Explain auto-summary in RIP and when it can cause routing issues.

• A. Automatically selects the shortest path by default.
• B. Applies only to static routes and never affects dynamic routing.
• C. Suppresses updates for networks smaller than a /24.
• D. Automatically summarizes routes to classful boundaries at major network borders; can misroute in discontiguous networks.

Answer: (D)

Auto-summarization in RIP happens when routing information crosses a major network boundary. The router then advertises the destination as a single classful network, rather than the specific subnets it actually knows. For example, all subnets within a classful boundary like 10.x.x.x are summarized to 10.0.0.0/8 when crossing into a different major network.

This behavior reduces the granularity of routing information, which can be efficient, but it can cause problems in discontiguous networks—where subnets of the same classful network are separated by other networks. Because the details about the exact subnets are hidden behind the classful summary, a router receiving the summary may not have the precise path to reach a particular subnet, leading to misrouting or traffic being sent toward the wrong exit. To avoid these issues in such designs, auto-summarization is often disabled so that the router propagates and uses the actual subnets across boundaries.

The other statements don’t capture this behavior: auto-summarization is not about choosing the shortest path, it applies to dynamic routing (not just static), it does affect dynamic routing, and it doesn’t simply suppress updates based on subnet size like /24.