Using Route Summarization

  • Article

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

With route summarization, or aggregation, in a hierarchical routing infrastructure, one route in a routing table represents many routes. A routing table entry for the highest level (the network) is also the route used for subnets and sub-subnets. In contrast, in a flat routing infrastructure, the routing table on every router in the network contains an entry for each network segment. When you use flat routing, the network IDs have no network/subnet structure and cannot be summarized. RIP-based Internet Packet Exchange (IPX) internetworks use flat network addressing and have a flat routing infrastructure.

Using route summarization, you can contain topology changes occurring in one area of the network within that area. Route summarization simplifies routing tables and reduces the exchange of routing information, but it requires more planning than does a flat routing infrastructure.

To support route summarization, your IP addressing scheme must meet the following requirements:

  • Classless routing protocols (those including subnet mask or prefix length information along with the IP address) must be used.

  • All IP addresses used in route summarization must share identical high-order bits.

  • The length of the prefix can be any number of bits up to 32 (for IPv4).