Interior Routing - RIP

RIP (Routing Information Protocol) is a distance-vector routing protocol used to exchange routing information within an autonomous system (AS). It determines the best path to a destination network based on the hop count (number of routers) between the source and destination.

How RIP Works:

1. Periodic Updates: Routers broadcast their routing tables to all neighbors every 30 seconds.

2. Hop Count: RIP uses hop count as a metric to determine the shortest path. A hop count of 16 indicates an unreachable network.

3. Routing Table: Each router maintains a routing table containing information about reachable networks, the next hop router, and the hop count.

4. Route Selection: When a packet needs to be forwarded, the router selects the route with the lowest hop count to the destination network.

RIP Features:

• Simplicity: Easy to configure and understand.

• Fast Convergence: Relatively quick to adapt to network changes.

• Scalability: Works well in small to medium-sized networks.

• Hop Count Limitation: Maximum hop count of 15 limits network size.

• Routing Loops: Can lead to routing loops if not properly configured.

RIP Versions:

• RIPv1: Original version with limitations like classful addressing and lack of authentication.

• RIPv2: Introduced classless addressing, authentication, and support for VLSM (Variable Length Subnet Masking).

Limitations of RIP:

• Scalability Issues: Not suitable for large networks due to the hop count limitation and broadcast updates.

• Slow Convergence: Can take time to recover from network changes.

• Security Concerns: Lack of authentication in RIPv1.

Replacement:

Due to its limitations, RIP has been largely replaced by more advanced protocols like OSPF (Open Shortest Path First) and EIGRP (Enhanced Interior Gateway Routing Protocol) for larger and more complex networks.

Despite its limitations, RIP is still used in smaller networks due to its simplicity.

RIP v1 is known as Classful Routing Protocol because it doesn’t send information of subnet mask in its routing update. 
RIP v2 is known as Classless Routing Protocol because it sends information of subnet mask in its routing update. 

Interior Gateway Protocols (IGPs) 
Interior Gateway Protocols (IGPs) are used to exchange routing information between routers within the same autonomous system (AS). An AS is a collection of networks under a single administrative control.

Types of IGPs

IGPs are primarily categorized into two types:

1. Distance-Vector Protocols: These protocols calculate the shortest path based on the number of hops (routers) between the source and destination.

• Examples: RIP (Routing Information Protocol), IGRP (Interior Gateway Routing Protocol)

  Link-State Protocols: These protocols build a complete map of the network topology and calculate the shortest path based on link costs.

• Examples: OSPF (Open Shortest Path First), IS-IS (Intermediate System to Intermediate System)

Key Characteristics of IGPs

• Dynamic Routing: Automatically adapt to network changes.

• Scalability: Can handle networks of varying sizes.

• Convergence: The speed at which the network converges to a stable state after changes.

• Complexity: Varying levels of configuration and management.

Comparison of Distance-Vector and Link-State Protocols

Popular IGPs

• RIP (Routing Information Protocol): Simple but has limitations in terms of scalability and convergence.

• IGRP (Interior Gateway Routing Protocol): Cisco-proprietary protocol that considers multiple metrics beyond hop count.

• OSPF (Open Shortest Path First): Widely used link-state protocol offering scalability and fast convergence.

• IS-IS (Intermediate System to Intermediate System): Link-state protocol often used in large networks.

• EIGRP (Enhanced Interior Gateway Routing Protocol): Cisco-proprietary protocol combining features of distance-vector and link-state.

Choosing the Right IGP

The choice of IGP depends on various factors such as network size, topology, desired level of security, and administrative preferences.

Exterior Routing - RIP

While ICMP isn't directly involved in routing, there are indeed Exterior Gateway Protocols (EGPs) used for routing between different autonomous systems (AS) on the internet. However, the focus of your question seems to be on understanding how a common Interior Gateway Protocol (IGP) called RIP (Routing Information Protocol) works in exterior routing scenarios.

Here's the thing: RIP, by its core design, is classified as an IGP. It's primarily used for routing within a single autonomous system, exchanging routing information between routers under the same administrative control. However, in some limited situations, RIP can be used for a basic form of exterior routing in a controlled environment.

RIP for Exterior Routing (Limited Use):

In very specific scenarios, like connecting two small, neighboring networks with limited routing needs, RIP can be configured for a rudimentary form of exterior routing. This is achieved through a concept called RIP route redistribution. Here's how it works:

1. RIP Routers on Each AS Boundary:  A router is placed on the border of each autonomous system (AS). This router is configured with two routing tables:

• One table for internal routing within its own AS using RIP.

• Another table for routes learned from the neighboring AS, which could be statically configured or learned from a different routing protocol used in the other AS.

Limitations of RIP for Exterior Routing:

There are significant limitations to using RIP for exterior routing:

• Limited Scalability: RIP suffers from scalability issues, meaning it's not suitable for large, complex networks with many routers and routes. As the number of routes and networks grows, RIP routing tables become unwieldy, and routing loops can occur.

• Security Concerns: RIP lacks robust security features, making it vulnerable to unauthorized route advertisements and potential routing disruptions.

• Slow Convergence:  RIP is known for its slow convergence time, meaning it can take a while for routing updates to propagate throughout the network after a topology change. This can lead to temporary outages or inefficient routing paths.

Alternatives for Exterior Routing:

For exterior routing between autonomous systems on the internet, more robust and scalable Exterior Gateway Protocols (EGPs) are typically used. Common EGPs include:

• Border Gateway Protocol (BGP):  The dominant EGP used for internet routing, known for its scalability, security features, and ability to handle complex routing policies.

• Exterior Interior Gateway Routing Protocol (EIGRP):  A Cisco proprietary routing protocol that can function as both an IGP and EGP in limited scenarios with controlled route redistribution.

Conclusion:

While RIP can be used for a basic form of exterior routing in very specific controlled environments, it's generally not recommended due to its limitations. For robust and scalable exterior routing between autonomous systems on the internet, EGPs like BGP are the preferred choice. Understanding the distinction between IGPs and EGPs, and their respective functionalities, is crucial for effective network design and routing implementation.

Exterior Routing Protocols

Exterior Gateway Protocols (EGPs) are used to exchange routing information between different Autonomous Systems (ASes). An AS is a collection of networks under a single administrative control. Unlike Interior Gateway Protocols (IGPs) which operate within an AS, EGPs handle inter-AS routing.   

Key Exterior Gateway Protocol: BGP

While there have been other EGPs in the past, Border Gateway Protocol (BGP) is the dominant protocol used today. It's responsible for the routing decisions on the internet.   

BGP Characteristics:

• Path Vector Protocol: BGP uses path attributes to determine the best path to a network.   

• Policy-Based Routing: Allows network administrators to influence routing decisions based on various factors like policy, economic considerations, and network performance.

• Scalability: Designed to handle large and complex networks.   

• Complex: BGP configuration and troubleshooting can be challenging.   

BGP Attributes:

• AS Path: Lists the ASes a route has traversed.   

• Next Hop: Specifies the next router to forward packets.

• Origin: Indicates how the route was learned.

• MED (Multi-Exit Discriminator): Used to influence path selection among multiple equal-cost paths.   

• Local Preference: Used to prefer routes within an AS.   

BGP Operations:

• Route Advertisement: Routers advertise their reachable networks to neighboring ASes.

• Route Selection: BGP routers use path attributes to select the best path to a destination.

• Path Withdrawal: Routers withdraw routes when they become unreachable.

BGP and Internet Routing

BGP is the cornerstone of the internet's routing infrastructure. It enables complex routing policies, allows for load balancing, and supports various network topologies. 

In summary, BGP is the primary protocol for inter-AS routing, providing the foundation for global network connectivity.