FHRPs

✅ First Hop Redundancy Protocols (FHRPs) – Technical Notes

🔹 1. What Are FHRPs?

First Hop Redundancy Protocols (FHRPs) are network protocols that provide gateway redundancy for end hosts in a LAN. They eliminate single points of failure by enabling multiple routers to act as a single virtual default gateway.

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🔹 1.1 Why Redundancy Is Important

• Prevents downtime in case the default gateway router fails.
• Maintains connectivity for end hosts to external networks.
• Crucial for business continuity in modern enterprises.

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🔹 1.2 The Problem Without FHRP

• Hosts rely on a single default gateway (via static IP or DHCP).
• They ARP for the gateway’s MAC and send traffic to it.
• If the default gateway (e.g., R1) fails:
  • Hosts continue sending packets to its MAC (which is now offline).
  • No automatic failover occurs → network outage.
  • Manual intervention would be needed — not acceptable in modern networks.

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🔹 1.3 The FHRP Solution – Virtual Gateway

• Routers share a Virtual IP (VIP) and Virtual MAC address.
• Hosts use the VIP as their default gateway.
• Roles:
  • Active router replies to ARP with the virtual MAC.
  • Standby router takes over automatically if active fails.
• Failover is seamless to end hosts (they don’t notice the change).

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🔹 1.4 Types of FHRPs (Cisco Supported)

Protocol	Type	Standard	Load Balancing
HSRP	Cisco-proprietary	No	Per subnet
VRRP	Industry-standard	RFC 3768 (IETF)	Per subnet
GLBP	Cisco-proprietary	No	Per host

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🔹 2. Key FHRP Concepts

🔸 2.1 Router Roles

Role	Protocol	Description
Active	HSRP	Forwards traffic for VIP
Standby	HSRP	Takes over if Active fails
Master	VRRP	Same role as Active
Backup	VRRP	Monitors Master
AVG (GLBP)	GLBP	Answers ARP requests
AVF (GLBP)	GLBP	Forwards traffic

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🔸 2.2 FHRP Communications

• Routers send Hello messages (multicast).
• If no hello is received within the Hold Time, failover occurs.
• Multicast is used instead of broadcast to reduce unnecessary LAN traffic.

Timer	Default Value
Hello	HSRP: 3s / VRRP: 1s
Hold	HSRP: 10s / VRRP: 3s

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🔸 2.3 Failover Process

1. Active router fails.
2. Standby/Backup router takes over.
3. Sends Gratuitous ARP (GARP) to update MAC tables in switches.
4. End hosts continue using the same VIP and virtual MAC.

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🔸 2.4 Preemption

• Allows a higher-priority router to re-take the active role upon recovery.
• Must be explicitly enabled (except in VRRP where it’s default).
• Optional preempt delay can be configured to wait for convergence.

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🔹 3. FHRP Comparison: HSRP vs VRRP vs GLBP

Feature	HSRP	VRRP	GLBP
Type	Cisco-proprietary	Open Standard (RFC 3768)	Cisco-proprietary
Terminology	Active / Standby	Master / Backup	AVG / AVFs
Multicast Address	v1: 224.0.0.2 / v2: 224.0.0.102	224.0.0.18	224.0.0.102
Virtual MAC Format	0000.0c07.acXX / 0c9f.fXXX	0000.5e00.01XX	0007.b400.XXYY
IPv6 Support	v2 only	Yes	Yes
Load Balancing	Per subnet	Per subnet	Per host (unique)
Preemption (Default)	Disabled	Enabled	AVG: Disabled / AVF: Enabled
Authentication	Plaintext / MD5	RFC: No (MD5 possible)	MD5

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🔹 4. Detailed Protocol Overview

🔸 4.1 HSRP (Hot Standby Router Protocol)

• Versions:
  • v1 (IPv4 only): 224.0.0.2, MAC: 0000.0c07.acXX
  • v2 (IPv4 + IPv6): 224.0.0.102, MAC: 0000.0c9f.fXXX
• No compatibility between v1 and v2.
• Load balancing is per subnet.
• Preemption is disabled by default.
• Uses UDP port 1985.
• Security: Basic MD5/Plaintext — vulnerable to LAN attacks.

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🔸 4.2 VRRP (Virtual Router Redundancy Protocol)

• Standardized by IETF (RFC 3768).
• Interoperable between vendors.
• MAC format: 0000.5e00.01XX.
• Preemption is enabled by default.
• Authentication not required per RFC, but MD5 is often supported.

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🔸 4.3 GLBP (Gateway Load Balancing Protocol)

• Cisco-proprietary.
• Load balancing on per-host basis:
  • Each AVF has its own virtual MAC.
  • AVG responds to ARP using round-robin, weighted, or host-dependent mode.
• Can support up to 4 AVFs per group.
• Preemption:
  • AVG: Disabled by default
  • AVF: Enabled by default
• Multicast: 224.0.0.102

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🔹 5. Basic HSRP Configuration (CCNA Level)

🔸 Configuration Requirements

• Each router needs its own unique IP for internal communication.
• All routers in the group share the same Virtual IP (VIP).
• HSRP group number and VIP must match on all routers.

🔸 Key Commands (Interface Mode)

interface GigabitEthernet0/1
 ip address 10.0.0.2 255.255.255.0
 standby version 2
 standby 1 ip 10.0.0.1
 standby 1 priority 120
 standby 1 preempt
 standby 1 authentication md5 key-string myHSRPkey
 standby 1 timers 1 3
 standby 1 preempt delay minimum 30
 no shutdown

🔸 Explanation

standby version 2: Enables HSRPv2.

standby 1 ip: Sets VIP.

priority: Higher = preferred active.

preempt: Enables role reclaim.

timers: Custom hello/hold timers.

authentication: Optional security.

preempt delay: Prevents premature failover during reconvergence.

🔸 Verification Commands

show standby brief
show standby

show standby brief: Quick overview.

show standby: Detailed HSRP state, VIP, timers, preemption, etc.

🔹 6. VRRP & GLBP Configuration (Notes)

Syntax is similar to HSRP.

VRRP and GLBP use a similar structure to HSRP.

Replace standby with vrrp or glbp in the commands.

Use vrrp or glbp instead of standby.

E.g., vrrp 1 ip 192.168.1.1 or glbp 1 ip 192.168.1.1

🔸 VRRP Example

interface GigabitEthernet0/1
 ip address 192.168.1.2 255.255.255.0
 vrrp 1 ip 192.168.1.1
 vrrp 1 priority 120
 vrrp 1 preempt

🔸 GLBP Example

interface GigabitEthernet0/1
 ip address 192.168.1.3 255.255.255.0
 glbp 1 ip 192.168.1.1
 glbp 1 priority 120
 glbp 1 preempt
 glbp 1 load-balancing round-robin

Note: VRRP and GLBP configurations are not typically required on CCNA, but understanding concepts is essential.

✅ Summary Comparison Table

Feature	HSRP	VRRP	GLBP
Proprietary	✅ Yes	❌ No	✅ Yes
Standard	❌ No	✅ RFC 3768	❌ No
Load Balancing	Per-subnet	Per-subnet	✅ Per-host
Preemption	❌ Disabled by default	✅ Enabled by default	AVG: ❌ / AVF: ✅
Default Hello/Hold	3s / 10s	1s / 3s	3s / 10s
Authentication	✅ Plaintext / MD5	❌ Not per RFC (but MD5 works)	✅ MD5
IPv6 Support	✅ (HSRPv2)	✅	✅
Use Case Focus	High availability	Interoperability	Load balancing + redundancy
Multicast Addr	v1: 224.0.0.2 / v2: 224.0.0.102	224.0.0.18	224.0.0.102

📌 Final Notes for Interview Prep

1. Understand router roles (Active/Standby/Master/Backup/AVG/AVF).

2. Know how failover and preemption work.

3. Be ready to compare HSRP, VRRP, and GLBP in terms of:

   1. Proprietary vs Standard

   2. Load balancing capability

   3. Configuration ease and use case

4. Know key default values and behaviors.

5. For CCNA: Focus on HSRP configuration, conceptual understanding of VRRP and GLBP.

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