| administrative-distance routing-protocol ipv6 redundancy-protocol |
1. Routing Protocol Fundamentals
Routing protocols fall into two categories:
- IGP (Interior Gateway Protocol): e.g., RIP, OSPF, EIGRP—used within a single AS
- EGP (Exterior Gateway Protocol): e.g., BGP—used between autonomous systems
2. Administrative Distance (AD)
A value from 0 to 255 used to rank route trustworthiness. Lower values are more preferred.
Directly Connected: AD=0
Static Route: AD=1
RIP: AD=120
EIGRP (internal): AD=90
OSPF: AD=110
3. Why Not Enable All Protocols?
- Multiple active protocols waste bandwidth and CPU cycles
- Example: EIGRP wins over RIP due to lower AD, but RIP still consumes resources
- Solution: Enable only the required protocol and upgrade older routers
4. Types of IGP Protocols
- Distance-Vector: e.g., RIP, IGRP (based on hop count)
- Link-State: e.g., OSPF, IS-IS (creates full network map)
- Hybrid: e.g., EIGRP
5. RIP Protocol
- Simple distance-vector protocol
- RIP v1: Broadcast only, no VLSM support
- RIP v2: Multicast, supports VLSM and MD5 authentication
| Feature | RIPv1 | RIPv2 |
|---|
| VLSM | No | Yes |
| Update Type | Broadcast | Multicast |
| Authentication | No | Yes (MD5) |
6. VLSM and Discontiguous Networks
VLSM allows different subnet masks to optimize IP address usage.
- Protocols that support VLSM and discontiguous networks: RIPv2, EIGRP, OSPF
- RIPv1 does not support these features
7. EIGRP Protocol
- Hybrid protocol using DUAL algorithm
- Maintains Neighbor Table, Topology Table, and Routing Table
- Uses Hello packets for neighbor discovery
- Supports IPv6 and discontiguous networks
8. Link-State Protocols: OSPF and IS-IS
OSPF
- Uses Dijkstra’s SPF algorithm
- Hierarchical design with Area 0 (backbone)
- Supports authentication, CIDR, and VLSM
IS-IS
- Uses CLNS instead of IP
- Supports IPv6 and VLSM without separate databases
- Best for large-scale ISP networks
9. BGP Protocol
- EGP protocol based on Path-Vector model
- Uses AS Path to determine best route
- Key protocol for connecting the Internet
- Supports route aggregation and classless routing
10. First-Hop Redundancy Protocols (FHRP)
Ensure default gateway availability using virtual routers
HSRP
- Cisco proprietary protocol
- Virtual MAC: 0000.0c07.ac0a
- Customizable hello and hold timers
(config-if)#standby 1 timers msec 200 msec 700
VRRP
- Open standard protocol
- Higher-priority router becomes active
- Compatible across vendors
