Core Issue
The Catalyst 6500/6000 series switches use the Policy Feature Card (PFC) on the supervisor engines to switch Layer 3 (L3) traffic in hardware, when combined with a Multilayer Switch Feature Card (MSFC) which programs the PFC. The PFC also provides new, enhanced policy based Quality of Service (QoS) and security features while providing the features commonly used in wiring closest designs such as protocol filtering, Internet Group Management Protocol (IGMP) snooping, and enhanced multicast packet replication. There are different types of PFCs available for different types of Catalyst 6000 family Supervisor engine modules. The PFC is available for Supervisor I and IA modules; PFC2 for the Supervisor II module; and PFC3 for the Supervisor 720 module.
Hot Standby Router Protocol (HSRP) provides redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop router failures. HSRP allows multiple routers on a single LAN to share a virtual IP and MAC address, which is configured as the default gateway on the hosts. From the group of routers configured in an HSRP group, the one with the highest priority functions as the active router, and the one with the second highest priority functions as the standby router. The active router assumes the role of forwarding packets sent to the virtual IP address. If the active router fails, the standby takes over as the new active router.
Resolution
L3 redundancy for the Multilayer Switch Feature Card (MSFC) is provided by configuring HSRP under the Switched Virtual Interfaces (SVI) for the VLANs on the MSFCs. The SVI acts as the routed interface for a VLAN. The maximum number of unique HSRP groups that can be configured on the MSFC depends on the type of PFC available on the Supervisor engine, which limits the number of HSRP MAC addresses supported. The switch with a PFC or PFC 3 supports 256 unique HSRP group mappings, but the one with a PFC 2 supports only up to 16 unique groups.
Though the number of unique groups supported is less in a PFC 2, the same group number can be used for different SVIs, and they will use the same HSRP MAC address. This does not create problems with switches that maintain a per VLAN MAC address table, but there might be issues if it is used with switches that use a system-wide MAC-address table, regardless of the VLAN.
For related information, refer to these documents:
