28.1.2 Route control when area is divided
Each area border router summarizes the routing information for each non-backbone area to which it belongs, and then sends it to all other routers in the backbone area. The summarized routing information for the backbone area and the summarized routing information sent from other areas to the backbone area are reported to the routers in non-backbone areas.
When a router uses this summarized routing information to determine the route to an address, the route will traverse the area border router that is the source of the summarized routing information. Therefore, a route between different areas always passes through the backbone area.
When advertising routing information to other areas, the area border router summarizes topology information derived from link states between routers and networks and the costs of those links.This information describes the cost between the area border router and specific routers and networks. The summarized information is called inter-area routing information.The information that describes routes to networks is advertised in Type 3 LSAs, and the information that describes routes to AS boundary routers is advertised in Type 4 LSAs.
- <Structure of this section>
(1) Route summarization at an area border router
The following table describes the summarization and suppression of routing information, and the summary information that the area border router reports to other areas.
|
Network addresses in area |
Summarization and suppression |
Summary reported to other areas |
|---|---|---|
|
3ffe:501:811:10::/60 3ffe:501:811:20::/61 3ffe:501:811:28::/61 3ffe:501:811:30::/60 |
No |
3ffe:501:811:10::/60 3ffe:501:811:20::/61 3ffe:501:811:28::/61 3ffe:501:811:30::/60 |
|
3ffe:501:811:10::/60 3ffe:501:811:20::/61 3ffe:501:811:28::/61 3ffe:501:811:30::/60 |
3ffe:501:811::/59 3ffe:501:811::20::/60 |
3ffe:501:811::/59 3ffe:501:811:20::/60 3ffe:501:811:30::/60 |
|
3ffe:501:811:10::/60 3ffe:501:811:20::/61 3ffe:501:811:28::/61 3ffe:501:811:30::/60 3ffe:501:811:ff00::/58 |
3ffe:501:811::/58 (suppressed) 3ffe:501:811:ff00::/56 |
3ffe:501:811:ff00::/56 |
You can use configuration commands to specify an address range that the area border router condenses into a single summarized route when summarizing the topology information for a given area. To specify an address range, use the area range command to specify a prefix and the prefix length. You can also specify a parameter that suppresses advertisement of the address range.
If the area contains even one network that matches the prefix range specified in the configuration command, the area border router summarizes all networks within the range into routing information with the prefix as its destination, and then reports it to other areas. The router does not report information for each network in the range outside area boundaries. The summarized routing information will adopt the highest cost among its constituent routes.
If you suppress advertisement, no summary LSAs for networks in the address range are advertised outside area boundaries, nor are the routes that are summarized under the prefix advertised to other areas. As a result, routes to addresses within the specified address range remain hidden from other areas.