Practice Q's - GLBP Flashcards
What command provides the output shown in the exhibit? (Click on the Exhibit(s) button.)
A. switch# show glbp
B. switch# show standby
C. switch# show glbp status
D. switch# show standby brief
Answer: A
Explanation:
The command show glbp displays the output in the exhibit. This command displays detailed information about GLBP groups on the switch. In this scenario, the switch is a member of GLBP group 100, and it is the active switch. The output indicates that the virtual IP address is 192.168.8.10 and that preemption is enabled.
The command show glbp status does not exist on Cisco routers.
The command show standby brief is used to display a summary of the HSRP groups to which the switch belongs. The summary information it provides includes the group number, priority, state, active device address, standby address, and group address. This command is for HSRP only.
The command show standby can be used to display detailed information about HSRP groups to which a switch belongs. This command is for HSRP only.
What command would display a single line of information for each virtual gateway or virtual forwarder on a switch?
A. switch# show glbp
B. switch# show glbp brief
C. switch# show standby
D. switch# show standby brief
Answer: B
Explanation:
A brief single line view of virtual forwarder and virtual gateway information is provided with the command show glbp brief. Virtual forwarders and virtual gateways are terms used for GLBP groups. A brief output of GLBP information is provided with the brief key word. This output includes the interface, priority, state, and address of GLBP interfaces on the switch.
The command show glbp displays detailed information about GLBP groups on the switch. This information includes the GLBP groups the switch is a member of, whether this is the active switch, the virtual IP address, and whether preemption is enabled.
The command show standby brief is used to display a summary of the HSRP groups to which the switch belongs. The summary information it provides includes the group number, priority, state, active device address, standby address, and group address. This command is for HSRP only. The command show standby can be used to display detailed information about HSRP groups to which a switch belongs. This command is for HSRP only.
Which command enables GLBP on an interface?
A. glbp
B. glbp 10 ip 192.168.1.1
C. standby mode glbp
D. switchport mode glbp
Answer: B
Explanation:
The glbp ip interface configuration command enables Group Load Balancing Protocol (GLBP). The syntax for this command is as follows:
switch(config-if)# glbp group-number ip ip-address
The following example activates GLBP for group 5 on Fast Ethernet interface 1/0. The virtual IP address to be used by the GLBP group is set to 10.5.5.5. The default gateway of each host should be set to the virtual IP address.
switch(config)# interface FastEthernet 1/0
switch(config-if)# ip address 10.5.5.1 255.255.255.0
switch(config-if)# glbp 5 ip 10.5.5.5
GLBP is a Cisco-designed protocol that provides for the dynamic use 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.
When a host issues an ARP to resolve its gateway’s MAC address, the active virtual gateway (AVG) will respond with the virtual MAC address of a selected active virtual forwarder (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, then another AVF can assume ownership of the virtual MAC.
Which protocol allows the network to fully use standby routers in a redundancy group without additional administrative burden?
A. HSRP
B. VRRP
C. GLBP
D. IRDP
Answer: C
Explanation:
Gateway Load Balancing Protocol (GLBP) allows the network to fully utilize standby routers in a redundancy group. Unlike HSRP and VRRP, GLBP allows automatic selection and simultaneous use of multiple gateways.
It also allows for router load balancing from a segment without using different host configuration as in HSRP. Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP) provide gateway redundancy, but only one router in a group can forward traffic for a redundancy group.
The bandwidth and resources associated with the non-actively forwarding routers are wasted.
GLBP allows this wasted bandwidth and resources to be utilized by providing automatic selection and use of multiple available gateways to destinations.
ICMP Router Discovery Protocol (IRDP) is an extension of the Internet Control Message Protocol (ICMP) that allows routers to advertise useful routes. IRDP does not require hosts to recognize routing protocols, nor does it require manual configuration.
The output displayed below is a result of what command?
- Interface Grp Fwd Pri State Address Active router Standby router*
- Vl10 10 - 254 Active 192.168.8.10 local unknown*
- Vl10 10 1 7 Active 0007.b400.0101 local -*
A. switch# show standby
B. switch# show glbp
C. switch# show standby brief
D. switch# show glbp brief
Answer: D
Explanation:
The output of the exhibit is provided with the command show glbp brief. This output includes the interface, priority, state, and address of GLBP interfaces on the switch. In this case, VLAN 10 is the active virtual gateway using IP address 192.168.8.10.
The command show glbp displays detailed information about GLBP groups on the switch. This information includes the GLBP groups the switch is a member of, whether this is the active switch, the virtual IP address, and whether preemption is enabled.
The command show standby brief is used to display a summary of the HSRP groups the switch is a member of. The summary information it provides includes the group number, priority, state, active device address, standby address, and group address. This command is for HSRP only.
The command show standby can be used to display detailed information about HSRP groups a switch is a member of. This command is for HSRP only.
Which load-balancing algorithm does GLBP use by default?
A. Random load-balancing
B. Weighted load-balancing
C. Round-robin load-balancing
D. Host-dependent load-balancing
Answer: C
Explanation:
Round-robin load-balancing is the default load-balancing method used by Gateway Load Balancing Protocol (GLBP).
The active virtual gateway (AVG) can be configured to use one of three load-balancing algorithms when selecting which active virtual forwarder (AVF) MAC address to use in its ARP response.
GLBP is a Cisco-designed protocol that provides for the dynamic use 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. When a host issues an ARP to resolve its gateway’s virtual IP address, the active virtual gateway (AVG) reply includes the virtual MAC address of a selected active virtual forwarder (AVF). The AVG is therefore responsible for performing load balancing, varying which virtual MAC it selects to use in the response.
The AVF will own that 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.
There is no random load balancing method.
GLBP and HSRP are Cisco-developed solutions. VRRP is defined in RFC 2338.
What command displays detailed information about the GLBP groups to which the switch belongs?
A. switch# show standby
B. switch# show glbp state
C. switch# show glbp
D. switch# show standby detail
Answer: C
Explanation:
The command show glbp displays detailed information about GLBP groups on the switch. This information includes the GLBP groups the switch is a member of, whether this is the active switch, the virtual IP address, and whether preemption is enabled. Below is an example of the command output.
The following can be learned from this output:
The show glbp state will only display the glbp state of the router (standby, listen etc). Detailed output is accomplished with the command show glbp.
The command show standby can be used to display detailed information about HSRP groups to which a switch belongs. This command is for HSRP only.
The command show standby detail provides the same output as show standby. It can be used to display detailed information about HSRP groups to which a switch belongs. This command is for HSRP only.
A company has the following network infrastructure. (Refer to the exhibit.)
Router A is a GLBP active virtual gateway with priority level set to 250. Routers B and C are configured with the default GLPB configurations. The configuration of the active virtual gateway needs to be changed such that if the AVG fails, Router C should be elected to be used as an active virtual gateway. As the network administrator, you have been asked to make corresponding changes to the configuration.
Which command would you use for this purpose on Router C, and where would the command be configured?
A. glbp 10 preempt (on Router B)
B. glbp 10 preempt (on Router C)
C. glbp 10 priority 200 (on Router B)
D. glbp 10 priority 200 (on Router C)
Answer: D
Explanation:
You would configure the glbp 10 priority 200 command on Router C to change the configuration as required. Gateway Load Balancing Protocol (GLBP) gateway priority determines the role that each GLBP gateway plays and what happens if the AVG fails.
In the given scenario, Router A is used as an active virtual gateway. If the AVG in a LAN topology fails, an election process takes place to determine which backup virtual gateway should take over. When you configure this command on Router C, Router C will be elected when Router A fails as an AVG.
Once the configuration change is made, it can be verified by examining the output if the show run command as shown below:
In the above output, it can be determined that the glpb priority 200 command has been applied to the gigabitEthernet0/0 interface on Router C. If the default priority of 100 had been applied, there would be no line in the output for priority. Because Router B is configured with the default configuration, it will have its priority set to the default level as 100.
You would not use the glbp 10 preempt command on Router B or the glbp 10 preempt command on Router C to change the configuration. You would use this command on a router to enable preemption. Preemption allows a virtual router that was once the AVG to assume its role as active virtual router when it comes back online if it has a higher priority than the current AVG.
Alternatively, it can enable a new router with a higher priority to take the role of AVG from the current AVG if the new router has a higher AVG.
You would use not the glbp 10 priority 200 command on Router B to change the configuration.
You would run this command if you needed Router B to be elected as the AVR instead of Router C, as running this command on Router B would configure it with higher priority than Router C.
Examine the configuration shown below in a partial output of the show run command executed from Router 5.
With this configuration, which of the following is true?
A. if POS 1/0 goes down, Router 5 will become the AVG.
B. if POS 1/0 goes down, Router 5 will relinquish its role as AVG.
C. if POS 1/0 and POS 2/0 go down, Router 5 will relinquish its role as AVG.
D. if POS 1/0 and POS 2/0 go down, Router 5 will become the AVG.
Answer: C
Explanation:
If POS 1/0 and POS 2/0 go down, Router 5 will relinquish its role as active virtual gateway (AVG).
The example indicates that Router 5 is configured a member of Gateway Load Balancing Protocol (GLBP) group 156 and that it is tracking the state of two of interfaces, POS 1/0 and 2/0. An initial weighting value of 120 has been assigned to the router in line 4, and upper and lower thresholds of 95 and 110 have been configured, respectively.
In lines 5 and 6, decrement values of 20 and 10 have assigned to the tracking of interface POS 1/0 and POS2/0, respectively. If either of the tracked interfaces goes down, the weight value of the router will be decreased by the amount of the decrement value.
If the weight falls below the lower threshold (95), the router will relinquish its role as the AVG. If either single interface that is being tracked goes down, the weight value will not fall below the lower threshold, and the router will
maintain its role of AVG. This occurs because 120 - 20 = 100, which is higher than 95 (the lowest threshold), and 120 - 10 = 110, which is also higher than 95. If both interfaces go down, the value will fall below the lower threshold (120 - 10 - 20 = 90, which is lower than 95), and the router will give up its role as the AVG.
The loss of interfaces will never cause a router to become the AVG. That can only occur if the router functioning as the current AVG in the group loses its interfaces.
The loss of either of the interfaces alone will not decrement the initial weighting value enough for it to fall below the lower threshold, and therefore will not cause the router to relinquish its role as AVG.
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
Answer: C,E
Explanation:
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:
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.
Which of the following features does GLBP provide, but not HSRP and VRRP? (Choose all that apply.)
A. Support for single active router
B. Support for automatic load balancing
C. Support for multiple gateways
D. Support for interface tracking
Answer: B,C
Explanation:
Support for automatic load balancing and support for multiple gateways are two features that are provided by Gateway Load Balancing Protocol (GLBP) but not by Hot Standby Routing Protocol (HSRP) or Virtual Router Redundancy Protocol (VRRP).
GLBP, HSRP, and VRRP provide a redundant and fault-tolerant solution in case of first-hop router failure in a network. The basic operation of these three protocols is the same.
In all three protocols, a group of routers on the same LAN is formed. One of the routers is selected as the active router and another as the standby router. The router with the highest priority is automatically selected as the active router. If the active router fails, the standby router assumes the responsibilities of the active router. The role of the active router is to forward the packets from the hosts to the virtual router (default gateway).
GLBP provides automatic load balancing between multiple routers by configuring multiple MAC addresses but a single virtual IP address. Every active virtual forwarder (AVF) in the group is configured with the virtual IP address but with different MAC addresses. All such AVFs can then participate in the packet-forwarding process. Multiple gateways then can share the load.
On the contrary, HSRP and VRRP do not support automatic load balancing. Both these protocols require additional configuration on all the routers that need to load balance. The additional configuration involves using multiple groups on the routers or assignment of different default gateways for the hosts.
Note that GLBP and VRRP are supported by both Cisco and non-Cisco routers, whereas, HSRP is supported only by Cisco routers.
Single active router and interface tracking both are supported by GLBP, HSRP, and VRRP.
What is the maximum number of virtual MAC addresses that GLBP allows per group?
A. 2
B. 4
C. 6
D. 8
Answer: B
Explanation:
GLBP Virtual MAC Address Assignment
A GLBP group allows up to four virtual MAC addresses per group. The AVG is responsible for
assigning the virtual MAC addresses to each member of the group. Other group members request
a virtual MAC address after they discover the AVG through hello messages. Gateways are
assigned the next MAC address in sequence. A virtual forwarder that is assigned a virtual MAC
address by the AVG is known as a primary virtual forwarder. Other members of the GLBP group
learn the virtual MAC addresses from hello messages. A virtual forwarder that has learned the
virtual MAC address is referred to as a secondary virtual forwarder.
Which gateway role is responsible for answering ARP requests for the virtual IP address in GLBP?
A. active virtual forwarder
B. active virtual router
C. active virtual gateway
D. designated router
Answer: C
Explanation:
GLBP Active Virtual Gateway
Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that
group. Other group members provide backup for the AVG in the event that the AVG becomes
unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group. Each
gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned
to it by the AVG. These gateways are known as active virtual forwarders (AVFs) for their virtual
MAC address.
The AVG is responsible for answering Address Resolution Protocol (ARP) requests for the
virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests with
different virtual MAC addresses.
Refer to the exhibit.
%GLBP-4-DUPADDR: Dulplicate address
Which option describes the reason for this message in a GLBP configuration?
A. Unavailable GLBP active forwarder
B. Incorrect GLBP IP address
C. HSRP configured on same interface as GLBP
D. Layer 2 loop
Answer: D
Explanation:
This section provides information you can use to troubleshoot your configuration.
%GLBP-4-DUPADDR: Duplicate address
The error message indicates a possible layer2 loop and STP configuration issues.
In order to resolve this issue, issue the show interface command to verify the MAC address of the
interface. If the MAC address of the interface is the same as the one reported in the error
message, then it indicates that this router is receiving its own hello packets sent. Verify the
spanning-tree topology and check if there is any layer2 loop. If the interface MAC address is
different from the one reported in the error message, then some other device with a MAC address
reports this error message.
Note: GLBP members communicate between each other through hello messages sent every 3
seconds to the multicast address 224.0.0.102 and User Datagram Protocol (UDP) port 3222
(source and destination). When configuring the multicast boundary command, permit the Multicast
address by permit 224.0.0.0 15.255.255.255.
