Summarize the key FHRP concepts in a comprehensive overview.
FHRPs (First Hop Redundancy Protocols) provide gateway redundancy using virtual router addresses. The main protocols are HSRP (Hot Standby Router Protocol) (Cisco), VRRP (Virtual Router Redundancy Protocol) (open), and GLBP (Gateway Load Balancing Protocol) (Cisco, load balancing). HSRP uses priority-based election with optional preemption and five operational states.
Core concepts summary:
1. The Problem: Hosts have a single default gateway. If it fails → total loss of connectivity. Even with a redundant router available, hosts can't automatically switch.
2. The Solution: FHRPs create a virtual router (virtual IP (Internet Protocol) + virtual MAC (Media Access Control)) backed by multiple physical routers. Hosts point to the virtual IP.
3. Protocol Options:
- HSRP — Cisco, active/standby, most common on Cisco
- VRRP — Open standard, master/backup, multi-vendor
- GLBP — Cisco, load balancing across all routers
4. HSRP Election: Highest priority wins (default 100, range 0-255). Tiebreaker: highest IP. Preemption disabled by default.
5. HSRP States: Initial → Learn → Listen → Speak → Standby/Active
6. HSRP Timers: Hello = 3s, Hold = 10s. Can be tuned for faster failover (minimum 1s/4s).
7. Failover process:
- Standby stops receiving Hellos
- Hold timer expires → standby becomes active
- Virtual IP and MAC transfer → hosts unaffected
8. Best practices:
- Always set explicit priorities (don't rely on IP tiebreaker)
- Enable preemption on the preferred router
- Consider GLBP if you want to utilize all routers' bandwidth
- Use VRRP in multi-vendor environments
Go deeper:
First-hop redundancy protocol (Wikipedia) — one overview tying together HSRP, VRRP and GLBP and the gateway-redundancy problem.