To facilitate load sharing, a single router may be a member of multiple HSRP
standby groups on a single segment. Multiple standby groups further enable
redundancy and load sharing within networks. 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.
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CAUTION:
Increasing the number of groups in which a router participates
increases the load on the router. This can have an impact on the performance of
the router.
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In Figure
, both router A
and router B are members of groups 1 and 2. However, 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.
Addressing HSRP Groups Across Trunk Links
Routers can
simultaneously provide redundant backup and perform load sharing across
different IP subnets.
For each standby group, an IP address and a single
well-known MAC address with a unique group identifier is allocated to the
group.
The IP address of a group is in the range of addresses belonging
to the subnet that is in use on the LAN. However, the IP address of the group
must differ from the addresses allocated as interface addresses on all routers
and hosts on the LAN, including virtual IP addresses assigned to other HSRP
groups.
In Figure
, two
HSRP-enabled routers participate in two separate VLANs using Inter-Switch Link
(ISL) or 802.1Q. Running HSRP over trunking allows users to configure
redundancy between multiple routers that are configured as front ends for VLAN
IP subnets. By configuring HSRP over ISLs, users can eliminate situations in
which a single point of failure causes traffic interruptions. This feature
inherently provides some improvement in overall networking resilience by
providing load balancing and redundancy capabilities between subnets and
VLANs.
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NOTE:
A Route Processor (RP) theoretically can support up to 32,650
subinterfaces; however, the actual number of supported interfaces is limited by
the capacity of the RP and the number of VLANs.
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Supporting Multiple Subnets with Multiple HSRP Groups
Routers can belong to multiple groups within multiple VLANs. As
members of multiple hot standby groups, routers can simultaneously provide
redundant backup and perform load sharing across different IP subnets.
Although multiple routers can exist in an HSRP group, only the active router
forwards the packets sent to the virtual router.
A single group can also
be configured to host the virtual router of multiple IP subnets. For example,
the default gateway virtual addresses of 172.16.10.1 and 172.16.20.1 can both
be associated with a single HSRP group number.
