Which protocol is considered an Interior Gateway Protocol (IGP)?

The protocol considered an Interior Gateway Protocol (IGP) is OSPF. OSPF, which stands for Open Shortest Path First, is designed specifically for routing within a single autonomous system or a local area network. It operates by using a link-state routing algorithm, allowing routers within the same area to share information about the network topology efficiently.

OSPF provides several key advantages, including fast convergence, support for large networks, and the ability to segment networks using areas, which enhances scalability and management. Additionally, it is widely used due to its open standards nature, allowing interoperability between different vendors' equipment.

In contrast, the other protocols listed serve different purposes. LACP (Link Aggregation Control Protocol) is used for bundling multiple Ethernet links into a single logical link for greater bandwidth and redundancy. STP (Spanning Tree Protocol) is primarily a protocol to prevent loops in Ethernet networks by managing the active paths and blocking redundant paths. PAGP (Port Aggregation Protocol) is similar to LACP, used for dynamically managing Port Aggregation on Cisco devices.

Understanding the roles and functionalities of these protocols helps clarify OSPF's unique position as an IGP, designed explicitly for managing internal routing within networks.