Interior Gateway Protocols (IGPs) are routing protocols used within an autonomous system (AS). An AS is essentially a collection of IP networks and routers under the control of a single organization that shares a common routing policy. IGPs distribute routing information within their AS to determine the best path for transmitting data packets between networks.
There are several types of IGPs:
RIP (Routing Information Protocol):
- Type: Distance-vector protocol.
- Metric: Uses hop count as its metric, with a maximum of 15 hops.
- Versions: RIP version 1 (RIPv1) and version 2 (RIPv2). RIPv2 supports VLSM (Variable Length Subnet Masking) and multicast for routing updates.
- Convergence: Can be slow, leading to loops in certain cases.
- Usage: Due to its limitations, RIP is less common in modern networks but might be found in simpler or older networks.
EIGRP (Enhanced Interior Gateway Routing Protocol):
- Type: Advanced distance-vector (often termed as a hybrid protocol because it has characteristics of both distance-vector and link-state protocols).
- Metric: Uses a composite metric based on bandwidth, delay, reliability, and load.
- Features: Supports fast convergence using Diffusing Update Algorithm (DUAL).
- Scalability: Scales well in large networks.
- Proprietary: Originally a Cisco proprietary protocol but now it’s an open standard.
OSPF (Open Shortest Path First):
- Type: Link-state protocol.
- Metric: Uses cost, which is inversely proportional to bandwidth.
- Features: Divides large networks into areas for better scalability and management. Uses Dijkstra’s algorithm to compute the shortest path tree.
- Convergence: Faster than RIP.
- Usage: One of the most widely used IGPs in larger networks.
IS-IS (Intermediate System to Intermediate System):
- Type: Link-state protocol.
- Metric: Uses cost like OSPF.
- Operation: Works on the data link layer, making it unique as it doesn’t rely on IP. Instead, it uses CLNP (Connectionless Network Protocol).
- Usage: Commonly used by telecommunications providers.
Key Concepts Associated with IGPs:
- Convergence: How quickly the routers in a network update their routing tables and come to a consensus about the network topology after a change.
- Metric: A standard by which the protocol determines the best path for data. Different protocols use different metrics.
- Distance Vector: Protocols that use distance (metric) and direction (vector) to decide the best path. They send their routing tables to their direct neighbors.
- Link-State: Protocols that create a complete map of the network to compute the best path.
Each IGP has its strengths and weaknesses, and the choice of which one to use often depends on the specific requirements of the network in question.