13.9 Default Gateway Router Redundancy

Question 1

Describe First Hop Redundancy Protocol (FHRP)

Answer 1

a fault-tolerant approach that ensures hosts can communicate outside their local subnets. Network hosts are typically configured with a single default gateway (the first-hop router) to allow them to communicate outside the local subnet. However, if the default gateway should fail, the hosts are limited to communicating only within the subnet, effectively disconnecting the hosts from the rest of the network.

Question 2

Describe Hot Standby Router Protocol (HSRP)

Answer 2

HRSP is a Cisco-proprietary FHRP that is designed to allow transparent failover of a first-hop IPv4 device. HSRP is used in a group of routers for selecting an active device and a standby device. The active device is the device that is used for routing packets; the standby device is the device that takes over when the active device fails or when pre-set conditions are met.

Question 3

Describe HSRP for IPv6

Answer 3

This is a Cisco-proprietary FHRP that provides the same functionality of HSRP, but in an IPv6 environment. An HSRP IPv6 group has a virtual MAC address derived from the HSRP group number and a virtual IPv6 link-local address derived from the HSRP virtual MAC address. Periodic router advertisements (RAs) are sent for the HSRP virtual IPv6 link-local address when the HSRP group is active. When the group becomes inactive, these RAs stop after a final RA is sent.

Question 4

Describe Virtual Router Redundancy Protocol version 2 (VRRPv2)

Answer 4

This is a non-proprietary election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on an IPv4 LAN. This allows several routers on a multiaccess link to use the same virtual IPv4 address. In a VRRP configuration, one router is elected as the virtual router master, with the other routers acting as backups in case the virtual router master fails.

Question 5

Describe VRRPv3

Answer 5

This provides the capability to support IPv4 and IPv6 addresses. VRRPv3 works in multi-vendor environments and is more scalable than VRRPv2.

Question 6

Describe Gateway Balancing Protocol (GLBP)

Answer 6

This is a Cisco-proprietary FHRP that protects data traffic from a failed router or circuit, like HSRP and VRRP, while also allowing load balancing (also called load sharing) between a group of redundant routers.

Question 7

Describe GLBP for IPv6

Answer 7

This is a Cisco-proprietary FHRP that provides the same functionality of GLBP, but in an IPv6 environment. GLBP for IPv6 provides automatic router backup for IPv6 hosts configured with a single default gateway on a LAN. Multiple first-hop routers on the LAN combine to offer a single virtual first-hop IPv6 router while sharing the IPv6 packet forwarding load.

Question 8

Describe ICMP Router Discovery Protocol (IRDP)

Answer 8

Specified in RFC 1256, IRDP is a legacy FHRP solution. IRDP allows IPv4 hosts to locate routers that provide IPv4 connectivity to other (non-local) IP networks.

Question 9

Router(config)#interface [type number]

Router(config-if)#standby [0-255] ip

Answer 9

Enters interface configuration mode and enables HSRP with a group number.

Question 10

Router(config-if)#standby [0-255] ip [address]

Answer 10

Configures the HSRP standby group with a virtual IP address.

Question 11

Router(config-if)#standby [0-255] preempt

Answer 11

Configures HSRP for preemption so the router may take over if it has a higher priority than the current active router.

Question 12

Router(config-if)#standby [0-255] priority [0-255]

Answer 12

Configures the HSRP group priority.

Question 13

Router(config-if)#standby [0-255] track [interface] [decrement_value]

Answer 13

Monitors the active router’s interface, which is used to forward traffic from the hosts and specifies the HSRP group priority amount that is decremented if the interface goes down.

Question 14

Router(config-if)#standby [0-255] timers msec [hello-value]

Router(config-if)#standby [0-255] timers msec [holdtime-value]

Answer 14

Configures the hello timer and hold timer values for HSRP.

Question 15

Router(config-if)#no standby [0-255] timers

Answer 15

Resets the hello timer and hold timer values back to their defaults, 3 and 10 seconds respectively.

Question 16

Router(config-if)#standby [0-255] authentication [value]

Router(config-if)#standby [0-255] authentication md5 key-string {0 | 7} [value]

Answer 16

Configures the authentication as plain text or encrypted text. This authenticates HSRP packets received from other routers in the group:
Specifying 0 means the key value is unencrypted.
Specifying 7 means the key value is encrypted. The key-string authentication key is automatically encrypted if the service password-encryption global configuration command is enabled.

Question 17

Router#show standby

Answer 17

Displays the gateway redundancy configuration and status of the configured interfaces.

Question 18

Router#debug standby

Answer 18

Displays HSRP state changes and debugging information regarding transmission and receipt of Hot Standby Protocol packets. Use this command to determine whether hot standby routers recognize one another and take the proper actions.