, of which one is within a backbone area and the other is within a non-backbone area, the path within non-backbone areas will be selected. If the paths from a device to an ASBR are all within non-backbone areas, the device first compares the costs of the paths. If the costs are the same, the device will select the path within the non-backbone area with a larger Area ID. If there are two paths from a device to an ASBR, of which one is an inter-area path that passes through the backbone area, and the other is a path within a non-backbone area, the device first compares the costs of the paths. If the costs of the paths are the same, the device will select the path that passes through the area with a larger Area ID. (The ID of the backbone area is 0, and therefore the inter-area path that passes through the backbone area will not be preferred.) By default, a device support the route selection rules defined in RFC 1583. (Route selection rules defined in RFC 1583: Route costs are compared first. If route costs are the same, the route that passes through an area with a larger Area ID is selected.)

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