HSRP, VRRP, and GLBP
IP hosts can use several methods of chief which absence router or absence aperture to use—
DHCP, BOOTP, ICMP Router Discovery Agreement (IRDP), chiral configuration, or alike by
running a acquisition agreement (although accepting hosts run a acquisition agreement is not accepted today).
The best archetypal methods—using DHCP or chiral configuration—result in the host alive a
single IP abode of its absence gateway. Hot Standby Router Agreement (HSRP), Basic Router
Redundancy Agreement (VRRP), and Aperture Amount Balancing Agreement (GLBP) represent a
chronological account of some of the best accoutrement for advantageous the issues accompanying to a host alive a
single IP abode as its aisle to get alfresco the subnet.
HSRP allows assorted routers to allotment a basic IP and MAC abode so that the end-user hosts do
not apprehend back a abortion occurs. Some of the key HSRP appearance are as follows:
■ Basic IP abode and basic MAC alive on the Master router
■ Standby routers accept for Hellos from the Alive router, behind to a 3-second accost interval
and 10-second asleep interval
■ Highest antecedence (IOS absence 100, ambit 1–255) determines the Alive router, with preemption
disabled by default
■ Supports tracking, whereby a router’s antecedence is decreased back a tracked article (interface
or route) fails
■ Up to 255 HSRP groups per interface, enabling an authoritative anatomy of amount balancing
■ Basic MAC of 0000.0C07.ACxx, area xx is the hex HSRP group
■ Basic IP abode charge be in the aforementioned subnet as the routers’ interfaces on the aforementioned LAN
■ Basic IP abode charge be altered from any of routers’ alone interface IP addresses
■ Supports clear-text and MD5 affidavit (through a key chain)
Example 5-2 shows a archetypal HSRP configuration, with two groups configured. Routers R1 and
R2 are absorbed to the aforementioned subnet, 10.1.1.0/24, both with WAN links (S0/0.1) abutting them
to the blow of an action network. Cisco IOS provides the tracking apparatus apparent in
Example 5-2 to admittance abounding processes, including HSRP, VRRP, and GLBP, to clue interface
states. A tracking article can clue based on the band agreement (shown here) or the IP acquisition table.
The archetype contains the capacity and account of the configuration.
Example 5-2 HSRP Configuration
! First, on Router R1, a tracking article charge be configured so that
! HSRP can clue the interface state.
track 13 interface Serial0/0.1 line-protocol
! Next, on Router R1, two HSRP groups are configured. R1 has a college priority
! in accumulation 21, with R2 accepting a college antecedence in accumulation 22. R1 is set to preempt
! in accumulation 21, as able-bodied as to clue interface s0/0.1 for both groups.
interface FastEthernet0/0
ip abode 10.1.1.2 255.255.255.0
standby 21 ip 10.1.1.21
HSRP, VRRP, and GLBP 137
standby 21 antecedence 105
standby 21 preempt
standby 21 clue 13
standby 22 ip 10.1.1.22
standby 22 clue 13
! Next, R2 is configured with a college antecedence for HSRP accumulation 22, and with
! HSRP tracking enabled in both groups. The tracking “decrement” acclimated by R2,
! back S0/0.1 fails, is set to 9 (instead of the absence of 10).
! A tracking article charge be configured first, as on R1.
track 23 interface Serial0/0.1 line-protocol
interface FastEthernet0/0
ip abode 10.1.1.1 255.255.255.0
standby 21 ip 10.1.1.21
standby 21 clue 23
standby 22 ip 10.1.1.22
standby 22 antecedence 105
standby 22 clue 23 cutback 9
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! On R1 below, for accumulation 21, the achievement shows that R1 is active, with R2
! (10.1.1.2) as standby.
! R1 is tracking s0/0.1, with a absence “decrement” of 10, acceptation that the
! configured antecedence of 105 will be decremented by 10 if s0/0.1 fails.
Router1#sh standby fa0/0
FastEthernet0/0 - Accumulation 21
State is Active
2 accompaniment changes, aftermost accompaniment change 00:00:45
Virtual IP abode is 10.1.1.21
Active basic MAC abode is 0000.0c07.ac15
Local basic MAC abode is 0000.0c07.ac15 (v1 default)
Hello time 3 sec, authority time 10 sec
Next accost beatific in 2.900 secs
Preemption enabled
Active router is local
Standby router is 10.1.1.2, antecedence 100 (expires in 7.897 sec)
Priority 105 (configured 105)
Track article 13 accompaniment Up cutback 10
IP back-up name is "hsrp-Fa0/0-21" (default)
! achievement omitted
! NOT SHOWN—R1 shuts bottomward S0.0.1, blurred its antecedence in accumulation 21 by 10.
! The alter beneath shows the bargain antecedence value. However, R2 does not become
! active, because R2’s agreement did not accommodate a standby 21 accroach command.
Router1# alter standby
*Mar 1 00:24:04.122: HSRP: Fa0/0 Grp 21 Accost out 10.1.1.1 Alive pri 95 vIP 10.1.1.21
Example 5-2 HSRP Agreement (Continued)
138 Chapter 5: IP Services
Because HSRP uses alone one Alive router at a time, any added HSRP routers are idle. To provide
load administration in an HSRP configuration, the abstraction of Assorted HSRP, or MHSRP, was developed.
In MHSRP, two or added HSRP groups are configured on anniversary HSRP LAN interface, area the
configured antecedence determines which router will be alive for anniversary HSRP group.
MHSRP requires that anniversary DHCP applicant and statically configured host is issued a absence gateway
corresponding to one of the HSRP groups and requires that they’re broadcast appropriately. Thus,
in an MHSRP agreement with two routers and two groups, all added things actuality equal, bisected of
the hosts should accept one HSRP accumulation abode as its absence gateway, and the added bisected of the
hosts should use the added HSRP accumulation address. If you now revisit Archetype 5-2, you will see that
it is an MHSRP configuration.
HSRP is Cisco proprietary, has been out a continued time, and is broadly popular. VRRP (RFC 3768)
provides a connected agreement to accomplish about the exact aforementioned function. The Cisco VRRP
implementation has the aforementioned goals in apperception as HSRP but with these differences:
■ VRRP uses a multicast basic MAC abode (0000.5E00.01xx, area xx is the hex VRRP
group number).
■ VRRP uses the IOS article tracking feature, rather than its own centralized tracking mechanism,
to clue interface states for failover purposes.
■ VRRP defaults to use pre-emption, but HSRP defaults to not use pre-emption. Both can be
configured to either use pre-emption or not.
■ The VRRP appellation Master agency the aforementioned affair as the HSRP appellation Active.
■ In VRRP, the VRRP accumulation IP abode is the interface IP abode of one of the VRRP routers.
GLBP is a newer Cisco-proprietary apparatus that adds load-balancing appearance in accession to gatewayredundancy
features. Hosts still point to a absence aperture IP address, but GLBP causes different
hosts to accelerate their cartage to one of up to four routers in a GLBP group. To do so, the GLBP Active
Virtual Aperture (AVG) assigns anniversary router in the accumulation a altered basic MAC address, following
the architecture 0007.B400.xxyy, area xx is the GLBP accumulation number, and yy is a altered cardinal for
each router (01, 02, 03, or 04). Back a applicant ARPs for the (virtual) IP abode of its default
gateway, the GLBP AVG replies with one of the four accessible basic MACs. By acknowledging to ARP
requests with altered basic MACs, the hosts in that subnet will in aftereffect antithesis the traffic
across the routers, rather than accelerate all cartage to the one alive router.
Cisco IOS accessories with GLBP abutment admittance configuring up to 1024 GLBP groups per physical
interface and up to four hosts per GLBP group.