![]() Router B will also assume the role of the AVG for the entire GLBP group. If Router A becomes unavailable, Client 1 will not lose access to the WAN because Router B will assume responsibility for forwarding packets sent to the virtual MAC address of Router A, and for responding to packets sent to its own virtual MAC address. Client 2 shares the same default gateway IP address but receives the gateway MAC address 0007.b400.0102 because Router B is sharing the traffic load with Router A. Client 1 has a default gateway IP address of 10.21.8.10 and a gateway MAC address of 0007.b400.0101. Router B is a member of the same GLBP group and is designated as the AVF for the virtual MAC address 0007.b400.0102. Router A is also an AVF for the virtual MAC address 0007.b400.0101. In Figure 1, Router A is the AVG for a GLBP group, and is responsible for the virtual IP address 10.21.8.10. Load sharing is achieved by the AVG replying to the ARP requests with different virtual MAC addresses. The AVG is responsible for answering Address Resolution Protocol (ARP) requests for the virtual IP address. ![]() ![]() These gateways are known as active virtual forwarders (AVFs) for their virtual MAC address. Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. The AVG assigns a virtual MAC address to each member of the GLBP group. Other group members provide backup for the AVG in the event that the AVG becomes unavailable. Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group. GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address 224.0.0.102, User Datagram Protocol (UDP) port 3222 (source and destination). Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets. GLBP provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. Although multiple virtual router groups can be configured for the same set of routers, the hosts must be configured for different default gateways, which results in an extra administrative burden. These standby routers have unused bandwidth that the protocol is not using. The other routers in the group are redundant until the active router fails. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. HSRP and VRRP protocols allow multiple routers to participate in a virtual router group configured with a virtual IP address. GLBP performs a similar, but not identical, function for the user as the HSRP and the VRRP. Other routers on the LAN may act as redundant GLBP routers that will become active if any of the existing forwarding routers fail. Multiple first hop routers on the LAN combine to offer a single virtual first hop IP router while sharing the IP packet forwarding load. The Gateway Load Balancing Protocol feature provides automatic router backup for IP hosts configured with a single default gateway on an IEEE 802.3 LAN. To configure GLBP, you need to understand the following concepts: For each GLBP forwarder to be configured, an additional MAC address is used. ![]()
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