Network Routing

Routing is the process of selecting paths in a network to send data from source to destination.

How Routers Work

Routers operate at Layer 3 (Network Layer) of the OSI model and make forwarding decisions based on IP addresses.

Routing Table

Routers maintain routing tables containing:

• Destination network
• Next hop router
• Interface to use
• Metric (cost) of the route

Routing Protocols

Static Routing

Manually configured routes that don’t change unless modified by an administrator.

Advantages:

• No bandwidth overhead
• Predictable behavior
• More secure

Disadvantages:

• Time-consuming to configure
• Not scalable for large networks
• No automatic failover

Dynamic Routing

Routers automatically discover and maintain routes using routing protocols.

Interior Gateway Protocols (IGP)

Used within an autonomous system (single organization’s network).

RIP (Routing Information Protocol)

• Distance-vector protocol
• Uses hop count as metric (max 15 hops)
• Simple but limited scalability
• Updates every 30 seconds

OSPF (Open Shortest Path First)

• Link-state protocol
• Uses cost as metric (based on bandwidth)
• Fast convergence
• Supports large networks
• Hierarchical design with areas

EIGRP (Enhanced Interior Gateway Routing Protocol)

• Cisco proprietary (hybrid protocol)
• Fast convergence
• Low bandwidth usage
• Supports unequal-cost load balancing

Exterior Gateway Protocols (EGP)

Used between different autonomous systems (between organizations).

BGP (Border Gateway Protocol)

• Path-vector protocol
• The routing protocol of the Internet
• Highly scalable
• Policy-based routing
• Prevents routing loops

Routing Metrics

Different protocols use various metrics to determine the best path:

• Hop count: Number of routers to destination (RIP)
• Bandwidth: Link capacity (OSPF)
• Delay: Time to traverse a link
• Load: Traffic level on a link
• Reliability: Error rates and link stability
• Cost: Administrative value assigned to a link

Advanced Routing Concepts

Load Balancing

Distributing traffic across multiple paths to optimize resource usage and improve performance.

Route Summarization

Combining multiple routes into a single advertisement to reduce routing table size.

Default Route

A route used when no specific route to a destination exists, typically pointing to the Internet gateway.

NAT (Network Address Translation)

Translates private IP addresses to public IP addresses, allowing multiple devices to share a single public IP.