GLBP (Gateway Load Balancing Protocol) is Cisco's first-hop redundancy protocol that does what HSRP and VRRP cannot: actively load-balance across multiple routers in a single group. Where HSRP and VRRP have one active forwarder per group, GLBP can have up to four active forwarders simultaneously, each handling a portion of the host traffic. The result is utilization across all the redundant routers instead of one sitting idle as standby.
This article walks through the GLBP architecture, the Active Virtual Gateway (AVG) and Active Virtual Forwarders (AVFs), the load balancing methods, configuration, and when GLBP is actually the right answer (it is more niche than the marketing suggests).
The Architecture
GLBP introduces two roles within a group:
| Role | Job |
|---|---|
| AVG (Active Virtual Gateway) | One per group; manages the protocol; assigns virtual MACs to AVFs; responds to ARP requests with the right virtual MAC |
| AVF (Active Virtual Forwarder) | Up to four per group; each owns a unique virtual MAC and forwards packets sent to that MAC |
How a packet flows:
- End host ARPs for the virtual IP gateway.
- The AVG (one router in the group) responds, but the MAC it returns rotates among the four virtual MACs assigned to the four AVFs.
- Different hosts ARP at different times and receive different virtual MACs, so each host pins to a specific AVF for its outbound traffic.
- Each AVF forwards the packets it receives via its virtual MAC.
- Failover: if an AVF fails, the AVG redirects its virtual MAC to a different AVF.
The result: per-host load distribution. With 100 hosts and 4 AVFs, roughly 25 hosts per AVF. Load distribution is statistical, not strictly equal, but the average across many hosts is close to even.
Load Balancing Methods
GLBP supports three load balancing modes, controlled by how the AVG hands out virtual MACs in ARP responses:
| Method | Behavior |
|---|---|
| Round-robin (default) | Cycle through AVFs in order. First host gets AVF1, second host gets AVF2, etc. |
| Weighted | AVFs with higher weight get more hosts. Useful when AVFs have different capacity. |
| Host-dependent | Same host always gets the same AVF (based on hash of source MAC). Provides stability across host reboots. |
Round-robin is the dominant default; the others are for specific edge cases.
Configuration
! Distribution switch 1 (becomes AVG)
interface GigabitEthernet1/0/24
ip address 192.168.1.2 255.255.255.0
glbp 1 ip 192.168.1.1
glbp 1 priority 110
glbp 1 preempt
glbp 1 load-balancing round-robin
glbp 1 weighting 100
glbp 1 weighting track 1 decrement 30
! Distribution switch 2 (AVF)
interface GigabitEthernet1/0/24
ip address 192.168.1.3 255.255.255.0
glbp 1 ip 192.168.1.1
glbp 1 priority 100
glbp 1 preempt
glbp 1 weighting 100
track 1 interface GigabitEthernet0/0/0 line-protocolNotice the AVF weighting separate from priority. Priority elects the AVG; weighting determines AVF eligibility (an AVF whose weighting drops below a threshold stops forwarding). This separation is what lets GLBP do load balancing.
Virtual MAC Address Format
GLBP virtual MACs follow a specific format: 0007.b400.GGFF where GG is the group number and FF is the AVF number (1-4).
| Group | AVF | Virtual MAC |
|---|---|---|
| 1 | 1 | 0007.b400.0101 |
| 1 | 2 | 0007.b400.0102 |
| 1 | 3 | 0007.b400.0103 |
| 1 | 4 | 0007.b400.0104 |
| 10 | 1 | 0007.b400.0a01 |
Different from HSRP (0000.0c07.acXX) and VRRP (0000.5e00.01XX). All three FHRP families use distinct OUIs.
The State Machine
| State | Meaning |
|---|---|
| Disabled | GLBP is configured but not yet running |
| Initial | Just started; no Hellos yet |
| Listen | Receiving Hellos but not active in any role |
| Speak | Sending Hellos and participating in election |
| Standby | Backup AVG (will take over if current AVG fails) |
| Active | Currently the AVG OR an AVF (depending on context) |
Verify with:
Switch# show glbp brief
Interface Grp Fwd Pri State Address Active router
Gi1/0/24 1 - 100 Active 192.168.1.1 local
Gi1/0/24 1 1 - Active 0007.b400.0101 local
Gi1/0/24 1 2 - Listen 0007.b400.0102 192.168.1.3
Gi1/0/24 1 3 - Listen 0007.b400.0103 192.168.1.4
Gi1/0/24 1 4 - Listen 0007.b400.0104 192.168.1.5
Switch# show glbp 1
GigabitEthernet1/0/24 - Group 1
State is Active
...
Forwarder 1
State is Active
...
Forwarder 2
State is Listen
...The first row is the AVG; subsequent rows are AVFs. "Active" forwarders are currently distributing traffic; "Listen" forwarders are backups.
When GLBP Is the Right Answer
GLBP is more niche than its marketing suggests. It is the right answer when:
- You have multiple physical paths upstream and want to use them simultaneously instead of one being idle as standby
- You are entirely Cisco-shop (GLBP is Cisco-only)
- You have homogeneous routers that can handle the additional GLBP complexity
- You want active/active without per-VLAN role tuning (the alternative is HSRP/VRRP with different active per VLAN, which works but is more configuration)
GLBP is not the right answer when:
- You have a multi-vendor environment (use VRRP)
- You have asymmetric routers (one fast, one slow); GLBP's per-host distribution does not handle asymmetry well without weighting
- You want simple, well-understood operations (HSRP is simpler)
- You are studying for VRRP/HSRP exam topics (both are more commonly tested)
In practice, most Cisco campus designs use HSRP with per-VLAN active distribution. GLBP shows up less often than the protocol's existence would suggest.
Anti-Patterns
- GLBP across non-Cisco gear. Does not work; not standardized.
- Skipping weighting tracking. An AVF with a dead WAN keeps forwarding to a black hole. Always track upstream interfaces and decrement weighting on failure.
- Weighting threshold too high or low. If too low, the AVF rarely loses its active status and tracking is ineffective. If too high, transient drops cause flapping. Tune based on the tracked interface's expected stability.
- Mixing GLBP with HSRP on the same hosts. Hosts get one default gateway IP. Pick one protocol per gateway and stick with it.
Summary
GLBP is Cisco's load-balancing FHRP. Up to four AVFs with different virtual MACs share the load; the AVG distributes hosts across them via ARP responses. Per-host load balancing without per-VLAN role tuning - the marketing case is real, but most Cisco campus designs use HSRP because it is simpler and well-understood.
Use GLBP when you specifically need active/active gateway forwarding in a Cisco-only network. Otherwise HSRP is the safer default. Bookmark this article alongside the FHRP cluster pillar and the HSRP vs VRRP vs GLBP comparison.