Which virtual router states are defined in the GLBP protocol? (Choose two.)
A. Backup gateway
B. Primary gateway
C. Active virtual gateway
D. Active secondary gateway
E. Active virtual forwarder
Correct Answer: CE
Explanation/Reference:
Answer:
Active virtual gateway and active virtual forwarder are the two states defined in the Gateway Load Balancing Protocol (GLBP). The active virtual gateway (AVG) is elected by the members of the GLBP group. The AVG creates the virtual MAC addresses that are assigned to each of the routers in the group. Each router is responsible for handling packets sent to its virtual MAC address. A GLBP router that forwards packets sent to its virtual MAC address is known as the active virtual forwarder (AVF). GLBP members communicate through hello messages sent every 3 seconds to the multicast address 224.0.0.102.
The election of the AVG can be influenced by use of the priority command. By default, all routers configured for GLBP have a priority of 100. A higher value indicates a higher priority. The configured priority of a router can be seen in the show run command as shown below:
In the above scenario, all other members of the group were left to the default, which can be determined on those routers by the absence of any priority entry in the show run command. In that case, this router would become the AVG. To remove a priority configuration, execute the nostandby priority command. When this command is executed, the router will revert to the default of 100. When all routers are left to the default, the router with the highest configured IP address will become the active router.
GLBP is a Cisco-designed protocol that provides for the dynamic utilization of redundant routers in a broadcast network. It differs from HSRP and VRRP in that it is not necessary to configure multiple groups to fully use redundant paths or routers. GLBP has a configurable load-balancing mechanism that will distribute the use of redundant gateways servicing a broadcast network, such as an Ethernet LAN. Each host will have its gateway set to the address of the AVG. When a host issues an ARP to resolve its gateway’s MAC Address, the AVG will respond with the virtual MAC address of a selected AVF. The AVG will perform load balancing by varying which virtual MAC it selects to use in the response. The AVF will own that assigned virtual MAC as long as the gateway is active. If an AVF becomes unable to provide service as gateway, another AVF can assume ownership of the virtual MAC.
Consider the partial output of the show run command for two routers participating in the GLBP group shown below:
In the above scenario, both routers have the same priority, so Router B will become AVG. Hosts will use a gateway address of 192.168.5.5 (the GLBP virtual address in line 4 of both outputs). When hosts send an ARP message for the MAC address of the gateway, Router B will reply with the MAC address of the next AVF.
The AVG can be configured to use one of three load-balancing algorithms:
Round-Robin Load-Balancing: Using round-robin load- balancing the AVG in turn points to each AVF virtual MAC address in its ARP reply (default method).
Weighted Load-Balancing: Using weighted load- balancing, the AVG selects an AVF virtual MAC address to use in the ARP reply proportionally based on the advertised weight value configured in a GLBP gateway.
Host Dependant Load-Balancing: Using host-dependant load- balancing, the AVG selects an AVF virtual MAC address to use in the ARP reply based on which one the host used previously. A host will use the same AVF as long as the GLBP group is unchanged.
GLBP allows better use of network resources by using the standby router through the load-balancing mechanism. The standby router is an available gateway for the network.
GLBP and HSRP are Cisco-developed solutions. VRRP is defined in RFC 2338.
Backup gateway, primary gateway, and active secondary gateway are not terms used when discussing GLBP.
Objective:
Infrastructure Services Sub-Objective:
Configure and verify first-hop redundancy protocols
References:
Cisco > Cisco IOS IP Application Services Configuration Guide, Release 12.4 > Part 1: First Hop Redundancy Protocols > Configuring GLBP
Cisco > First Hop Redundancy Protocols Configuration Guide, Cisco IOS Release 12.4 > Configuring GLBP > GLBP Active Virtual Gateway
Explanation/Reference:
Answer:
Active virtual gateway and active virtual forwarder are the two states defined in the Gateway Load Balancing Protocol (GLBP). The active virtual gateway (AVG) is elected by the members of the GLBP group. The AVG creates the virtual MAC addresses that are assigned to each of the routers in the group. Each router is responsible for handling packets sent to its virtual MAC address. A GLBP router that forwards packets sent to its virtual MAC address is known as the active virtual forwarder (AVF). GLBP members communicate through hello messages sent every 3 seconds to the multicast address 224.0.0.102.
The election of the AVG can be influenced by use of the priority command. By default, all routers configured for GLBP have a priority of 100. A higher value indicates a higher priority. The configured priority of a router can be seen in the show run command as shown below:
In the above scenario, all other members of the group were left to the default, which can be determined on those routers by the absence of any priority entry in the show run command. In that case, this router would become the AVG. To remove a priority configuration, execute the nostandby priority command. When this command is executed, the router will revert to the default of 100. When all routers are left to the default, the router with the highest configured IP address will become the active router.
GLBP is a Cisco-designed protocol that provides for the dynamic utilization of redundant routers in a broadcast network. It differs from HSRP and VRRP in that it is not necessary to configure multiple groups to fully use redundant paths or routers. GLBP has a configurable load-balancing mechanism that will distribute the use of redundant gateways servicing a broadcast network, such as an Ethernet LAN. Each host will have its gateway set to the address of the AVG. When a host issues an ARP to resolve its gateway’s MAC Address, the AVG will respond with the virtual MAC address of a selected AVF. The AVG will perform load balancing by varying which virtual MAC it selects to use in the response. The AVF will own that assigned virtual MAC as long as the gateway is active. If an AVF becomes unable to provide service as gateway, another AVF can assume ownership of the virtual MAC.
Consider the partial output of the show run command for two routers participating in the GLBP group shown below:
In the above scenario, both routers have the same priority, so Router B will become AVG. Hosts will use a gateway address of 192.168.5.5 (the GLBP virtual address in line 4 of both outputs). When hosts send an ARP message for the MAC address of the gateway, Router B will reply with the MAC address of the next AVF.
The AVG can be configured to use one of three load-balancing algorithms:
Round-Robin Load-Balancing: Using round-robin load- balancing the AVG in turn points to each AVF virtual MAC address in its ARP reply (default method).
Weighted Load-Balancing: Using weighted load- balancing, the AVG selects an AVF virtual MAC address to use in the ARP reply proportionally based on the advertised weight value configured in a GLBP gateway.
Host Dependant Load-Balancing: Using host-dependant load- balancing, the AVG selects an AVF virtual MAC address to use in the ARP reply based on which one the host used previously. A host will use the same AVF as long as the GLBP group is unchanged.
GLBP allows better use of network resources by using the standby router through the load-balancing mechanism. The standby router is an available gateway for the network.
GLBP and HSRP are Cisco-developed solutions. VRRP is defined in RFC 2338.
Backup gateway, primary gateway, and active secondary gateway are not terms used when discussing GLBP.
Objective:
Infrastructure Services Sub-Objective:
Configure and verify first-hop redundancy protocols
References:
Cisco > Cisco IOS IP Application Services Configuration Guide, Release 12.4 > Part 1: First Hop Redundancy Protocols > Configuring GLBP
Cisco > First Hop Redundancy Protocols Configuration Guide, Cisco IOS Release 12.4 > Configuring GLBP > GLBP Active Virtual Gateway
