HSRP (Hot Standby Router Protocol) – In-Depth Guide

1. HSRP Overview

HSRP is a Cisco proprietary First Hop Redundancy Protocol (FHRP) that provides gateway redundancy. If the active router fails, a standby router takes over automatically, ensuring network continuity.

Multiple routers form an HSRP group sharing a virtual IP address.
One router is Active, others remain in Standby.
Hosts configure the virtual IP as their default gateway.

Example Scenario:

Two routers (R1 & R2) form HSRP Group 1 with virtual IP 192.168.1.254. Hosts point their default gateway to this virtual IP.

2. HSRP Versions

Version	Key Differences	When to Use
HSRP v1	Group numbers 0–255, IPv4 only, MAC `00:00:0c:07:ac:XX`	Legacy setups, IPv4-only networks
HSRP v2	Group numbers 0–4095, IPv6 support, MAC `00:00:0c:9f:fX:XX`	Modern networks, IPv6 required, multiple groups

3. HSRP States and State Machine

State	Description
Initial	Startup, no HSRP activity yet
Learn	Learns virtual IP
Listen	Listening for hello messages
Speak	Exchanges hello messages, election process starts
Standby	Ready to take over if Active fails
Active	Currently forwarding packets

Timers: Hello (default 3s), Hold (default 10s).

4. HSRP Priority and Preemption

Default Priority: 100
Higher priority routers become Active

Configuring Priority and Preemption:

standby [group] priority [value]
standby [group] preempt

5. HSRP Group and Virtual MAC/IP

Group Number: Identifies HSRP instance.

Virtual MAC Address Format:

HSRP v1: 00:00:0c:07:ac:XX
HSRP v2: 00:00:0c:9f:fX:XX

Example Configuration:

interface g0/1
 standby 1 ip 192.168.1.254
 standby 1 priority 110
 standby 1 preempt

6. Timers and Convergence

Hello Timer: Interval for hello messages (3s default).
Hold Timer: Wait before declaring router down (10s default).

Adjust timers carefully to balance fast convergence and network load:

standby [group] timers [hello] [hold]

7. HSRP Interface Tracking

Decreases priority if a key interface fails:

standby [group] track [interface] [decrement value]

8. Authentication

Secures HSRP traffic between routers.

Plain-text: standby [group] authentication [string]

MD5: More secure:

standby [group] authentication md5 key-string [string]

9. Load Sharing Techniques

Use multiple HSRP groups with different virtual IPs for load distribution.

10. Integration with Other Protocols

Protocol	Description
VRRP	Standard-based alternative to HSRP
GLBP	Cisco protocol supporting load balancing with one virtual IP

11. HSRP over NBMA and Point-to-Point Links

NBMA: Ensure broadcast mapping or static configuration.
Point-to-Point: Usually not needed due to direct links.

12. Show and Debug Commands

show standby: Detailed HSRP status
show standby brief: Brief status
debug standby events: Monitor state transitions
debug standby packets: Inspect HSRP traffic

13. Troubleshooting HSRP

Issue	Possible Cause	Solution
Split-brain	Both routers Active	Check timers, priority, preemption
No failover	Priority/preempt misconfigured	Verify priority and preempt settings
State inconsistencies	HSRP version mismatch, interface down	Check configuration and interfaces

14. Advanced Topics

BFD integration: Faster failure detection.
Graceful Restart: Minimal disruption during failover.

Sample Configuration:

interface GigabitEthernet0/1
 ip address 192.168.10.2 255.255.255.0
 standby 10 ip 192.168.10.254
 standby 10 priority 110
 standby 10 preempt

When to Use HSRP

Enterprise networks
Data centers
Multi-router campuses

HSRP (Hot Standby Router Protocol) Quiz

1. What is the main purpose of HSRP?

A. Provide gateway redundancy to ensure continuous network access B. Increase bandwidth between routers C. Encrypt routing protocol messages D. Manage routing table updates

Correct answer is A. HSRP provides gateway redundancy by electing an Active and Standby router to maintain network availability.

2. Which of the following is TRUE about HSRP versions?

A. HSRP v1 supports IPv6 natively B. HSRP v2 uses the MAC format 00:00:0c:07:ac:XX C. HSRP v2 is limited to 255 groups D. HSRP v2 supports IPv6 and more group numbers (0–4095)

Correct answer is D. HSRP v2 supports IPv6 and a larger range of group numbers for modern network deployments.

3. What is the default priority value for HSRP routers?

A. 150 B. 100 C. 50 D. 200

Correct answer is B. The default priority in HSRP is 100, used in the election of Active routers.

4. Which HSRP state indicates the router is currently forwarding packets as the default gateway?

A. Standby B. Speak C. Active D. Learn

Correct answer is C. The Active state means the router is forwarding packets as the current default gateway.

5. What command enables a router interface to participate in HSRP group 10 with virtual IP 192.168.10.254?

A. standby 10 ip 192.168.10.254 B. hsrp group 10 ip 192.168.10.254 C. ip hsrp 10 address 192.168.10.254 D. hsrp enable 10 ip 192.168.10.254

Correct answer is A. The 'standby [group] ip [virtual-ip]' command configures the virtual IP address for HSRP group participation.

6. What effect does the 'standby [group] preempt' command have?

A. Prevents router from taking Active role B. Disables HSRP on the interface C. Forces the router to ignore hello packets D. Allows router with higher priority to take Active role if available

Correct answer is D. Preemption enables a router with higher priority to take over the Active role when it becomes available.

7. Which timers control HSRP failover detection?

A. Hello and Dead timers B. Hello and Hold timers C. Start and Stop timers D. Join and Leave timers

Correct answer is B. HSRP uses Hello (default 3 seconds) and Hold (default 10 seconds) timers to detect failures and trigger failover.

8. What is the purpose of HSRP interface tracking?

A. To disable HSRP on an interface B. To increase the priority permanently C. To decrease router priority if a tracked interface goes down D. To monitor CPU usage of the interface

Correct answer is C. Interface tracking reduces the router's priority if the tracked interface fails, enabling failover.

9. Which of the following is a Cisco proprietary alternative to HSRP that supports true load balancing?

A. GLBP (Gateway Load Balancing Protocol) B. VRRP (Virtual Router Redundancy Protocol) C. OSPF (Open Shortest Path First) D. EIGRP (Enhanced Interior Gateway Routing Protocol)

Correct answer is A. GLBP supports true load balancing by allowing multiple routers to share a single virtual IP.

10. What show command gives detailed HSRP status on an interface?

A. show standby brief B. show standby C. show hsrp interface D. show standby statistics

Correct answer is B. 'show standby' provides detailed HSRP status for interfaces.

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