ensure cisco router redundancy with hsrp

7/29/2019 Ensure Cisco Router Redundancy With HSRP

1/3

Ensure Cisco router redundancy with HSRP

By David "Davis CCIE, MCSE+I, SCSA"

April 20, 2006, 7:50pm PDT

What happens if your Internet router goes down and you lose all Internet access? Is that acceptable for

your organization? You can probably get away with it for about two minutes, but you need to have a

better plan than just calling a support desk.

That's why it's important to include redundancy in your network. Consider adding a backup router to

your current router that can take over at a moment's notice. All you need is the hardware, and the Cisco

software can take care of the rest. Let's examine how to configure this using the Hot Standby Router

Protocol (HSRP).

What is HSRP?

HSRP is a Cisco proprietary protocol for redundancy. It provides nearly 100 percent router availability

and redundancy. So, if one router goes down, a backup router takes over the routing functions of the

primary one.

However, there are other available industry protocols supported by Cisco. One industry standard is

theVirtual Router Redundancy Protocol(VRRP). Another HSRP alternative is theGateway Load

Balancing Protocol(GLBP), another Cisco proprietary solution.

A sample network

Before we discuss how to configure HSRP, let's take a look at the network we'll use for this example. To

help you better understand how HSRP works, here's a basic network diagram:

In our sample network, we've configured the PC's default gateway to IP address 10.1.1.3. However, that

IP address doesn't point to a real device; instead, it serves as the virtual IP address for whichever router

is the primary.

How does HSRP work?

When using HSRP, routers can either be primary or standby. If the primary router doesn't send out the

HELLO packet to the standby router for a period of time, the standby router assumes the primary router

is down and thus takes over. The standby router then assumes responsibility for the virtual IP address

and begins responding to the virtual Ethernet MAC address to which the virtual IP address is pointing.

The primary and standby routers exchange HSRP HELLO packets so that each knows the other router is

there. These HELLO packets use multicast 224.0.0.2 and UDP port 1985. The most basic form of HSRP
http://www.cisco.com/en/US/products/hw/vpndevc/ps2284/products_tech_note09186a0080094490.shtmlhttp://www.cisco.com/en/US/products/hw/vpndevc/ps2284/products_tech_note09186a0080094490.shtmlhttp://www.cisco.com/en/US/products/hw/vpndevc/ps2284/products_tech_note09186a0080094490.shtmlhttp://techrepublic.com.com/5100-1035_11-5888533.htmlhttp://techrepublic.com.com/5100-1035_11-5888533.htmlhttp://techrepublic.com.com/5100-1035_11-5888533.htmlhttp://techrepublic.com.com/5100-1035_11-5888533.htmlhttp://techrepublic.com.com/5100-1035_11-5888533.htmlhttp://techrepublic.com.com/5100-1035_11-5888533.htmlhttp://www.cisco.com/en/US/products/hw/vpndevc/ps2284/products_tech_note09186a0080094490.shtml


2/3

has been available since IOS 10.0, but there have been newer features released in the 11 and 12

versions of the IOS.

What determines the active router? First, you can configure a priority number to determine it, and then

it's by the highest IP address. The default priority number is 100; a higher priority number signifies the

preferred router.

Of course, when setting up router redundancy, you aren't limited to just two routers. In fact, you can set

up groups of routers that work together and have multiple "standby" routers.

How do you configure HSRP?

You can accomplish almost all HSRP configuration in the router's Interface Configuration Mode using

the standbycommand. Let's look at the steps I took to configure the network shown in the diagram.

For Router 1:

1. Configure the IP address on the Ethernet interface.2. Configure the standby IP address.3. Configure standby preempt. (With preempt, Router 1 will always be the primary router as long

as it's available.)

For Router 2:

1. Configure the IP address on the Ethernet interface.2. Configure the standby IP address.3. Configure standby priority to be less than 100. (In this case, it's 99.)

Now, let's look at the configuration for our sample network.

Router 1

(show running-config output) interface Ethernet0/0 ip address 10.1.1.1 255.255.255.0 standby ip

10.1.1.3 standby preempt Router1# show standby Ethernet0/0 - Group 0 State is Active 2 state

changes, last state change 00:00:29 Virtual IP address is 10.1.1.3 Active virtual MAC address is

0000.0c07.ac00 Local virtual MAC address is 0000.0c07.ac00 (default) Hello time 3 sec, hold time 10

sec Next hello sent in 0.692 secs Preemption enabled Active router is local Standby router is

10.1.1.2, priority 99 (expires in 8.097 sec) Priority 100 (default 100) IP redundancy name is "hsrp-

Et0/0-0" (default) Router1#

Router 2

(show running-config output) interface Ethernet0/0 ip address 10.1.1.2 255.255.255.0 standby ip

10.1.1.3 standby priority 99 Router2# show standby Ethernet0/0 - Group 0 Local state is Standby,


3/3

priority 99 Hellotime 3 sec, holdtime 10 sec Next hello sent in 1.014 Virtual IP address is 10.1.1.3

configured Active router is 10.1.1.1, priority 100 expires in 7.159 Standby router is local 4 state

changes, last state change 00:02:02 Router2#

You can use the show standbycommand when in Privileged Mode to check the status of HSRP. This

command tells you which router is active and which is standby, as well as a number of other statistics.

On the PC, the default IP address should point to 10.1.1.3not either of the routers. This way, if one of

the routers goes down, the other will take over. And you may even be able to use this redundancy to

take production routers down during the day because the HSRP failover time is less than 10 seconds.

HSRP is a valuable tool for ensuring high availability and router redundancy. Of course, there are also

several HSRP options that I didn't address in this article. For more information, check out theCisco HSRP

FAQ.

Miss a column?

Check out theCisco Routers and Switches Archive, and catch up on David Davis' most recent columns.

Want to learn more about router and switch management?Automatically sign up for our free Cisco

Routers and Switches newsletter, delivered each Friday!

David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE,

MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network

administrators for a privately owned retail company and performs networking/systems consulting on a

part-time basis.
http://www.cisco.com/warp/public/619/3.htmlhttp://www.cisco.com/warp/public/619/3.htmlhttp://www.cisco.com/warp/public/619/3.htmlhttp://www.cisco.com/warp/public/619/3.htmlhttp://techrepublic.com.com/1200-1035-5685066.htmlhttp://techrepublic.com.com/1200-1035-5685066.htmlhttp://techrepublic.com.com/1200-1035-5685066.htmlhttp://nl.com.com/MiniFormHandler?brand=techrepublic&list_id=e115http://nl.com.com/MiniFormHandler?brand=techrepublic&list_id=e115http://nl.com.com/MiniFormHandler?brand=techrepublic&list_id=e115http://nl.com.com/MiniFormHandler?brand=techrepublic&list_id=e115http://nl.com.com/MiniFormHandler?brand=techrepublic&list_id=e115http://nl.com.com/MiniFormHandler?brand=techrepublic&list_id=e115http://techrepublic.com.com/1200-1035-5685066.htmlhttp://www.cisco.com/warp/public/619/3.htmlhttp://www.cisco.com/warp/public/619/3.html

ensure cisco router redundancy with hsrp

Documents