Cisco CCNA ICND2 (Interconnecting Cisco Networking Devices Part 2)Exam Cram

4. Infrastructure Services

4.1 HSRP

Short for Hot Standby Routing Protocol, a proprietary protocol from Cisco. HSRP is a routing protocol that provides backup to a router in the event of failure. Using HSRP, several routers are connected to the same segment of an Ethernet, FDDI or token-ring network and work together to present the appearance of a single virtual router on the LAN. The routers share the same IP and MAC addresses, therefore in the event of failure of one router, the hosts on the LAN are able to continue forwarding packets to a consistent IP and MAC address. The process of transferring the routing responsibilities from one device to another is transparent to the user.

HSRP configuration requires only one command on the two or more routers that want to share default router responsibilities with HSRP interface sub command:

standby <group-no> ip <virtualipadd>

The first value defines HSRP group number which must match on all the routers within the group. The group number lets one router support multiple HSRP groups at a time on the same interface and it allows router to identify each other based on the group. The command also configures the virtual ip address shared by the routers in the same group. The virtual ip address is the address the hosts in the VLAN use as their default-gateway.

Note: If the group-number is not entered, then it will default to a group number of 0. 

Example:

R1>enable
R1#configure teminal
R1(config)#interface
fastethernet 0/0
R1(config-if)#standby
11 ip 192.168.10.100

The above command configures router R1 as a member of HSRP standby group with group number 11 and virtual ip address as 192.168.10.100 on interface fa0/0.

The command :

standby <group-number> preempt

is used to force an interface to resume active router state. Note that the priority of the router should be higher than the current Active router.

Members of HSRP group

1. Virtual router: virtual router is what is seen by the end user device. The virtual router has its own IP and MAC addresses.

2. Active router: Forwards packets sent to the virtual router. An active router assumes the IP and MAC addresses of the virtual router.

3. Standby router: Standby router monitors the state of HSRP by using Hello massages. It assumes the role of Active router, should the current Active router fail.

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When an Active router fails in HSRP environment, Standby router assumes the Active router role. This new Active router will remain as Active router even if the failed Active router comeback to service, irrespective of the priority levels.

To enable the previous Active router to resume its activity as Active router by taking over the role from a lower priority Active router, use the command

Rtr(config-if)#standby <group-number> preempt

The hosts served by HSRP router use the IP address of virtual router as the default IP address.

Each router in a standby group can be assigned a priority value. The range of priority values is between 0 and 255 (including 0 and 255). The default priority assigned to a router in a standby group is 100. The router with numerically higher priority value will become Active router in the HSRP standby group.

The command used to set the router's priority in standby group is:

R(config-if)#standby <group-number> priority <priority-value>

HSRP Features:

1. Within the standby group of routers, the router with the highest standby priority in the group becomes the active router. For example, a router with a priority of 100 will become active router over a router with a priority of 50. The active router forwards packets sent to the virtual router. It maintains its active state by using Hello messages.

2. The default HSRP standby priority is100. If the standby priorities of routers participating in HSRP are same, the router with the highest IP address becomes the Active router.

3. HSRP authentication is carried out in clear text.

4. An HSRP router status can be displayed by using the command :

RouterA# show standby

The above command displays the router priority, state (active/standby), group number among other things.

The following is sample output of the "show standby" command

images/showstandby.jpg

5. To enable HSRP debugging, use the command :

RouterA#debug standby

6. To disable debugging, use the command :

RouterA# no debug standby

7. All routers in an HSRP standby group can send and/or receive HSRP message. Also, HSRP protocol packets are addressed to all-router address (224.0.0.2) with a TTL of 1.

8.The HSRP messages are encapsulated in the data portion of UDP packets.

9. In HSRP, the MAC address used by virtual router is made up of the following three components:

  • Vendor ID: The first three bytes of the MAC address correspond to the vendor ID.
  • HSRP ID: The next two bytes of the MAC address correspond to HSRP code. It is always 07.ac. Therefore, the virtual router MAC address will have 07.ac in the fourth and fifth bytes.
  • Group ID: The last byte of the MAC address is the group's identification number.
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If there is a tie, the router with highest ip address becomes active while the router with second highest ip becomes standby.

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