Common Routing Protocols
Routing protocols are essential for routers to dynamically learn and share information about network paths. Below is a detailed explanation of the most common routing protocols: OSPF, EIGRP, BGP, and RIP.
1. OSPF (Open Shortest Path First)
Type: Link-State Routing Protocol (Interior Gateway Protocol - IGP).
Purpose: Used within an autonomous system (AS) to determine the best path for data.
Key Features:
Uses the Dijkstra algorithm to calculate the shortest path.
Divides the network into areas to reduce routing overhead.
Supports VLSM (Variable Length Subnet Masking) and CIDR (Classless Inter-Domain Routing).
Fast convergence and scalable for large networks.
Metric: Cost (based on bandwidth).
Hello Protocol: Sends hello packets to discover and maintain neighbor relationships.
Areas:
Backbone Area (Area 0): Central area that connects all other areas.
Non-Backbone Areas: Other areas connected to the backbone.
Use Case: Commonly used in enterprise networks.
2. EIGRP (Enhanced Interior Gateway Routing Protocol)
Type: Advanced Distance-Vector Routing Protocol (Interior Gateway Protocol - IGP).
Purpose: Used within an autonomous system (AS) to provide fast and efficient routing.
Key Features:
Cisco-proprietary (though partially opened as an informational RFC).
Uses the DUAL (Diffusing Update Algorithm) for loop-free path calculation.
Supports VLSM and CIDR.
Fast convergence and low network overhead.
Metric: Composite metric (based on bandwidth, delay, reliability, load, and MTU).
Hello Protocol: Sends hello packets to maintain neighbor relationships.
Use Case: Commonly used in Cisco environments.
3. BGP (Border Gateway Protocol)
Type: Path-Vector Routing Protocol (Exterior Gateway Protocol - EGP).
Purpose: Used between autonomous systems (AS) to route traffic across the internet.
Key Features:
The protocol that powers the global internet routing system.
Uses AS Path as a key metric to determine the best route.
Supports policy-based routing (e.g., traffic engineering).
Highly scalable and designed for large networks.
Metric: Based on attributes like AS Path, Next Hop, and Local Preference.
Neighbor Relationships:
eBGP (External BGP): Between routers in different ASes.
iBGP (Internal BGP): Between routers in the same AS.
Use Case: Used by ISPs and large enterprises to connect to the internet.
4. RIP (Routing Information Protocol)
Type: Distance-Vector Routing Protocol (Interior Gateway Protocol - IGP).
Purpose: Used within small to medium-sized networks.
Key Features:
Simple and easy to configure.
Uses hop count as the metric (maximum of 15 hops).
Sends periodic routing updates every 30 seconds.
Supports RIPv1 (classful) and RIPv2 (classless with VLSM support).
Metric: Hop count.
Limitations:
Slow convergence.
Not suitable for large networks due to the 15-hop limit.
Use Case: Used in small networks with minimal complexity.
Comparison of Routing Protocols
Feature
OSPF
EIGRP
BGP
RIP
Type
Link-State
Advanced Distance-Vector
Path-Vector
Distance-Vector
Metric
Cost (based on bandwidth)
Composite (bandwidth, delay, etc.)
AS Path, Next Hop, Local Preference
Hop Count
Convergence
Fast
Very Fast
Slow
Slow
Scalability
High
High
Very High
Low
Use Case
Enterprise networks
Cisco environments
Internet routing
Small networks
VLSM/CIDR Support
Yes
Yes
Yes
RIPv2 only
Administrative Distance
110
90 (internal), 170 (external)
20 (eBGP), 200 (iBGP)
120
Key Terms
Administrative Distance (AD):
A measure of the trustworthiness of a routing protocol.
Lower AD values are preferred.
Convergence:
The time it takes for all routers in a network to agree on the best paths.
Autonomous System (AS):
A collection of IP networks and routers under the control of a single organization.
Summary
OSPF: Best for large enterprise networks due to its scalability and fast convergence.
EIGRP: Ideal for Cisco environments with fast convergence and low overhead.
BGP: Essential for internet routing and large-scale networks.
RIP: Suitable for small, simple networks but limited in scalability.
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