📘 CCNA 200-301 v1.1
3.5 Describe the purpose, functions, and concepts of first hop redundancy protocols
🔹 1. What are First Hop Redundancy Protocols (FHRPs)?
Definition:
A First Hop Redundancy Protocol (FHRP) is a group of protocols that provide gateway redundancy in a network.
Why it’s needed:
In a local area network (LAN), hosts (computers, printers, etc.) use a default gateway to send traffic to destinations outside their local network (like another subnet or the internet).
If that gateway (the router) goes down, hosts cannot communicate outside the LAN — even if another router is available.
FHRPs solve this problem by allowing multiple routers to share a single virtual IP address as the default gateway.
If one router fails, another router automatically takes over — keeping network communication up and running without manual changes.
🔹 2. Purpose of FHRPs
The main purpose is redundancy for the default gateway.
This ensures network availability and no interruption when a router fails.
In short:
- Without FHRP: One default gateway → single point of failure.
- With FHRP: Multiple routers act together → no single point of failure.
🔹 3. Basic Concept and Operation
Let’s understand how FHRPs work conceptually:
- Multiple routers are configured on the same LAN segment (for example, two routers connected to the same VLAN).
- They share a virtual IP address (the IP used by end devices as the default gateway).
- They also share a virtual MAC address.
- One router is elected as the active router (or master).
- This router is responsible for forwarding packets sent to the virtual IP.
- The other router(s) act as standby (or backup) routers.
- They monitor the active router’s status.
- If the active router fails or stops responding, a standby router takes over automatically, becoming the new active router.
- Hosts don’t need any configuration change — they keep using the same virtual gateway IP address.
This ensures continuous connectivity even if one router fails.
🔹 4. Common Types of FHRPs
Cisco supports several FHRPs.
For CCNA 200-301, you must know their names, differences, and basic functions.
🟢 1. HSRP (Hot Standby Router Protocol)
- Cisco proprietary (only works on Cisco devices).
- Uses a virtual IP and MAC address shared between routers.
- One router is Active, one is Standby.
- Uses Hello messages to check if routers are alive.
- If the active router fails, standby takes over immediately.
HSRP Key Points for CCNA:
- Hello messages: Sent every 3 seconds by default.
- Hold time: 10 seconds (if no Hello is heard, failover occurs).
- Virtual MAC address:
0000.0C07.ACxx(xx = HSRP group number). - Uses UDP port 1985 for communication.
- Only one router actively forwards at a time.
🟢 2. VRRP (Virtual Router Redundancy Protocol)
- Open standard (works on different vendors, not only Cisco).
- Similar concept to HSRP.
- One router is Master, others are Backups.
- Virtual IP can be the same as one of the routers’ physical IPs.
- Master router sends advertisements to announce its presence.
VRRP Key Points for CCNA:
- Advertisement interval: 1 second.
- Default priority: 100 (higher value = preferred master).
- Virtual MAC address:
0000.5E00.01xx(xx = VRRP group ID). - Uses IP protocol 112.
- Supports preemption (higher-priority router can take over).
🟢 3. GLBP (Gateway Load Balancing Protocol)
- Cisco proprietary.
- Provides both redundancy and load balancing.
- Multiple routers can actively forward traffic at the same time.
- Uses one virtual IP, but different virtual MAC addresses per router.
- Clients are assigned different virtual MAC addresses, balancing the load.
GLBP Roles:
- AVG (Active Virtual Gateway): Assigns virtual MAC addresses to other routers.
- AVF (Active Virtual Forwarders): Forward traffic for their assigned MAC addresses.
GLBP Key Points for CCNA:
- Provides both redundancy and load sharing.
- Uses UDP port 3222.
- Hello interval: 3 seconds; hold time: 10 seconds.
- Virtual MAC:
0007.B4xx.xxxx.
🔹 5. Comparison Table (for quick understanding)
| Feature | HSRP | VRRP | GLBP |
|---|---|---|---|
| Type | Cisco proprietary | Open standard | Cisco proprietary |
| Function | Redundancy | Redundancy | Redundancy + Load balancing |
| Active device | Active/Standby | Master/Backup | AVG + AVFs |
| Default Hello time | 3 sec | 1 sec | 3 sec |
| Protocol used | UDP 1985 | IP 112 | UDP 3222 |
| Load balancing | ❌ | ❌ | ✅ |
| Virtual MAC format | 0000.0C07.ACxx | 0000.5E00.01xx | 0007.B4xx.xxxx |
🔹 6. FHRP Priority and Preemption
Each router in an FHRP group has a priority value (0–255).
- Higher priority = preferred active/master router.
- If a higher priority router joins later, preemption decides whether it should take back control.
Preemption:
- Allows a higher-priority router to take over again when it becomes available.
- Example: Router A (priority 110) fails, Router B (priority 100) takes over.
When Router A comes back, with preemption enabled, it reclaims the active role.
🔹 7. FHRP Group Numbers
Each FHRP group uses a group ID to identify which routers belong together.
This ID is used in the virtual MAC and IP configuration.
Example:
HSRP group 1
Virtual IP: 192.168.10.1
Router1: Active (priority 110)
Router2: Standby (priority 100)
All devices in the same VLAN using 192.168.10.1 as their gateway will automatically be protected by this group.
🔹 8. Summary of FHRP Benefits
| Benefit | Description |
|---|---|
| Gateway redundancy | If one router fails, another takes over instantly |
| High availability | Keeps network uptime consistent |
| No reconfiguration needed on clients | Clients always use the same gateway IP |
| Fast failover | Failover happens automatically in seconds |
| Load balancing (GLBP) | Distributes traffic among multiple routers |
🔹 9. Exam Tips (CCNA Focus Points)
✅ Know the purpose — to prevent a single point of failure for the default gateway.
✅ Know three FHRP types: HSRP, VRRP, GLBP.
✅ Know basic operation and roles (Active/Standby, Master/Backup, AVG/AVF).
✅ Know protocol numbers, hello timers, and MAC formats.
✅ Understand priority and preemption concepts.
✅ Remember HSRP and GLBP are Cisco proprietary, while VRRP is an open standard.
🔹 Example in IT Environment (No real-life analogies)
Imagine a company VLAN with:
- 2 routers connected to the same switch (Router1 and Router2)
- Users configured with default gateway: 192.168.10.1
If Router1 (Active) fails:
- Router2 (Standby) takes over instantly.
- Users continue working — they don’t even notice the change.
- When Router1 comes back online, depending on priority/preemption, it may become Active again.
This is how redundancy for gateways works in real IT networks.
✅ In short:
First Hop Redundancy Protocols (FHRPs) protect users’ network connectivity by ensuring there’s always a working default gateway, even if one router fails.
Cisco provides HSRP, VRRP, and GLBP for this purpose — each with slightly different features but the same goal: network reliability and uptime.
