Glbp Overview; Glbp Active Virtual Gateway; Glbp Virtual Mac Address Assignment; Chapter 18 Configuring Glbp - Cisco Nexus 7000 Series Configuration Manual

Information About GLBP
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GLBP Overview

GLBP
IEEE 802.3 LAN. Multiple routers on the LAN combine to offer a single virtual first-hop IP gateway
while sharing the IP packet forwarding load. Other routers on the LAN may act as redundant GLBP
gateways that become active if any of the existing forwarding gateways fail.
GLBP performs a similar function to the Hot Standby Redundancy Protocol (HSRP) and the Virtual
Router Redundancy Protocol (VRRP). HSRP and VRRP allow multiple routers to participate in a virtual
group configured with a virtual IP address. These protocols elect one member as the active router to
forward packets to the virtual IP address for the group. The other routers in the group are redundant until
the active router fails.
GLBP performs an additional load balancing function that the other protocols do not provide. GLBP load
balances over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC
addresses. GLBP shares the forwarding load among all routers in a GLBP group instead of allowing a
single router to handle the whole load while the other routers remain idle. You configure each host with
the same virtual IP address, and all routers in the virtual group participate in forwarding packets. GLBP
members communicate between each other using periodic hello messages.

GLBP Active Virtual Gateway

GLBP prioritizes gateways to elect an active virtual gateway (AVG). If multiple gateways have the same
priority, the gateway with the highest real IP address becomes the AVG. The AVG assigns a virtual MAC
address to each member of the GLBP group. Each member is the active virtual forwarder (AVF) for its
assigned virtual MAC address, forwarding packets sent to its assigned virtual MAC address.
The AVG also answers Address Resolution Protocol (ARP) requests for the virtual IP address. Load
sharing is achieved when the AVG replies to the ARP requests with different virtual MAC addresses.
In Cisco NX-OS Release 4.1(2) and later, packets received on a routed port destined for the GLBP virtual
Note
IP address will terminate on the local router, regardless of whether that router is the active GLBP router
or a redundant GLBP router. This includes ping and telnet traffic. Packets received on a Layer 2 (VLAN)
interface destined for the GLBP virtual IP address will terminate on the active router.

GLBP Virtual MAC Address Assignment

The AVG assigns the virtual MAC addresses to each member of the group. The group members request
a virtual MAC address after they discover the AVG through hello messages. The AVG assigns the next
MAC address based on the load-balancing algorithm selected (see the
Tracking" section on page
the primary virtual forwarder. The other members of the GLBP group that learn the virtual MAC
addresses from hello messages are secondary virtual forwarders.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
18-2
provides automatic gateway
18-5). A gateway that is assigned with a virtual MAC address by the AVG is
backup for IP hosts configured with a single default gateway on an
Chapter 18 Configuring GLBP
"GLBP Load Balancing and
OL-20002-02