Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
Route summarization is an advanced routing feature that enables a router to advertise multiple contiguous subnets as a single, larger subnet. Summarization, which is also known as supernetting, combines several smaller subnets into one larger subnet. This enables routers on the network to maintain a single summarized route in their routing tables. Therefore, fewer routes are advertised by the routers, which reduces the amount of bandwidth required for routing update traffic. Route summarization is most efficient when the subnets can be summarized within a single subnet boundary and are contiguous, meaning that all of the subnets are consecutive. Summarization is typically performed between the enterprise campus core and the enterprise edge. Advanced routing features, such as summarization, route filtering, and redistribution, can greatly impact the functionality and scalability of a network and, thus, should be carefully considered during the network design process.
Redundancy, not route summarization, is the repetition built into a network design to protect the network from unnecessary vulnerabilities or downtime that might be caused by having a single point of failure.
Simply put, redundancy is having a backup plan in place that can be used in the event that the primary plan becomes unavailable. For example, multiple physical links between two switches could be used to promote redundancy.
Redistribution, not route summarization, is an advanced routing feature that increases the scalability of a network design by facilitating the coexistence of multiple routing protocols. For example, to join networks at multiple locations where one is running Enhanced Interior Gateway Routing Protocol (EIGRP) and the other is running Open Shortest Path First (OSPF), Cisco recommends that you configure two-way redistribution with route map filters at each location. Redistribution is typically performed by routers between the enterprise campus core and the enterprise edge.
Route filtering, not route summarization, is an advanced routing feature that can be used in conjunction with redistribution to block route advertisements that could create routing loops. Routing loops occur when a topology change or a delayed routing update results in two routers pointing to each other as the next hop to a destination. For example, Router1 has a path to Router2 that begins with Router3, and Router3 has a path to Router2 that begins with Router1. Since both Router1 and Router3 send data to each other that is intended for Router2, they will continuously bounce the data back and forth between them, thus forming a loop. In order to prevent this loop, a route filter could be used to stop the path from Router1 to Router2 from being advertised to Router3. Consequently, when Router3 receives data from Router1 that is intended for Router2, the only route available is its own path directly to Router2. Because route filtering is often used in conjunction with redistribution, route filtering is typically performed by routers between the enterprise campus core and the enterprise edge.
Reference:
Cisco: OSPF Design Guide: OSPF and Route Summarization