Introducing GLBP

This topic introduces GLBP.

Allows automatic selection and use of multiple, available gateways to destination

Provides automatic detection and rerouting in the event of failure to any gateway

Fully utilizes resources (available bandwidth) without administrative burden

© 2004, Cisco Systems, Inc. All rights reserved. BCMSN v2.1—6-53

While HSRP and VRRP provide gateway resiliency, the standby members of the redundancy group are underutilized along with their upstream bandwidth. Only the active router for the HSRP group handles traffic for the virtual MAC. Bandwidth and resources associated with the standby router are not fully utilized.

Cisco designed GLBP to allow automatic selection and simultaneous use of multiple, available gateways, and to provide automatic detection and failover to a redundant path in the event of failure to any active gateway. With GLBP, you can fully utilize resources without the extra administrative burden of configuring multiple groups and managing multiple default gateway configurations.

GLBP Operation

GLBP supports these operational modes for load balancing:

■ Weighted load-balancing algorithm: The amount of load directed to an active virtual forwarder is dependent upon the weighting value advertised by the gateway containing that active virtual forwarder.

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: Each virtual forwarder MAC address takes turns being included in address resolution replies for the virtual IP address.

GLBP allows automatic selection and simultaneous use of multiple, available gateways. The members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group. Other members of the group provide backup for the AVG if it should become unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group. These gateways become the active virtual forwarder (AVF) for that virtual MAC address and forwards packets sent to the virtual MAC address.

In the default mode, GLBP will attempt to balance traffic on a per-host basis using a round-robin scheme, as shown in the figure. When a device sends an ARP message for the gateway IP address, the AVG will return a MAC address based on a load-balancing algorithm. When a second device sends an ARP message, the AVG returns the next virtual MAC from the list of available gateways.

Note Only the AVG responds to ARP requests. The AVG is responsible for managing the members of the GLBP group.

GLBP Operation (Cont.)

© 2004, Cisco Systems, Inc. All rights re

© 2004, Cisco Systems, Inc. All rights re

Now, clients A and B send their off-net traffic to separate routers; this is because they each have cached a different MAC address for the single virtual gateway IP address (in this case, Each GLBP router is an active virtual forwarder for the MAC address that it has been assigned.

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