What is “no auto-summary” command in EIGRP?

In EIGRP (Enhanced Interior Gateway Routing Protocol), the “no auto-summary” command is used to disable the automatic summarization of routes at network boundaries. By default, EIGRP performs auto-summarization, which means that it summarizes network addresses at classful boundaries (class A, B, or C) when advertising routes to neighbouring routers.

When the “no auto-summary” command is configured under the EIGRP routing process, it prevents automatic summarization from occurring. This allows more specific (subnet-level) routes to be advertised and exchanged between routers, providing greater flexibility in routing and preserving the full network topology.

Disabling auto-summary is often recommended in scenarios where Variable Length Subnet Masking (VLSM) is used, as it allows for more efficient utilization of IP address space and enables the routing protocol to advertise subnets more accurately.

By using the “no auto-summary” command in EIGRP, you ensure that routes are propagated with their original subnet masks, enabling more precise routing decisions and finer control over network traffic flows.