Optimizing HSRP
Load sharing

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.

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.

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.

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.

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.