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.