Multilayer Switching: VRRP and GLBP

continued… VVRP has preemption enabled by default.


The primary purpose of the Gateway Load Balancing Protocol (GLBP) is just that – load balancing!

Cisco proprietary

GLBP uses a round robin format to distribute traffic to all group routers.

Multiple gateways in use at the same time.

GLBP also allows standard configuration of the hosts, who will all have their gateway address set to the virtual router’s address.

Key Concept:

The key to GLBP is that when a host sends an ARP request for the MAC of the virtual router, one of the physical routers will answer with its own MAC address. The host will then have the IP address of the GLBP virtual router and the MAC address of a physical router in the group.

 The Active Virtual Gateway (AVG) will be the router with the highest GLBP priority, and this router will send back ARP responses containing different virtual MAC addresses.

The three hosts will have the same Layer 3 address for their gateway, but a different L2 address.

The routers receiving and forwarding traffic received on this virtual MAC address are Active Virtual Forwarders (AVFs).

GLBP routers use Hellos to detect whether other routers in their group are available or not.

GLBP groups can have up to four members.

If the AVG fails, the router serving as the standby AVG will take over.

GLBP offers three different forms of MAC address assignment, the default being round-robin. With round-robin assignments, a host that sends an ARP request will receive a response containing the next virtual MAC address in line.

If a host or hosts need the same MAC gateway address every time it sends an ARP request, host-dependent load balancing is the way to go.

Weighted MAC assignments affect the percentage of traffic that will be sent to a given AVF. The higher the assigned weight, the more often that particular router’s virtual MAC will be sent to a requesting host.


MLS(config-if)# glbp 5 priority 150
MLS(config-if)# glbp 5 preempt

GLBP Weighting

A router can be configured to give up its role as the AVF if its overall weight drops below a configured value. The default weight of a GLBP AVF is 100. The router is configured with upper and lower weight thresholds, and should the router’s weight fall below its lower threshold, it gives up the role of AVF. When the router’s GLBP weight exceeds the higher threshold, it resumes the role of GLBP AVF. Before configuring the GLBP-specific commands, we configure track statements to number and name the interfaces being tracked.

Author: drbabbers - my personal journey to ccie