GLBP Configuration on Cisco IOS
Learn how one virtual gateway IP can use multiple active forwarders, then configure selection, weighting, tracking, and failover.
Gateway Redundancy Learning Path
Start with the virtual-gateway idea, configure each major protocol, then compare and troubleshoot complete designs.
In This Lesson
Learn why GLBP separates gateway coordination from packet forwarding. Then configure the AVG, AVFs, weighting, and tracking so load sharing does not hide a weak failure design.
Quick Learning Map
Keep these three decisions in view as you work through the detailed lesson.
Elect the AVG
One router coordinates the virtual gateway and answers ARP requests.
Assign forwarding roles
AVFs use different virtual MAC addresses to share host traffic.
Track and rebalance
Weighting removes unhealthy forwarders while healthy paths continue.
GLBP Configuration on Cisco IOS at a Glance
Use this summary to establish the big picture before moving into commands, examples, and troubleshooting.
Coordinator
The AVG manages the group and assigns virtual MAC addresses.
Forwarders
Multiple AVFs can carry traffic for the same virtual IP.
Trade-off
Load sharing adds useful capacity but also more behavior to verify.
What makes GLBP different?
Gateway Load Balancing Protocol (GLBP) provides first-hop redundancy while allowing multiple routers to forward client traffic for one virtual IP. The Active Virtual Gateway (AVG) answers ARP requests and assigns virtual MAC addresses. Active Virtual Forwarders (AVFs) forward frames sent to those MACs.
One virtual IP
Every client can use 192.168.10.1 as its default gateway.
Several virtual MACs
The AVG distributes virtual MAC answers so different clients use different AVFs.
Forwarder takeover
A surviving member can take responsibility for a failed forwarder’s virtual MAC.
AVG and AVF election
The member with the highest gateway priority is preferred as AVG; an IP-address tie-breaker applies when priorities match. Forwarder roles use weighting and related election logic. AVG and AVF are separate jobs, so a router can coordinate ARP replies while multiple group members forward data.
Configure GLBP
DSW1
DSW1(config)# interface vlan 10 DSW1(config-if)# ip address 192.168.10.2 255.255.255.0 DSW1(config-if)# glbp 10 ip 192.168.10.1 DSW1(config-if)# glbp 10 priority 110 DSW1(config-if)# glbp 10 preempt DSW1(config-if)# glbp 10 load-balancing round-robin DSW1(config-if)# no shutdown
DSW2
DSW2(config)# interface vlan 10 DSW2(config-if)# ip address 192.168.10.3 255.255.255.0 DSW2(config-if)# glbp 10 ip 192.168.10.1 DSW2(config-if)# glbp 10 priority 100 DSW2(config-if)# glbp 10 preempt DSW2(config-if)# glbp 10 load-balancing round-robin DSW2(config-if)# no shutdown
Common load-balancing methods are round-robin, weighted, and host-dependent. Feature and platform support varies, so validate GLBP availability and exact syntax on the target software.
Weighting and tracking
Gateway priority controls AVG preference. Weighting
DSW1(config)# track 10 interface GigabitEthernet1/0/48 line-protocol DSW1(config)# interface vlan 10 DSW1(config-if)# glbp 10 weighting 110 lower 90 upper 105 DSW1(config-if)# glbp 10 weighting track 10 decrement 25
Verify GLBP
DSW1# show glbp brief Interface Grp Fwd Pri State Address Active router Standby router Vl10 10 - 110 Active 192.168.10.1 local 192.168.10.3 Vl10 10 1 - Active 0007.b400.0a01 local - Vl10 10 2 - Listen 0007.b400.0a02 192.168.10.3 - DSW1# show glbp DSW1# show track Client# arp -a
Sample several clients rather than one. Their ARP tables should show the same virtual IP associated with potentially different GLBP virtual MACs. Then fail each member and confirm that the surviving device takes over the affected forwarder.
GLBP troubleshooting
| Symptom | Focus |
|---|---|
| All hosts use one router | Load-balancing method, ARP cache age, AVF state, platform support |
| AVG is healthy but some hosts fail | Inspect each AVF and its virtual MAC ownership |
| Failed uplink keeps forwarding role | Track state, weighting decrement, lower threshold |
| Peers form different groups | VLAN, group, virtual IP, authentication, timers |
Load Sharing Is a Design Choice
GLBP can distribute hosts across multiple active virtual forwarders, but that does not automatically mean the network is balanced. The selected algorithm, host distribution, uplink capacity and traffic patterns all affect the result. A busy server VLAN may need a different approach from a user access VLAN.
Tracking is valuable because gateway availability is not the same as end-to-end reachability. If a router still has a local interface but has lost its useful upstream path, tracking can reduce its weighting or remove it from forwarding service. Select tracked objects that represent the service users actually depend on.
During verification, check the AVG, each AVF, weights, virtual MAC assignments and client traffic. Test both a forwarder failure and a gateway failure, then confirm that hosts recover without receiving an unexpected default gateway.
GLBP FAQs
Does GLBP require multiple default-gateway IPs?
No. Clients use one virtual IP; the AVG varies the virtual MAC returned in ARP replies.
Is GLBP always better than HSRP or VRRP?
No. Confirm platform support, operational familiarity, convergence requirements, and whether host-based sharing is actually needed.
What happens when an AVF fails?
A surviving member can assume the failed forwarder’s virtual MAC so clients already using it retain gateway service.
Next: Compare and Troubleshoot FHRP