What is GLBP (Gateway Load Balancing Protocol)?
Learn what GLBP is, how the AVG and AVFs load-balance gateway traffic, and how it compares to HSRP/VRRP — interview Q&A.
Expected Interview Answer
GLBP (Gateway Load Balancing Protocol) is a Cisco-proprietary first-hop redundancy protocol that, unlike HSRP or VRRP, actively load-balances outbound traffic across all routers in the group simultaneously, while still providing the same automatic failover if any of them goes down.
GLBP groups multiple routers around a single virtual IP, but instead of only one router forwarding traffic, one router is elected the Active Virtual Gateway (AVG) and is responsible for handing out — via ARP replies — one of several virtual MAC addresses to different hosts on the LAN, cycling through them (typically round-robin, but weighted or host-dependent methods exist). Each router owning one of those virtual MACs is called an Active Virtual Forwarder (AVF), so several routers actually forward traffic concurrently even though hosts all still point to one shared gateway IP. If an AVF fails, the AVG reassigns its virtual MAC to a surviving router, and hosts that had ARP-cached the failed MAC seamlessly continue through the new owner without any ARP change on their end. This gives GLBP a real advantage over HSRP/VRRP: it uses all available uplink bandwidth instead of leaving Standby/Backup routers idle. The tradeoff is added configuration complexity and it remains Cisco-only, so mixed-vendor networks typically fall back to VRRP.
- Uses all group routers for forwarding instead of leaving one idle
- Still provides automatic, transparent failover like HSRP/VRRP
- Hosts need no ARP changes on failover — the AVG reassigns the virtual MAC
- Supports multiple load-balancing methods (round-robin, weighted, host-dependent)
AI Mentor Explanation
GLBP is like a stadium that, instead of opening only one ticket gate while three others sit idle (as HSRP/VRRP effectively do), assigns every arriving group of fans to a different gate in rotation so all four are actively processing people at once — a coordinator at the entrance (the AVG) decides which gate each new group heads to. If one gate’s staff has to shut down, the coordinator quietly redirects new arrivals to the remaining gates without anyone needing new tickets. Fans already inside keep going wherever they were told, exactly like a host continuing to use its already-learned virtual MAC. This is the real capacity gain GLBP has over a single-active-gateway approach.
Step-by-Step Explanation
Step 1
AVG election
Routers in the GLBP group elect one router as the Active Virtual Gateway, responsible for answering ARP requests for the shared virtual IP.
Step 2
AVF assignment
The AVG assigns each router in the group its own virtual MAC address, making each one an Active Virtual Forwarder.
Step 3
Load distribution
The AVG hands out different virtual MACs to different hosts (round-robin, weighted, or host-dependent) so traffic spreads across all AVFs.
Step 4
Failover
If an AVF fails, the AVG reassigns its virtual MAC to a surviving router, and hosts continue using the same MAC transparently.
What Interviewer Expects
- Correct definition: Cisco-proprietary protocol that load-balances across all group routers
- Knows the AVG (elects/assigns) vs AVF (actually forwards) role split
- Explains why GLBP outperforms HSRP/VRRP in uplink utilization
- Aware it remains Cisco-only, unlike the standards-based VRRP
Common Mistakes
- Thinking GLBP is functionally identical to HSRP/VRRP with only naming differences
- Confusing the AVG role (controller) with the AVF role (actual forwarder)
- Not knowing multiple virtual MACs, not just one, are in play
- Assuming GLBP is an open standard like VRRP
Best Answer (HR Friendly)
“GLBP is Cisco’s answer to a limitation in protocols like HSRP and VRRP — instead of having only one router actively doing all the work while the backups sit idle, GLBP spreads outbound traffic across every router in the group at the same time, while still automatically covering for any router that fails. It gets more value out of the hardware that is already there, at the cost of being a bit more complex to configure and tied to Cisco equipment.”
Code Example
interface GigabitEthernet0/1
ip address 10.0.0.2 255.255.255.0
glbp 1 ip 10.0.0.1
glbp 1 priority 150
glbp 1 load-balancing round-robin
! On a peer router that becomes an additional AVF:
interface GigabitEthernet0/1
ip address 10.0.0.3 255.255.255.0
glbp 1 ip 10.0.0.1
! Check GLBP status and virtual MAC assignments
show glbp brief
# Interface Grp Fwd Pri State Address Active router Standby router
# Gi0/1 1 - 150 Active 10.0.0.1 local 10.0.0.3
# Gi0/1 1 1 150 Active 0007.b400.0101 local -
# Gi0/1 1 2 100 Listen 0007.b400.0102 10.0.0.3 -Follow-up Questions
- What is the difference between the AVG and an AVF in GLBP?
- How does GLBP’s round-robin load-balancing method work?
- Why might an organization choose VRRP over GLBP despite GLBP’s load balancing?
- What happens to hosts using a failed AVF’s virtual MAC?
MCQ Practice
1. What is the main advantage of GLBP over HSRP and VRRP?
Unlike HSRP/VRRP where only one router forwards at a time, GLBP distributes traffic across multiple Active Virtual Forwarders.
2. In GLBP, what role does the AVG (Active Virtual Gateway) perform?
The AVG owns the virtual IP for ARP purposes and hands out different virtual MACs to the group’s Active Virtual Forwarders.
3. Is GLBP an open, vendor-neutral standard?
GLBP, like HSRP, is a Cisco-proprietary protocol; VRRP is the open standard alternative.
Flash Cards
What is GLBP? — A Cisco-proprietary first-hop redundancy protocol that load-balances traffic across all group routers.
What does the AVG do? — Elects itself, answers ARP for the virtual IP, and assigns virtual MACs to Active Virtual Forwarders (AVFs).
How does GLBP differ from HSRP/VRRP? — Multiple routers actively forward traffic simultaneously instead of only one Active/Master router.
Is GLBP vendor-neutral? — No — it is Cisco-proprietary, unlike the open-standard VRRP.