Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
When designing a campus network, Cisco recommends that you use a redundant triangle topology and summarize routes from the distribution layer to the core layer. In a redundant triangle topology, each core layer device has direct paths to redundant distribution layer devices, as shown in the diagram below:

This topology ensures that a link or device failure in the distribution layer can be detected immediately in hardware. Otherwise, a core layer device could detect only link or device failures through a software-based mechanism such as expired routing protocol timers. Additionally, the use of equal-cost redundant links enables a core layer device to enter both paths into its routing table. Because both equal-cost paths are active in the routing table, the core layer device can perform load balancing between the paths when both paths are up. When one of the equal-cost redundant links fails, the routing protocol does not need to reconverge, because the remaining redundant link is still active in the routing table. Thus traffic flows can be immediately rerouted around the failed link or device.
You should summarize routes from the distribution layer to the core layer. With route summarization, contiguous network addresses are advertised as a single network. This process enables the distribution layer devices to limit the number of routing advertisements that are sent to the core layer devices. Because fewer advertisements are sent, the routing tables of core layer devices are kept small and access layer topology changes are not advertised into the core layer.
Cisco does not recommend that you use a redundant square topology. In a redundant square topology, not every core layer device has redundant direct paths to distribution layer devices, as shown below:

Because a redundant square topology does not provide a core layer device with redundant direct paths to the distribution layer, the device will enter only the path with the lowest cost into its routing table. If the lowest cost path fails, the routing protocol must converge in order to select an alternate path from the remaining available paths. No traffic can be forwarded around the failed link or device until the routing protocol converges.
You should create routing protocol peer relationships on only the transit links of Layer 3 devices. A transit link is a link that directly connects two or more Layer 3 devices, such as a multilayer switch or a router. By default, a Layer 3 device sends routing protocol updates out of every Layer 3 interface that participates in the routing protocol. These routing updates can cause unnecessary network overhead on devices that directly connect to a large number of networks, such as distribution layer switches. Therefore, Cisco recommends filtering routing protocol updates from interfaces that are not directly connected to Layer 3 devices.
Reference:
Cisco: Campus Network for High Availability Design Guide: Using Triangle Topologies