HSRP Overview
This affiliate discusses the use of HSRP in the campus network. The campus arrangement is
important because that is area the switching belvedere is best prevalent. To accept the campus
network accessible at all times, a akin of back-up charge be congenital in. This back-up is where
HSRP comes in. The campus archetypal has a hierarchical attending to it and HSRP fits in at the switch
block layer, accouterment an alive and a standby aisle to the administration layer. HSRP not only
allows for a band of redundancy, but additionally has the adequacy of accouterment amount sharing.
Issues with Acceptable Methods
This area deals with assorted acceptable methods that accept been used, but for one acumen or
another are abortive in some abortion modes. These methods accommodate absence gateways, proxy
ARP, RIP, and IRDP.
Default Gateways
In Figure 9-2, Router A is amenable for acquisition packets for Subnet A, and Router B is
responsible for administration packets on Subnet B. If Router A goes bottomward or contrarily becomes
unavailable to the PC, the acquisition protocols acclimated amid the blow of the routers will converge
at some point and connectivity will abide for at atomic allotment of the network.
Unfortunately, the PC doesn’t accept the adequacy to aggregate and barter acquisition information.
Therefore, back Router A went down, it doesn’t apperceive about any added way to avenue information
because it is bound to a distinct absence router. These accessories about are configured with a
single absence aperture IP address. If the router that is the absence aperture fails, the accessory is
limited to communicating alone on the bounded IP arrangement articulation and is finer disconnected
from the blow of the network. Alike if a bombastic router exists that could serve as a default
gateway, no activating adjustment exists for these accessories to about-face to a new absence aperture IP
address.
Providing bombastic paths and bombastic accessories in key locations of the arrangement to prevent
the abortion of any distinct accessory does not agreement the availability of basic arrangement assets to
the end user.
In this example, the end-user base is configured to accelerate packets for the Application Server to
Router 192.168.20.1. Router A and Router B apperceive about all of anniversary others IP interfaces through
the use of activating acquisition protocols. However, no way exists for the PC to dynamically switch
to a new absence aperture if the present absence aperture fails.
HSRP Overview 307
Figure 9-2 Absence Aperture Illustration
Proxy ARP
Proxy ARP is sometimes acclimated in a network. Proxy ARP is acclimated back the router is responding
with its own MAC abode as a proxy for some added host on a altered subnet. So what does
that buy you? Because the router knows area the destination MAC is amid or via the routing
table, sending packets to the router will absolutely get you to the absolute destination.
In a proxy ARP situation, the host considers the destination to be on the aforementioned subnet as itself.
The rub comes back the router actuality acclimated as a proxy fails. This creates a botheration in that the
host base will abide to accelerate packets destined for the far end station. The packets will
unfortunately be dropped.
How is this botheration rectified? A reboot of the host computer will fix the botheration by clearing
the ARP cache, or a force of addition ARP appeal will aces up a failover router. Unfortunately,
there is a cogent adjournment in communications, either to reboot the host or to admit an ARP.
This adjournment depends absolutely on the ARP abeyance period.
PC
Router A
192.168.20.1
Router B
192.168.20.2
Subnet B
192.168.60.x
Subnet A
192.168.50.x
Application server
Campus
Network
308 Affiliate 9: Overview of Hot Standby Router Protocol
Routing Advice Agreement (RIP)
One of the aboriginal acquisition protocols accessible in networking was the Acquisition Advice Protocol
(RIP) to ascertain routers. In this case, the workstation holds a acquisition table, which lists routes
and associated abutting hops that accept a aisle to the destination. It is again up to the workstation to
choose the best path.
RIP is a ambit agent acquisition protocol, which agency it relies on the cardinal of hops. RIP has
a limitation of 15 hops. The added limitation of application RIP—and about any ambit vector
protocols—is that arrangement aggregation is slow. In an ambiguous network, this can account many
problems.
ICMP Router Discovery Agreement (IRDP)
Some newer IP hosts use the ICMP Router Discovery Agreement (IRDP) to acquisition a new router
when a avenue becomes unavailable.
A host that runs IRDP listens for accost multicast letters from the adopted absence router. As
long as the end-user base detects these accost messages, the MAC abode for the router
generating the accost letters is acclimated as the destination MAC abode by the end-user station.
As anon as an end user no best detects the accost letters from the adopted router, the enduser
station switches to an alternating router.
IRDP is adopted over RIP and absence gateways because RIP takes best to converge. Default
gateways do not accommodate an another avenue if the absence aperture becomes unavailable.
Some IP hosts use ICMP and IRDP to acquisition a new aisle back the primary router becomes
unavailable. IRDP is not a acquisition agreement like RIP or the Interior Aperture Acquisition Protocol
(IGRP). IRDP is an addendum to ICMP that provides a apparatus for routers to acquaint useful
default routes. IRDP offers several advantages over added methods of advertent addresses of
neighboring routers. IRDP does not crave hosts to admit acquisition protocols, nor does it
require chiral agreement by an administrator.
A host that uses IRDP listens for accost multicast letters from the adopted absence router. The
IRDP-based advertisements are advised accurate alone for a predefined lifetime value. If a new
advertisement is not apparent during that lifetime, the router abode is advised invalid and the
host removes the agnate absence route. The IRDP agreement allows for capricious timing
values. A lifetime amount is included in the attack of every IRDP advertisement and applies to
all addresses included in the packet. A host will use the router abode alone for the cardinal of
lifetime abnormal afterwards the best contempo advertisement.
Advertisements are beatific every seven to ten minutes; the absence lifetime is 30 minutes. However,
the router has complete ascendancy over the breach and lifetime ethics and appropriately can ascendancy the
period of time the addresses are advised valid.
HSRP Overview 309
IRDP has two abstracted breach times: a minimum and a best advertisement interval. All
unsolicited advertisements are beatific in the window of time authentic by these two values. IRDP is
covered in greater detail in RFC 1256.
Hot Standby Router Protocol
So what botheration are we aggravating to solve? We’ve looked at solutions that try to break the problem
of a arrangement failure, but for assorted affidavit don’t necessarily fix the problem. Enter Hot
Standby Router Protocol. HSRP addresses the botheration acquired by first-hop failures generally
having changeless absence aperture addresses on hosts. Previously, a abortion at the absence gateway
address would leave the host clumsy to acquaint alfresco of its own subnet. Now with
HSRP, the absence aperture is a virtual-router address. A abortion of the alive router would result
in a about-face to the standby router, and packets would abide to be forwarded.
Cisco routers use HSRP, which enables end stations to abide communicating throughout the
network alike back the absence aperture becomes unavailable.
With HSRP, a set of routers works calm to represent a distinct basic standby router. The
standby router accumulation functions as a distinct router configured with a basic IP and MAC address,
distinct from the concrete routers in the network.
Because the routers in the standby accumulation avenue packets beatific to a basic address, packets are still
routed through the arrangement alike back the router originally forwarding the packets fails.
HSRP allows one router to automatically accept the action of the additional router if the second
router fails. HSRP is decidedly advantageous back the users on one subnet crave connected access
to assets in the network.
If the primary or advance router of a accumulation of HSRP routers fails, a standby router in the aforementioned group
begins to advanced cartage for the HSRP group.
The routers adjudge aural the accumulation which router assiduously cartage for the basic address. At
regular intervals, the routers barter advice to actuate which routers are still present
and able to advanced traffic.
When routers are configured to be allotment of an HSRP group, the routers admit their own native
MAC address, additional the HSRP accumulation MAC address.
Routers whose Ethernet controllers alone admit a distinct MAC abode will use the HSRP
MAC abode back assuming as the alive router and the burn-in abode (BIA) back in
standby approach or not speaking.
NOTE The router additionally sends a chargeless ARP back it becomes alive in adjustment to accomplish the end stations
aware of the MAC abode change.
310 Affiliate 9: Overview of Hot Standby Router Protocol
HSRP Accumulation Members
As apparent in Figure 9-3, the HSRP accumulation consists of the afterward members:
• Alive router
• Standby router
• Basic router
• Added routers
Figure 9-3 HSRP Accumulation Members
Each of these associates will be discussed in detail after in this chapter.
To facilitate amount sharing, a distinct router may be a affiliate of assorted HSRP standby groups
on a distinct segment. Anniversary standby accumulation emulates a distinct basic router. There is a absolute of 255
standby groups on any accustomed LAN.
NOTE Some platforms do not abutment assorted HSRPs because of the distinct MAC abode per
interface restriction. This brake can be aerial with the use of the standby use-bia
command.
Figure 9-4 illustrates that both Router A and Router B are associates of Groups 10 and 20.
However, Router A is the alive forwarding router for Accumulation 10 and the standby router for
Group 20. Router B is the alive forwarding router for Accumulation 20 and the standby router for
Group 10. Both Groups are associates of VLAN 100.
Standby Basic router
router
Active
router
HSRP Overview 311
Figure 9-4 HSRP Accumulation Associates Example
Addressing HSRP Groups Beyond ISL Links
As declared earlier, HSRP routers can accommodate for back-up and amount administration beyond the same
subnets, but what about altered subnets?
As you are no agnosticism able-bodied aware, an ISL block provides the alteration for assorted VLANs
simultaneously.
NOTE HSRP over ISL was alien in IOS 11.3 and is not accessible with any beforehand IOS version.
For anniversary standby group, an IP abode and a distinct acclaimed MAC abode with a unique
group identifier is allocated to the group.
The IP abode of a accumulation is in the ambit of addresses acceptance to the subnet in use on the LAN.
However, the IP abode of the accumulation charge alter from the addresses allocated as interface
addresses on all routers and hosts on the LAN, including basic IP addresses assigned to other
HSRP groups.
Group 20
Group 10
Router B
Router A
VLAN 100
312 Affiliate 9: Overview of Hot Standby Router Protocol
Running HSRP over ISL allows users to configure back-up amid assorted routers that
are configured as advanced ends for VLAN IP subnets. By configuring HSRP over ISLs, users can
eliminate situations in which a distinct point of abortion causes cartage interruptions. This feature
inherently provides some advance in all-embracing networking animation by accouterment load
balancing and back-up capabilities amid subnets and VLANs.
To configure HSRP over an ISL articulation amid VLANs, accomplish the afterward tasks:
Step 1 Define the encapsulation type.
Step 2 Configure the IP address.
Step 3 Enable HSRP.
The aboriginal two accomplish were discussed in Affiliate 7, “InterVLAN Routing.” The accomplish to enable
HSRP are discussed after in this chapter.
CAUTION Although a avenue processor can apparently abutment up to 32,650 subinterfaces, applied usage
is dictated by the robustness of the avenue processor and the cardinal of VLANs. Please monitor
the CPU appliance of the avenue processor to actuate your applied limitations.
Multiple HSRP Groups
Routers can accord to assorted groups aural assorted VLANs. As associates of assorted HSRP
groups, routers can accompanying accommodate bombastic advancement and accomplish amount administration across
different IP subnets. Some of the added characteristics of assorted HSRP groups accommodate the
following:
• Although assorted routers can abide in an HSRP group, alone the alive router assiduously the
packets beatific to the basic router.
• A abstracted HSRP accumulation is configured for anniversary abstracted VLAN.
• Assorted standby groups may coexist on a LAN segment. Anniversary accumulation operates
independently of added groups. Anniversary standby accumulation emulates a distinct basic router.
• Individual routers may participate in assorted groups. The router maintains abstracted state
and timers for anniversary group.
• For anniversary standby group, an IP abode and a distinct acclaimed MAC abode with a
unique accumulation identifier is allocated to the group.
• There can be up to 255 standby groups on any LAN.
• If assorted groups are acclimated on a distinct LAN, amount agreeable can be accomplished by distributing
hosts amid altered standby groups.