8.1.5 Concurrent Routing Protocol Operation
You can implement various static and dynamic routing protocols concurrently on the Switch.
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(1) Selecting Learned Route Priority
In an environment running more than one routing protocol concurrently, each protocol uses its own route selection algorithm to select the best route to a given destination. Both summarized routes and directly connected routes are treated as a protocol route as with routes learned by routing protocols. This can result in the Switch learning multiple different routes to the same destination. In this case, the switch compares the distance of each route and applies the routing information with the highest priority.
In the Switch, you can use configuration commands to set the default distance (priority) that applies to each static route and to each piece of routing information generated by dynamic routing protocols (such as RIP). A route with a smaller distance has a higher priority. The following table describes the default distances for each protocol.
|
Route |
Default distance |
|---|---|
|
Directly connected route |
0 (fixed value) |
|
Static route |
2 |
|
BGP4 route learned from external peer |
20 |
|
OSPF (internal AS route) |
110 |
|
OSPF (external AS route) |
110 |
|
RIP Route |
120 |
|
Summarized route |
130 |
|
BGP4 route learned from internal peer |
200 |
|
BGP4 route learned from inter-AS peer |
200 |
|
Route imported from another VRF or the global network |
210 |
(2) Advertised route
In an environment running more than one routing protocol, each routing protocol only advertises the routing information learned by that protocol. It does not advertise routing information it learns from other routing protocols.
In the Switch, you can use route filtering to configure a routing protocol to advertise routing information it learns from other routing protocols, or to exclude specific routes from advertisement. Information of inactive routes cannot be advertised in other routing protocols.
For details about route filtering, see 14 Route Filtering (IPv4).
(a) Route Advertisement in RIP
RIP-1 and RIP-2 implement the same routing protocol. Each advertises the routes it learns from the other.
(b) Route Advertisement in OSPF
The OSPF routing protocol operates independently in each OSPF domain. For this reason, a number of internal and external AS routes taken from different OSPF domains might exist for a given destination address. When OSPF routes share the same distance, the route with the smaller domain number takes priority. You can change the default distances for internal and external AS OSPF routes (intra-area and inter-area routes).
Under normal circumstances, routes are not advertised between the various OSPF domains configured on the Switch. However, you can configure route filtering to enable advertisement of internal and external OSPF routes to other OSPF domains.
(c) Route Advertisement in BGP4
When route filtering is disabled, the switch advertises the BGP4 routes it learns from a given AS to other ASs. In this case, the switch advertises the best BGP4 route as selected by the BGP4 routing protocol even if another routing protocol defines a route to the same destination.
If route filtering is enabled, the switch advertises the routing information that represents the route with the highest priority, selected according to its distance.