configuring multiple groups and managing multiple default gateway configurations as is required with HSRP and VRRP. GLBP has the following functions:

Active virtual gateway (AVG): Members of a GLBP group elect one gateway to be the AVG for that group. Other group members provide backup for the AVG if the AVG becomes unavailable. The AVG assigns a virtual MAC address to each member of the group.
Active virtual forwarder (AVF): Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. These gateways are known as AVFs for their virtual MAC address.
Communication: GLBP members communicate with each other using hello messages sent every 3 seconds to the multicast address 224.0.0.102, User Datagram Protocol (UDP) port 3222.

GLBP has the following features:

Load sharing: Traffic from LAN clients can be shared by multiple routers.
Multiple virtual routers: Up to 1,024 virtual routers (GLBP groups) can be on each physical interface of a router, and there can be up to four virtual forwarders per group.
Preemption: You can preempt an AVG with a higher priority backup virtual gateway. Forwarder preemption works in a similar way, except that it uses weighting instead of priority and is enabled by default.
Efficient resource utilization: Any router in a group can serve as a backup, which eliminates the need for a dedicated backup router because all available routers can support network traffic.

GLBP provides upstream load sharing by utilizing the redundant uplinks simultaneously. It uses link capacity efficiently, thus providing peak-load traffic coverage. By making use of multiple available paths upstream from the routers or Layer 3 switches running GLBP, output queues may also be reduced. HSRP and VRRP use only a single path; other paths are idle, unless multiple groups and gateways are configured. The single path may encounter higher output queue rates during peak times, which leads to lower performance from higher jitter rates. The impact of jitter is lessened and overall performance is improved with GLBP, because more upstream bandwidth is available and additional upstream paths are used.

Content 5.3 Configuring Layer 3 Redundancy with VRRP and GLBP 5.3.5 Identifying the GLBP Operations Process GLBP allows automatic selection and simultaneous use of all available gateways in the group. The members of a GLBP group elect one gateway to be the AVG for that group. Other members of the group provide backup for the AVG if it becomes unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group. All routers become AVFs for frames addressed to that virtual MAC address. As clients send Address Resolution Protocol (ARP) requests for the address of the default gateway, the AVG sends these virtual MAC addresses in the ARP replies. A GLBP group can have up to four group members. GLBP supports the following operational modes for load balancing traffic across multiple default routers servicing the same default gateway IP address:

Weighted load-balancing algorithm: The amount of load directed to a router is dependent upon the weighting value advertised by that router.
Host-dependent load-balancing algorithm: A host is guaranteed to use the same virtual MAC address as long as that virtual MAC address is participating in the GLBP group.
Round-robin load-balancing algorithm: As clients send ARP requests to resolve the MAC address of the default gateway, the reply to each client contains the MAC address of the next possible router in round-robin fashion. All routers’ MAC addresses take turns being included in address resolution replies for the default gateway IP address.

GLBP automatically manages the virtual MAC address assignment, determines who handles the forwarding, and ensures that each station has a forwarding path for failures to gateways or tracked interfaces. If failures occur, the load-balancing ratio is adjusted among the remaining AVFs so that resources are used in the most efficient way. As shown in Figure , GLBP attempts to balance traffic on a per-host basis using the round-robin algorithm. Figure describes how GLBP balances traffic using the round-robin algorithm. In Figure , clients A and B have each resolved a different MAC address for the default gateway, so they send their routed traffic to separate routers, although they both have the same default gateway address configured. Each GLBP router is an AVF for the virtual MAC address to which it has been assigned. Like HSRP, GLBP can be configured to track interfaces. In Figure , the WAN link from router R1 is lost, and GLBP detects the failure. Because interface tracking was configured on R1, the job of forwarding packets for virtual MAC address 0000.0000.0001 is taken over by the secondary virtual forwarder for the MAC, which is router R2. Therefore, the client sees no disruption of service nor does it need to resolve a new MAC address for the default gateway. GLBP is supported on select Cisco Catalyst platforms. Figure illustrates the GLBP interface commands. Figure describes the command parameters. Figure describes the steps needed to configure GLBP. The following example configures GLBP on two multilayer switches: SwitchA(config)#interface vlan7
SwitchA(config-if)#ip address 10.1.7.5 255.255.255.0
SwitchA(config-if)#glbp 7 ip 10.1.7.1
SwitchA(config-if)#glbp 7 priority 150
SwitchA(config-if)#glbp 7 timers msec 250 msec 750 SwitchB(config)#interface vlan7
SwitchB(config-if)#ip address 10.1.7.6 255.255.255.0
SwitchB(config-if)#glbp 7 ip 10.1.7.1
SwitchB(config-if)#glbp 7 priority 100
SwitchB(config-if)#glbp 7 timers msec 250 msec 750 SwitchA#show glbp 7
Vlan7 - Group 7
State is Active
2 state changes, last state change 23:50:33
Virtual IP address is 10.1.7.1
Hello time 250 msec, hold time 750 msec
Next hello sent in 40 msecs
Redirect time 600 sec, forwarder time-out 7200 sec
Authentication text "stringabc"
Preemption enabled, min delay 60 sec
Active is local
Standby is unknown
Priority 254 (configured)
Weighting 105 (configured 110), thresholds: lower 95, upper 105
Track object 2 state Down decrement 5
Load balancing: host-dependent
There is 1 forwarder (1 active)
Forwarder 1
State is Active
1 state change, last state change 23:50:15
MAC address is 0007.b400.0101 (default)
Owner ID is 0005.0050.6c08
Redirection enabled
Preemption enabled, min delay 60 sec

Content 5.4 High Availability Lab Exercise 5.4.1 Lab 5-1 Hot Standby Router Protocol
Lab Activity Lab Exercise: Lab 5-1 Hot Standby Router Protocol Configure inter-VLAN routing with HSRP to provide redundant, fault tolerant routing to the internal network.

Content Summary Device, link, or hardware component redundancy at strategic points in the network leads to high availability. Hot Standby Router Protocol (HSRP) provides router redundancy to network hosts and can be optimized in several ways. Virtual Router Redundancy Protocol (VRRP) and Gateway Load Balancing Protocol (GLBP) were derived from HSRP and provide additional redundancy features.