VRRP (Virtual Router Redundancy Protocol) – Complete Overview
1. Purpose and Overview
VRRP is an open-standard First Hop Redundancy Protocol (FHRP) providing gateway redundancy for hosts. It eliminates single points of failure by allowing a backup router to take over seamlessly if the primary router fails.
VRRP vs. HSRP
| Aspect | VRRP (RFC 5798) | HSRP (Cisco proprietary) |
|---|---|---|
| Standard | IETF Open Standard | Cisco Only |
| Versions | v2 (IPv4), v3 (IPv4 & IPv6) | v1, v2 (IPv4/IPv6) |
| Virtual IP | Usually matches Master's real IP | Not assigned to real interface |
| Virtual MAC | 00-00-5E-00-01-XX | 00-00-0C-07-AC-XX |
| Default timers | 1 sec Advertisement | 3 sec Hello |
2. VRRP Terminology
- Master Router: Currently forwards traffic for the virtual IP.
- Backup Router: Ready to take over if Master fails.
- VRID: Virtual Router Identifier.
- Virtual IP: Gateway IP for clients.
3. VRRP Operation and Election
- Priority range: 1–254 (default 100).
- Highest priority router becomes Master.
- Supports preemption by default.
4. VRRP States
| State | Description |
|---|---|
| Initialize | No VRRP participation yet |
| Backup | Listening for advertisements |
| Master | Forwarding for virtual IP, sending advertisements |
5. Timers and Advertisements
- Advertisement Interval: Default 1 second.
- Master Down Interval: Typically 3× Advertisement Interval plus skew time.
6. Failover Behavior
If Master fails, Backup assumes Master role with minimal disruption.
7. Authentication (VRRPv2 only)
Optional plaintext/simple authentication (not secure; recommended only in trusted networks).
8. Load-Sharing Techniques
Multiple VRRP groups per subnet allow load distribution.
9. Example VRRP Configuration (Cisco IOS)
interface g0/1 ip address 192.168.10.2 255.255.255.0 vrrp 1 ip 192.168.10.254 vrrp 1 priority 120 vrrp 1 preempt
When to Use VRRP
- Multi-vendor environments needing an open standard.
- Networks needing simple gateway redundancy.
GLBP (Gateway Load Balancing Protocol) – Conceptual Overview
1. Purpose and Overview
GLBP is a Cisco proprietary FHRP providing both gateway redundancy and automatic load balancing among multiple routers.
GLBP vs. HSRP/VRRP
| Feature | HSRP/VRRP | GLBP |
|---|---|---|
| Redundancy | Yes | Yes |
| Load balancing | No | Yes |
| Virtual MACs | One per group | Multiple per group (one per AVF) |
2. GLBP Terminology
- AVG (Active Virtual Gateway): Assigns virtual MACs and manages the GLBP group.
- AVF (Active Virtual Forwarder): Forward traffic assigned by AVG.
3. Load-Balancing Methods
- Round-Robin: Sequential MAC assignment.
- Host-Dependent: Persistent MAC-to-host assignment.
- Weighted: Based on manually set weights.
4. Timers and Failover
- Hello: 3 seconds default.
- Hold: 10 seconds default.
5. Authentication
GLBP supports secure MD5 authentication:
glbp 1 authentication md5 key-string SECRET
6. Example GLBP Configuration (Cisco IOS)
interface g0/1 ip address 192.168.20.2 255.255.255.0 glbp 1 ip 192.168.20.254 glbp 1 priority 120 glbp 1 preempt glbp 1 load-balancing round-robin glbp 1 authentication md5 key-string SECRET
When to Use GLBP
- When both redundancy and load balancing are required.
- Large LANs with multiple redundant gateways.
Summary Table
| Feature | HSRP | VRRP | GLBP |
|---|---|---|---|
| Standard | Cisco | Open Standard | Cisco |
| Load Balance | No | No | Yes |
| Preemption | Optional | Default | Optional |
| Authentication | Plain/MD5 | Plain/simple | MD5 |
| Virtual MACs | 1 per group | 1 per group | Multiple per group |