Section 3.3: Summarization
Summarization allows the representation of a series of networks in a single summary address. At the top of the hierarchical design, the subnets in the routing table are more generalized. The subnet masks are shorter because they have aggregated the subnets lower in the network hierarchy. These summarized networks are often referred to as supernets, particularly when seen in the Internet as an aggregation of class addresses. They are also known as aggregated routes. The summarization of multiple subnets within a few subnets has several advantages. These include: reducing the size of the routing table; simplifying the recalculation of the network as the routing tables are smaller; network overhead scalability; and hiding network changes.
3.3.1: Automatic Summarization
All routing protocols employ some a type of summarization. RIP and IGRP automatically summarize at the NIC or natural class boundary as the subnet mask is not sent in the routing updates. When a routing update is received, the router checks if it has an interface in the same class network. If it has one, it applies the mask configured on the interface to the incoming routing update. With no interface configured in the same NIC network, there is insufficient information and the routing protocol uses the first octet rule to determine the default subnet mask for the routing update.
3.3.2: Manual Summarization
Both EIGRP and Open Shortest Path First (OSPF) send the subnet mask along with the routing update. This feature allows the use of VLSM and summarization. When the routing update is received, it assigns the subnet mask to the particular subnet. When the routing process performs a lookup, it searches the entire database and acts on the longest match, which is important because it allows for the granularity of the hierarchical design, summarization, and discontiguous networks.
A discontiguous network is a network in which a different NIC number separates two instances of the same NIC number. This can happen either through intentional design or through a break in the network topology. If the network is not using a routing protocol that supports VLSM, this will create a routing problem because the router will not know where to send the traffic. Without a subnet mask, a routing protocol that supports VLSM resolves the address down to the NIC number, which appears as if there is a duplicate address. This will incorrectly lead to the appearance of intermittent connectivity symptoms.
If there are discontiguous networks in the organization, it is important that summarization is turned off or not configured. Summarization may not provide enough information to the routing table on the other side of the intervening NIC number to be capable of appropriately routing to the destination subnets, especially with EIGRP, which automatically summarizes at the NIC boundary. In OSPF and EIGRP, manual configuration is required for any sophistication in the network design. However, because EIGRP can perform summarization at the interface level, it is possible to select interfaces that do not feed discontiguous networks for summarization.
If summarization is not possible, you can either turn summarization off and understand the scaling limitations that have now been set on the network, or you can readdress the network.