subnet, the active router is forwarding all the
packets off that subnet while the standby router is not
forwarding any packets. To facilitate load sharing, a single
router may be a member of multiple HSRP groups on the same
segment. Multiple standby groups further enable redundancy and
load sharing. While a router is actively forwarding traffic for
one HSRP group, the router can be in standby or listen state
for another group. Each standby group emulates a single virtual
router. There can be up to 255 standby groups on any LAN, but
the maximum number of standby groups need be no more than the
number of routers on a segment. In most cases, two standby
groups are sufficient. CAUTION: Increasing the number of
groups in which a router participates increases the load on the
router, which can impact the router’s performance. In Figure ,
both router A and B are members of groups 1 and 2. Router A is
the active forwarding router for group 1 and the standby router
for group 2. Router B is the active forwarding router for group
2 and the standby router for group 1. The following example
shows how multiple HSRP groups can be configured on the same
segment to facilitate load sharing. To be useful, half the
hosts on the segment need to use 172.16.10.110 as a default
gateway, while the other half need to use 172.16.10.120.
RouterA#show running-config
Building
configuration...
Current configuration:
!
<output omitted>
interface Vlan10
ip address
172.16.10.32 255.255.255.0
no ip redirects
standby 1
priority 150
standby 1 ip 172.16.10.110
standby 2
priority 50
standby 2 ip 172.16.10.120 RouterB#show
running-config
Building configuration...
Current configuration:
!
<output omitted>
interface Vlan10
ip address 172.16.10.33 255.255.255.0
no ip redirects
standby 1 priority 50
standby 1 ip
172.16.10.110
standby 2 priority 150
standby 2 ip
172.16.10.120 RouterA#show standby brief
P indicates
configured to preempt.
|
Interface Grp Prio P State
Active Standby Virtual IP
Vl10 1 150 Active local
172.16.10.33 172.16.10.110
Vl10 2 50 Standby 172.16.10.33
local 172.16.10.120 Routers can simultaneously provide
redundant backup and perform load sharing across different IP
subnets. In Figure , two HSRP-enabled routers participate in
two separate VLANs, using ISL or 802.1Q. Running HSRP over
trunks allows users to configure redundancy among multiple
routers that are configured as front ends for VLAN IP subnets.
By configuring HSRP over trunks, users can eliminate situations
in which a single point of failure causes traffic
interruptions. This feature provides some improvement in
overall networking resilience by providing load balancing and
redundancy capabilities between subnets and VLANs. For a VLAN,
configure the same device to be both the spanning tree root and
the HSRP active router. This approach ensures that the Layer 2
forwarding path leads directly to the Layer 3 active router,
thereby achieving maximum load balancing efficiency on the
routers and trunks. A standby group, an IP address, and a
single well-known MAC address with a unique group identifier
should be allocated to the group for each VLAN. Although up to
255 standby groups can be configured, the number of group
identifiers used should be kept to a minimum. If you are
configuring two distribution layer switches, you typically need
only two standby group identifiers. The following example shows
how multiple HSRP groups can be configured on two HSRP-enabled
routers participating in two separate VLANs RouterB#show
running-config
Building configuration...
Current configuration:
!
<output omitted>
interface Vlan10
ip address 172.16.10.32 255.255.255.0
no ip redirects
standby 1 priority 150
standby 1 ip
172.16.10.110
interface Vlan20
ip address 172.16.20.32
255.55.255.0
no ip redirects
standby 2 priority 50
standby 2 ip 172.16.20.120
RouterB#show
running-config
Building configuration...
Current configuration:
!
<output omitted>
interface Vlan10
ip address 172.16.10.33 255.255.255.0
no ip redirects
standby 1 priority 50
standby 1 ip
172.16.10.110
interface Vlan20
ip address 172.16.20.33
255.255.255.0
no ip redirects
standby 2 priority
150
standby 2 ip 172.16.20.120
Content
5.2 Optimizing HSRP 5.2.4
HSRP Debug Commands The commands in Figure are used to
debug HSRP operations. Figure describes the debug
commands.
Content 5.2 Optimizing
HSRP 5.2.5 Debugging HSRP Operations The
Cisco IOS implementation of HSRP supports the debug
command, which displays HSRP state changes and information
regarding the transmission and receipt of HSRP packets. To
enable HSRP debugging, enter the following command in
privileged EXEC mode: Switch#debug standby Figure
provides a description of debug standby fields.
CAUTION: Because debugging output is assigned high
priority in the CPU process, this command can render the system
unusable. Example: Debugging with Two Active Routers
The example in Figure displays output on distribution router
1DSW1. Router 1DSW1 is also receiving an HSRP hello from
172.16.1.112 for the same VLAN and same virtual IP address but
with a different standby group number. Hence, both routers are
active for the same virtual IP address. The debug
standby command is being used to troubleshoot the problem.
The standby group number is not consistent, so the two routers
have not formed a standby group. Example: Debugging Active
Router Negotiation This example displays the debug
standby command output as the 1DSW1 router with IP address
172.16.1.111 initializes and negotiates for the role of active
router. *Mar 8 20:34:10.221: SB11: Vl11 Init: a/HSRP
enabled
*Mar 8 20:34:10.221: SB11: Vl11 Init ->
Listen
*Mar 8 20:34:20.221: SB11: Vl11 Listen: c/Active
timer expired (unknown)
*Mar 8 20:34:20.221: SB11: Vl11
Listen -> Speak
*Mar 8 20:34:20.221: SB11: Vl11 Hello
out 172.16.11.111 Speak pri 100 ip 172.16.11.115
*Mar 8
20:34:23.101: SB11: Vl11 Hello out 172.16.11.111 Speak pri 100
ip 172.16.11.115
*Mar 8 20:34:25.961: SB11: Vl11 Hello out
172.16.11.111 Speak pri 100 ip 172.16.11.115
*Mar 8
20:34:28.905: SB11: Vl11 Hello out 172.16.11.111 Speak pri 100
ip 172.16.11.115
*Mar 8 20:34:30.221: SB11: Vl11 Speak:
d/Standby timer expired (unknown)
*Mar 8 20:34:30.221:
SB11: Vl11 Standby router is local
*Mar 8 20:34:30.221:
SB11: Vl11 Speak -> Standby
*Mar 8 20:34:30.221: SB11:
Vl11 Hello out 172.16.11.111 Standby pri 100 ip
172.16.11.115
*Mar 8 20:34:30.221: SB11: Vl11 Standby:
c/Active timer expired (unknown)
*Mar 8 20:34:30.221: SB11:
Vl11 Active router is local
*Mar 8 20:34:30.221: SB11: Vl11
Standby router is unknown, was local
*Mar 8 20:34:30.221:
SB11: Vl11 Standby -> Active
*Mar 8 20:34:30.221:
%STANDBY-6-STATECHANGE: Vlan11 Group 11 state Standby ->
Active
*Mar 8 20:34:30.221: SB11: Vl11 Hello out
172.16.11.111 Active pri 100 ip 172.16.11.115 To disable the
debugging feature, use either the no debug standby or
the no debug all command. Example: Debugging First
and Only Router on Subnet Because 1DSW1 (172.16.11.111) is
the only router on the subnet, and it is not configured for
preempt, it goes through five HSRP states before becoming the
active router. Notice that at Mar 8 20:34:10.221 the interface
comes up, and 1DSW1 enters the listen state. The router stays
in listen state for a hold time of 10 seconds. 1DSW1 then goes
into speak state at Mar 8 20:34:20.221 for 10 seconds. When the
router is speaking, it sends its state out every 3 seconds,
according to its hello interval. After 10 seconds in speak
state, the router has determined that there is no standby
router and enters the standby state at Mar 8 20:34:30.221. The
router has also determined that there is not an active router;