Dynamic Routing Protocols #

Dynamic routing protocols automatically discover network routes and adapt to topology changes. They exchange routing information between routers to maintain optimal path selection and network convergence.

RIP (Routing Information Protocol) #

Distance-vector protocol using hop count as the routing metric:

graph TD
    A[Router A] -->|1 hop| B[Router B]
    B -->|2 hops| C[Router C]
    A -.->|Direct| C

    D[RIP Update] --> E[Router advertisement every 30 seconds]
    F[Maximum 15 hops] --> G[16 = unreachable]

RIP v2 Configuration #

Cisco IOS:

Router(config)# router rip
Router(config-router)# version 2
Router(config-router)# network 192.168.1.0
Router(config-router)# network 192.168.2.0
Router(config-router)# no auto-summary

Key Features:

• Algorithm: Bellman-Ford
• Metric: Hop count (1-15)
• Update Timer: 30 seconds
• Invalid Timer: 180 seconds
• Hold Down: 180 seconds
• Flush Timer: 240 seconds

RIP Limitations #

• Slow convergence
• Count-to-infinity problem (solved by split horizon, poison reverse)
• Limited scalability (max 15 hops)
• Bandwidth intensive (periodic updates)

OSPF (Open Shortest Path First) #

Link-state protocol using Dijkstra’s algorithm for shortest path calculation:

graph TD
    A[Router A] -->|Link Cost| B[Router B]
    B -->|Link Cost| C[Router C]
    A -->|Link Cost| C

    D[OSPF Database] --> E[LSA Flooding]
    F[Dijkstra SPF] --> G[Shortest Path Tree]

OSPF Areas #

Hierarchical design to improve scalability:

graph TD
    A[Backbone Area 0] --> B[Area 1]
    A --> C[Area 2]
    A --> D[Area 3]

    E[ABR: Area Border Router] --> A
    E --> B

OSPF Configuration #

Single Area:

Router(config)# router ospf 1
Router(config-router)# network 192.168.1.0 0.0.0.255 area 0
Router(config-router)# network 10.0.0.0 0.0.0.3 area 0

Multi-Area:

# Backbone router
Router(config)# router ospf 1
Router(config-router)# network 192.168.1.0 0.0.0.255 area 0

# Area router
Router(config)# router ospf 1
Router(config-router)# network 192.168.2.0 0.0.0.255 area 1

OSPF Features #

• Metric: Cost (bandwidth-based by default)
• Convergence: Fast (link-state updates)
• Scalability: Area hierarchy
• Authentication: MD5 or clear text
• DR/BDR: Designated Router election on multi-access segments

BGP (Border Gateway Protocol) #

Path-vector protocol for inter-domain routing:

graph TD
    A[AS 100] -->|BGP Peer| B[AS 200]
    B -->|BGP Peer| C[AS 300]

    D[BGP Attributes] --> E[AS Path]
    D --> F[Next Hop]
    D --> G[Local Preference]
    D --> H[MED]

eBGP vs iBGP #

eBGP (External BGP):

• Between different Autonomous Systems
• Advertises routes between organizations

iBGP (Internal BGP):

• Within same Autonomous System
• Full mesh requirement for consistent routing

BGP Configuration #

Basic eBGP:

Router(config)# router bgp 65001
Router(config-router)# neighbor 192.168.1.2 remote-as 65002
Router(config-router)# network 10.1.1.0 mask 255.255.255.0

BGP Attributes:

# Local preference (higher = preferred)
Router(config)# router bgp 65001
Router(config-router)# bgp default local-preference 200

# AS path prepending (discourage selection)
Router(config-router)# neighbor 192.168.1.2 route-map PREPEND out
Router(config)# route-map PREPEND permit 10
Router(config-route-map)# set as-path prepend 65001 65001

BGP Path Selection #

BGP uses attributes to select best path:

1. Highest Weight (Cisco proprietary)
2. Highest Local Preference
3. Locally originated routes
4. Shortest AS Path
5. Lowest Origin type (IGP < EGP < Incomplete)
6. Lowest MED (Multi-Exit Discriminator)
7. eBGP over iBGP
8. Lowest IGP metric to next-hop
9. Oldest route (for stability)
10. Lowest Router ID

Protocol Comparison #

Route Redistribution #

Injecting routes from one protocol into another:

# Redistribute OSPF into RIP
Router(config)# router rip
Router(config-router)# redistribute ospf 1 metric 2

# Redistribute static routes into OSPF
Router(config)# router ospf 1
Router(config-router)# redistribute static subnets

# Redistribute BGP into OSPF
Router(config)# router ospf 1
Router(config-router)# redistribute bgp 65001 subnets

Troubleshooting #

RIP Issues #

# Check RIP neighbors
show ip rip database

# Debug RIP updates
debug ip rip

# Verify RIP routes
show ip route rip

OSPF Issues #

# Check OSPF neighbors
show ip ospf neighbor

# Verify OSPF database
show ip ospf database

# Check OSPF interface
show ip ospf interface

# Debug OSPF packets
debug ip ospf packet

BGP Issues #

# Check BGP peers
show ip bgp summary

# Verify BGP routes
show ip bgp

# Debug BGP updates
debug ip bgp updates

# Check BGP neighbors
show ip bgp neighbors

Routing protocols form the intelligence layer of modern networks, enabling automatic route discovery, optimal path selection, and network resilience through dynamic adaptation to topology changes.