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HSRP Questions

October 29th, 2018 Go to comments

Note: If you are not sure about HSRP, please read our HSRP tutorial.

Question 1


First Hop Redundancy Protocol (FHRP) is a protocol that enables two or more devices to work together in a group, sharing a single IP address, the virtual IP address. One router is elected to handle all requests sent to the virtual IP address. With HSRP, this is the active router. An HSRP group has one active router and at least one standby router.

Question 2

Question 3


Virtual Router Redundancy Protocol (VRRP) is one of the First Hop Redundancy Protocols that is supported by Cisco. Unlike HSRP and GLBP (which are Cisco proprietary protocols), VRRP is an industry standard protocol.

Question 4


From this paragraph:

“A set of routers that run HSRP works in concert to present the illusion of a single default gateway router to the hosts on the LAN. This set of routers is known as an HSRP group or standby group. A single router that is elected from the group is responsible for the forwarding of the packets that hosts send to the virtual router. This router is known as the active router. Another router is elected as the standby router. If the active router fails, the standby assumes the packet forwarding duties. Although an arbitrary number of routers may run HSRP, only the active router forwards the packets that are sent to the virtual router IP address.

In order to minimize network traffic, only the active and the standby routers send periodic HSRP messages after the protocol has completed the election process. Additional routers in the HSRP group remain in the Listen state. If the active router fails, the standby router takes over as the active router. If the standby router fails or becomes the active router, another router is elected as the standby router.”

Reference: https://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/10583-62.html#anc6

-> There is exactly one active router and one standby router in an HSRP group. Answer A is surely a correct but other answers are not correct. Answers C, D and E are wrong terminologies so they are surely not correct. Therefore answer B is a best choice left (although it is not totally correct).

Question 5


HSRP consists of 6 states:

State Description
Initial This is the beginning state. It indicates HSRP is not running. It happens when the configuration changes or the interface is first turned on
Learn The router has not determined the virtual IP address and has not yet seen an authenticated hello message from the active router. In this state, the router still waits to hear from the active router.
Listen The router knows both IP and MAC address of the virtual router but it is not the active or standby router. For example, if there are 3 routers in HSRP group, the router which is not in active or standby state will remain in listen state.
Speak The router sends periodic HSRP hellos and participates in the election of the active or standby router.
Standby In this state, the router monitors hellos from the active router and it will take the active state when the current active router fails (no packets heard from active router)
Active The router forwards packets that are sent to the HSRP group. The router also sends periodic hello messages

Please notice that not all routers in a HSRP group go through all states above. In a HSRP group, only one router reaches active state and one router reaches standby state. Other routers will stop at listen state.

Question 6


There are three First Hop Redundancy Protocols which are HSRP, VRRP and GLBP. HSRP and GLBP are Cisco proprietary protocols while VRRP is a standards-based protocol.

Question 7


HSRP election is based on a priority value (0 to 255) that is configured on each router in the group. By default, the priority is 100. The router with the highest priority value (255 is highest) becomes the active router for the group. If all router priorities are equal or set to the default value, the router with the highest IP address on the HSRP interface becomes the active router. Below is an example of assigning HSRP priority of 200 to R1:

R1(config-if)# standby 1 priority 200

Question 8


The “preempt” command enables the HSRP router with the highest priority to immediately become the active router. For example if we have a new router joining an HSRP of 1 and we want this router becomes the active router immediately (provided it had the highest HSRP priority) then we will need this additional command:

New_Router(config-if)#standby 1 preempt

Question 9


Both HSRP version 1 & version 2 support preempt command -> Answer D is not correct.

In HSRP version 1, group numbers are restricted to the range from 0 to 255. HSRP version 2 expands the group number range from 0 to 4095 -> A is correct.

Question 10


When all other routers in the group fail, the local router will not receive any HSRP Hello messages so it will become “active”. Notice that in this case the “preempt” command is not necessary. The “preempt” command is only useful when the local router receives a HSRP Hello message from the active HSRP router with a lower priority (then the local router will decide to take over the active role).

Question 11


The virtual MAC address of HSRP version 1 is 0000.0C07.ACxx, where xx is the HSRP group number in hexadecimal based on the respective interface. For example, HSRP group 10 uses the HSRP virtual MAC address of 0000.0C07.AC0A. HSRP version 2 uses a virtual MAC address of 0000.0C9F.FXXX (XXX: HSRP group in hexadecimal)

For more information about HSRP operation, please read our HSRP tutorial.

Question 12


With HSRP, two or more devices support a virtual router with a fictitious MAC address and unique IP address. There are two version of HSRP.

+ With HSRP version 1, the virtual router’s MAC address is 0000.0c07.ACxx , in which xx is the HSRP group. Therefore C is correct.
+ With HSRP version 2, the virtual MAC address is 0000.0C9F.Fxxx, in which xxx is the HSRP group.

Note: Another case is HSRP for IPv6, in which the MAC address range from 0005.73A0.0000 through 0005.73A0.0FFF.

(Good resource for HSRP: http://www.cisco.com/en/US/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_hsrp.html)

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