10.1.2 Route selection criteria
From information learned by each protocol about routes to the same destination, the Switch selects the best route according to the independent route selection procedures associated with each protocol. If there are a number of alternative routes in the generated information, their distances are compared and the routing information that has the highest priority is selected.
Under RIP, priority rules govern the selection of the best route to a given destination among the routes learned and advertised by different routers. Route selection conditions are listed in order of priority as follows:
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Selects the route with the lowest metric.
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Selects routes whose aging time is less than or equal to 1/2 second of the timer value (if the metric values are the same).
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Selects the route with the smallest next-hop address.
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Selects the route whose next-hop address in the routing information matches the address of the gateway from which the information originated.#2
This condition applies if routing information containing the same next-hop address is learned by different neighboring routers on the network.
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In all other cases, ignores the most recently learned route.
When the routes to a given destination learned by each protocol (OSPF, BGP4, and static) result in multiple entries, their distances are compared, and the route that has the highest priority is set in the routing table.
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(1) Generating a Secondary Route
Using the generate-secondary-route configuration command, you can generate up to two route entries (primary route and secondary route) to the same destination learned by two different neighboring routers. The following table describes the conditions for generating a secondary route.
|
Conditions |
Generating secondary route |
|
|---|---|---|
|
Configuration commands Specification of the generate-secondary-route configuration command |
Distance |
|
|
NG |
- |
Not generated |
|
OK |
Different values for primary route and secondary route |
Not generated |
|
OK |
Same values for primary route and secondary route |
Generated |
(Legend) OK: With configuration NG: Without configuration-: Not applicable
When a second-priority route is specified, the priority of route information to the same destination is determined according to the priority of route selection. The route selection conditions for the secondary route are listed in the following order of precedence: However, if the next-hop addresses are the same, only the first-priority route is generated.
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Selects the route with the next lowest metric.
-
Selects routes whose aging time is less than or equal to 1/2 second of the timer value (if the metric values are the same).
-
Selects the route with the next smallest next-hop address#1
If a new route is learned while a secondary route is already registered, this condition does not apply.
-
Selects the route whose next-hop address in the routing information matches the address of the gateway from which the information originated.#2
This condition applies if routing information containing the same next-hop address is learned by different neighboring routers on the network.
-
Selects the route that has previously been the first priority.
-
In all other cases, ignores the most recently learned route.