OSI Layer 3: Routing Quiz

OSI Layer 3: Routing

• Routing is the process of determining the path a data packet takes from its source to its destination on a network.
• Routers use routing tables to make decisions based on criteria like IP destination, subnet mask, and metric.
• Routing occurs in two ways: direct routing (hosts on the same LAN) and indirect routing (hosts on different networks requiring one or more routers).

Learning Outcomes

• Students should master the basics of routing.
• Students should differentiate between static and dynamic routing.
• Students should describe characteristics and operation of main dynamic routing protocols.
• Students should configure and implement basic routing protocols.
• Students should use diagnostic tools and commands to check and optimize routing.

What is Routing?

• Routing is the process of determining the path a data packet should take from a source to a destination on a network.
• Routers make decisions based on routing tables using criteria such as IP destination, subnet mask, and metric.
• Routing can be direct when both hosts are on the same local area network (LAN), or indirect when hosts reside on different networks requiring a router.

Why is Routing Important?

• Routing is essential for inter-network communication enabling data exchange between sub-networks, VLANs, and WANs (internet).
• Path optimization is key; routing selects the most efficient routes, reducing transit times, and congestion.
• Fault tolerance is critical; the dynamic protocols (OSPF, EIGRP) adapt paths in the event of failure to ensure network resilience.

The Default Gateway

• The default gateway is the IP address of a router enabling a device to connect to networks beyond its local area network (LAN).
• If the destination address is outside the local network, the device forwards the packet to the default gateway.
• The default gateway then forwards the packet to the appropriate route to the target network.
• Default gateway configuration is critical for accessing external resources.

Routing Table

• The routing table is the fundamental element used by a router to make routing decisions.
• It contains information on various paths a router can use to reach destinations.
• Each entry in a routing table has the network destination, subnet mask, gateway (next hop), interface and metric.

Static Routing

• Static routing means that routes are defined manually by the network administrator, not changing unless modified explicitly.
• Advantage: Simple configuration for small networks; total control; no bandwidth consumption.
• Disadvantage: Difficult maintenance for larger networks; no automatic updates for changing topologies; unsuitable for dynamic environments.

Dynamic Routing

• Dynamic routing uses protocols that automatically learn routes available on the network, adapting to changes.
• Advantages: Adaptability and Simplified maintenance for large networks; and resilience to failures.
• Disadvantages: Resource consumption and complexity (requiring in-depth knowledge). Routing loops (errors in configurations) can render parts of the network inaccessible.

Dynamic Routing Protocols

• Protocols such as RIP, OSPF, and EIGRP exchange routing information between routers.
• This process automatically updates routing tables and enables adaptation to changes in network topology to choose the best paths.

Routing Protocols

• Interior Gateway Protocols (IGPs): Protocols operating within an Autonomous System (e.g., RIP, OSPF, EIGRP)
• Exterior Gateway Protocols (EGPs): Protocols used for routing between Autonomous Systems on large networks like the internet (e.g., BGP).

RIP (Routing Information Protocol)

• RIP uses a distance-vector algorithm.
• It's based on the number of hops to determine the best route.
• Limited to 15 hops, restricting its use to small networks.
• Updates routing tables every 30 seconds.
• Easy to configure, but limited to small networks and slow convergence.

OSPF (Open Shortest Path First)

• OSPF is a link-state protocol using the SPF algorithm.
• Networks divided into hierarchical zones for optimized routing.
• Updates only when changes occur, reducing bandwidth usage.
• Offers rapid convergence and scalability, appropriate for large networks.

EIGRP (Enhanced Interior Gateway Routing Protocol)

• EIGRP is a hybrid protocol combining distance vector and link-state elements.
• A composite metric (bandwidth, delay, reliability, load) evaluates the best route.
• Offers fast convergence and low bandwidth consumption but is a proprietary Cisco protocol, limiting interoperability.

BGP (Border Gateway Protocol)

• BGP is the main protocol used for routing between Autonomous Systems (ASes) on the internet.
• Uses prefix paths and routing policy; not strict metrics like other protocols.
• Enables routing between different ASes, particularly for internet routing.
• Crucial for global routing. It is complex to configure, requiring extensive expertise.

Verification Commands

• show ip route: Displays the router's routing table (known routes, sources)
• show ip protocols: Displays information about routing protocols configured on the router (e.g., OSPF, RIP, EIGRP).
• show ip ospf neighbor: Checks OSPF neighbor relationships and their status.
• show ip bgp summary: Displays a summary of BGP sessions, including connections and exchanged information.

Troubleshooting Routing Problems

• Analyze the routing table to diagnose missing or incorrect routes.
• Check neighbor relationships to find potential configuration errors affecting OSPF or EIGRP.
• Ensure proper configuration of BGP sessions within Autonomous Systems.
• Use commands like ping and traceroute to verify network connectivity.

Conclusion

• Routing is essential for efficient data flow between diverse networks.
• Dynamic routing protocols optimize and adapt to network changes.
• Detailed understanding is essential for managing and troubleshooting network connectivity.