Table of Contents Link to heading

• GLBP: What It Solves That HSRP and VRRP Cannot
• GLBP Fundamentals
• GLBP Roles: AVG and AVF
• Load-Balancing Algorithms
• GLBP Weighting and AVF Eligibility
• Pre-emption
• Configuration
  • Basic GLBP Configuration
  • Load-Balancing Algorithm Selection
  • Weighting and Interface Tracking
  • Verification
• GLBP vs HSRP vs VRRP
• Operational Considerations

GLBP: What It Solves That HSRP and VRRP Cannot Link to heading

HSRP and VRRP both provide gateway redundancy, but neither provides true load balancing with a single virtual IP. In HSRP and VRRP, only one router is active at a time — the standby/backup router sits idle, consuming resources but forwarding no traffic. Load sharing in HSRP/VRRP requires multiple groups and splitting hosts across different virtual IPs, which is an operational workaround rather than a true solution.

GLBP (Gateway Load Balancing Protocol) solves this by:

1. Using a single virtual IP for all hosts in the subnet — no host splitting required
2. Assigning multiple virtual MAC addresses (up to four) — one per active router
3. Distributing ARP replies to different virtual MACs based on the configured algorithm — different hosts get different MACs, causing them to use different physical gateways

The result: all routers forward traffic simultaneously, and hosts automatically use their assigned gateway MAC without any manual configuration.

GLBP Fundamentals Link to heading

GLBP Roles: AVG and AVF Link to heading

GLBP uses two distinct roles:

AVG — Active Virtual Gateway:

• There is exactly one AVG per GLBP group
• The AVG owns the virtual IP address and responds to ARP requests
• The AVG assigns virtual MAC addresses (0007.b400.XXYY) to each AVF
• The AVG controls which virtual MAC is returned in each ARP reply — this is how load balancing is implemented
• AVG election: highest priority wins; highest IP breaks ties

AVF — Active Virtual Forwarder:

• There are up to four AVFs per GLBP group
• Each AVF owns one virtual MAC address and forwards traffic for hosts that ARPed for that MAC
• The AVG itself is always an AVF (AVF #1)
• Other group members are also AVFs, each with their own virtual MAC

Load-Balancing Algorithms Link to heading

Round-robin (default): Each incoming ARP request is answered with the next virtual MAC in rotation. Over time, hosts are evenly distributed across all AVFs. New hosts are balanced immediately.

Weighted: Traffic distribution is proportional to each AVF’s weight value. An AVF with weight 200 receives twice the ARP assignments of an AVF with weight 100. Useful when routers have different capacity.

Host-dependent: Uses the requesting host’s MAC address to deterministically select a virtual MAC. The same host always receives the same gateway virtual MAC — useful when consistent gateway assignment is required (e.g., for firewall policy tracking).

GLBP Weighting and AVF Eligibility Link to heading

Each GLBP member has a weight value (default 100, range 1–254). The weight determines:

1. Whether the router participates as an AVF (weight must be above the lower threshold)
2. Traffic distribution when the weighted algorithm is used

Weight thresholds:

• Lower threshold: If a router’s weight falls below this, it stops being an AVF and gives up its virtual MAC
• Upper threshold: A router does not become an AVF again until its weight recovers above this

glbp 1 weighting 100 lower 20 upper 80

Weight is typically reduced by interface tracking — when a tracked interface (WAN uplink) goes down, the router’s GLBP weight is decremented, potentially causing it to stop being an AVF.

Pre-emption Link to heading

GLBP has two types of pre-emption:

• AVG pre-emption: Determines whether a higher-priority router takes back the AVG role — disabled by default (same as HSRP)
• AVF pre-emption: Determines whether a router with recovered weight reclaims its AVF role — enabled by default

! Enable AVG pre-emption with delay
glbp 1 preempt delay minimum 60

Configuration Link to heading

Basic GLBP Configuration Link to heading

! On R1 (primary — higher priority)
interface GigabitEthernet0/1
 ip address 10.10.10.2 255.255.255.0
 glbp 1 ip 10.10.10.1
 glbp 1 priority 120
 glbp 1 preempt delay minimum 60

! On R2 (secondary)
interface GigabitEthernet0/1
 ip address 10.10.10.3 255.255.255.0
 glbp 1 ip 10.10.10.1
 glbp 1 priority 100

With this configuration:

• R1 becomes the AVG (priority 120 > 100) and AVF #1
• R2 becomes AVF #2
• The virtual IP 10.10.10.1 is shared by both
• ARP replies rotate between the two virtual MACs — half the hosts use R1, half use R2

Load-Balancing Algorithm Selection Link to heading

! Round-robin (default — no configuration needed)
glbp 1 load-balancing round-robin

! Weighted — traffic proportional to GLBP weight
glbp 1 load-balancing weighted

! Host-dependent — consistent gateway per host MAC
glbp 1 load-balancing host-dependent

Weighting and Interface Tracking Link to heading

! Track WAN uplink
track 1 interface GigabitEthernet0/0 line-protocol

interface GigabitEthernet0/1
 glbp 1 ip 10.10.10.1
 glbp 1 priority 120
 glbp 1 preempt delay minimum 60
 glbp 1 weighting 100 lower 20 upper 80
 glbp 1 weighting track 1 decrement 60

If GigabitEthernet0/0 goes down, the weight decrements by 60 (from 100 to 40). Since 40 is still above the lower threshold of 20, the router remains an AVF. If a second tracked interface also fails, weight drops to below 20, and the router gives up its AVF role.

Verification Link to heading

show glbp
show glbp brief
show glbp GigabitEthernet0/1
show glbp GigabitEthernet0/1 detail

show glbp brief shows the group, virtual IP, state (Active/Standby/Listen for AVG role), weight, and which virtual MAC each router owns. Look for all expected AVFs showing as Active and the correct virtual MACs assigned.

GLBP vs HSRP vs VRRP Link to heading

Operational Considerations Link to heading

ARP cache timeout: GLBP load balancing works at ARP time — a host is assigned a gateway MAC when it ARPs. If a host has a long ARP cache timeout, it will continue using its originally assigned gateway for an extended period even if the load has shifted. ARP cache entries are typically 4 hours on Windows and 1–2 hours on Linux. In practice, GLBP distribution normalises over time as ARP entries expire and are refreshed.

Asymmetric traffic: In GLBP, outbound traffic from a host goes to its assigned AVF; return traffic from external networks arrives on whichever router the routing protocol delivers it to — which may not be the same router. This asymmetric forwarding is normal and handled correctly by both routers, but be aware of it in stateful firewall deployments where connection tracking requires traffic to pass through the same device in both directions.

Maximum four AVFs: GLBP supports a maximum of four simultaneous active forwarders per group. In most LAN deployments, two or three active gateways is sufficient and provides an excellent utilisation-to-complexity ratio.