these messages. The source address is the
interface IP address of the sending router. An HSRP group may
contain other routers that are group members but are not in an
active or standby state. These routers monitor the hello
messages sent by the active and standby routers to ensure that
active and standby routers exist for the HSRP group of which
they are a member. These routers do forward packets addressed
to their own specific IP addresses, but they do not forward
packets addressed to the virtual router. These routers issue
speak messages at every hello interval time. Figure describes
some of the terms used with HSRP. When the active router fails,
the other HSRP routers stop seeing hello messages from the
active router. The standby router then assumes the role of the
active router. If other routers are participating in the group,
they contend to be the new standby router. If both the active
and standby routers fail, all routers in the group contend for
the active and standby router roles. Because the new active
router assumes both the IP and MAC addresses of the virtual
router, the end stations see no disruption in service. The
end-user stations continue to send packets to the virtual
router MAC address, and the new active router delivers the
packets to the destination.
Content 5.1
Configuring Layer 3 Redundancy with HSRP 5.1.5
Describing HSRP States A router in an HSRP group can be
in one of the following states: initial, learn, listen, speak,
standby, or active. Figure describes the different HSRP
states. When a router exists in one of these states, it
performs the actions required for that state. Not all HSRP
routers in the group transition through all states. For
example, if there are three routers in the group, the router
that is not the standby or active router remains in the listen
state. All routers begin in the initial state, indicating that
HSRP is not running. This state is entered via a configuration
change, such as when HSRP is disabled on an interface, or when
an HSRP-enabled interface is first brought up, such as when the
no shutdown command is issued. After the initial state,
the interface moves to the learn state. The interface is
expecting to see HSRP packets and from these packets determine
the virtual IP and active HSRP router for the group. Once the
interface has seen HSRP packets and determined the virtual IP,
it moves to the listen state. The purpose of the listen state
is to determine if there are already active or standby routers
for the group. If the active and standby routers are
functional, the interface remains in this state. However, if
hellos are not seen from either router, the interface moves to
the speak state. In the speak state, the routers are actively
participating in the election of the active router, standby
router, or both. The routers look at each other’s hello packets
to determine which router should assume which role. Three
timers are used in HSRP: active, standby, and hello. If a hello
is not received from an active HSRP router within the active
timer, the router transitions to a new HSRP state. Figure
describes the HSRP timers. In the standby state , because the
router is a candidate to become the next active router, it
sends periodic hello messages. It also listens for hello
messages from the active router. There can only be one standby
router in the HSRP group. In the active state , the router is
currently forwarding packets that are sent to the virtual MAC
address of the group. It also replies to ARP requests directed
to the virtual router’s IP address. The active router sends
periodic hello messages. There must be one active router in
each HSRP group.
Content 5.1 Configuring Layer
3 Redundancy with HSRP 5.1.6 Describing HSRP
Configuration Commands Figure illustrates common HSRP
configuration commands. Figure describes the essential commands
used to configure and verify HSRP.
Content 5.1
Configuring Layer 3 Redundancy with HSRP 5.1.7
Enabling HSRP The following command enables HSRP on an
interface: Switch(config-if)#standby
group-number ip ip-address Figure
describes the command parameters for configuring an HSRP group
on an interface. When HSRP is running, the end-user stations
must not discover the actual MAC addresses of the routers in
the standby group. Any protocol that informs a host of a
router’s actual address must be disabled. Enabling HSRP on a
Cisco router interface automatically disables Internet Control
Message Protocol (ICMP) redirects on that interface, which
ensures that the addresses of the participating HSRP routers
are not discovered. After the standby ip command is
issued, the interface changes to the appropriate state, and the
router issues an HSRP message. To remove an interface from an
HSRP group, enter the no standby group ip
command. The following example states that interface VLAN11 is
a member of HSRP group 11, the virtual router IP address for
the group is 172.16.11.115, and ICMP redirects are disabled. To
verify the HSRP configuration, use the show
running-config command: Switch#show
running-config
Building configuration... Current
configuration:!
<output omitted>
interface
Vlan11
ip address 172.16.11.113 255.255.255.0
no ip
redirects
standby 11 ip 172.16.11.115
! Another way to
verify the HSRP configuration is with the show standby brief
command, which displays abbreviated information about the
current state of all HSRP operations on the device. To display
the status of the HSRP router, use one of these commands:
Switch#show standby [interface
[group]] [active | init | listen
| standby] [brief]
Switch#show
standby delay [type-number] If the optional
interface parameters are not included, the show standby
command displays HSRP information for all interfaces. The
following example shows the output of the show standby
command: Switch#show standby Vlan11 11
Vlan11 -
Group 11
Local state is Active, priority 110
Hellotime
3 holdtime 10
Next hello sent in 00:00:02.944
Hot
standby IP address is 172.16.11.115 configured
Active
router is local
Standby router is 172.16.11.114 expires in
00:00:08
Standby virtual mac address is 0000.0c07.ac01 This
is the output when you use the brief parameter:
Switch#show standby brief
Interface Grp Prio P State
Active addr Standby addr Group addr
Vl11 11 110 Active
local 172.16.11.114 172.16.11.115 Notice that the group address
172.16.11.115 is on the same subnet as the standby and active
router IP addresses.
Content 5.2
Optimizing HSRP 5.2.1 Describing HSRP
Optimization Options The options illustrated in Figures and
make it possible to optimize HSRP operation in the campus
network. Each standby group has its own active and standby
routers. The network administrator can assign a priority value
to each router in a standby group, allowing the administrator
to influence the active and standby router selection. To set
the priority value of a router (default is 100), enter this
command in interface configuration mode:
Switch(config-if)#standby group-number
priority priority-value Figure describes the
variables for the standby command. During the election process,
the router with the highest priority in an HSRP group becomes
the active router. In the case of a tie, the router with the
highest configured IP address is chosen. To reinstate the
default standby priority value, use the no standby
priority command. The following example states that
interface VLAN10 has a priority value of 150 in HSRP group 1.
If this priority value is the highest number in that HSRP
group, the routing device on which this interface resides is
the active router for that group. Switch#show
running-config
Building configuration...
Current configuration:
!
<output omitted>