GLBP
One issue with both HSRP and VRRP is that only the primary router is in
use, the others must wait for the primary to fail before they are used. These
two protocols use groups to get around that limitation. However, Gateway
Load Balancing Protocol (GLBP) allows the simultaneous use of up to four
gateways, thus maximizing bandwidth. With GLBP, there is still one virtual
IP address. However, each participating router has a virtual MAC address,
and different routers’ virtual MAC addresses are sent in answer to ARPs sent
to the virtual IP address. GLBP can also use groups up to a maximum of
1024 per physical interface.
The load sharing is done in one of three ways:
■ Weighted load balancing—Traffic is balanced proportional to a configured
weight.
■ Host-dependent load balancing—A given host always uses the same
router.
■ Round-robin load balancing—Each router MAC is used to respond to
ARP requests in turn.
GLBP routers elect an Active Virtual Gateway (AVG). It is the only router to
respond to ARPs. It uses this capacity to balance the load among the GLBP
routers. The highest priority router is the AVG; the highest configured IP
address is used in case of a tie.
The actual router used by a host is its Active Virtual Forwarder (AVF).
GLBP group members multicast hellos every 3 seconds to IP address
224.0.0.102, UDP port 3222. If one router goes down, another router
answers for its MAC address.
Configure GLBP with the interface command glbp group-number ip virtual-
IP-address, as shown:
Router(config-if)#glbp 39 ip 10.0.0.1
To ensure deterministic elections, each router can be configured with a priority.
The default priority is 100:
Router(config-if)#glbp 39 priority 150
Hello and hold (or dead) timers can be configured for each interface with the
command glbp group-number timers [msec] hello-time [msec] hold-time.
Values are in seconds unless the msec keyword is used.
GLBP can also track interfaces; if an interface goes down, another router
answers for the first router’s MAC address.