Cisco ICS 7700 Series Integrated Communication Systems

test ior

Document ID: 27880


This document addresses the frequently asked questions on Hot Standby Router Protocol (HSRP) group support or limitations on the Catalyst 3550s. For more information about HSRP and configuration examples, refer to Understanding and Troubleshooting HSRP Problems in Catalyst Switch Networks.



Q. What is the maximum number of HSRP groups supported?

A.  The Catalyst 3550 can only support HSRP running on 16 interfaces. This is because in hardware, we allocate a VLAN ID and MAC address for each interface configured for HSRP. There is only space for 16 entries in hardware.

Q. Does the limit apply to all software images?
A. Yes. The limitation is due to the hardware design. However, releases before  12.1(12c)EA1 do allow this number to be exceeded. When more than 16 HSRP groups are configured and are active at the same time, users might experience connectivity problems in some VLANs. In 12.1(12c)EA1 and later, the parser does not allow you to enter such a configuration anymore and displays a warning message : "%More than 16 standby group entries not supported in this platform."

Q. Is this limitation the same as with Catalyst 6000 Supervisor 2 based systems? Does the same workaround apply?


A. No. On the Catalyst 6000 Supervisor 2 based systems, you can use up to 16 unique HSRP group IDs, on as many interfaces as you like. Shoud you need more, you may use the same  HSRP group ID on multiple interfaces. This limitation of 16 HSRP group IDs does not mean that you can have only 16 HSRP processes or 16 VLAN interfaces with HSRP enabled.  That is quite different from the Catalyst 3550 which can only support HSRP running on 16 interfaces, regardless of whether the same or different HSRP group IDs are configured.

Q. Do the HSRP group IDs have to be contiguous? Are there any other requirements?

A. The HSRP group IDs do not have to be contiguous. You can pick any 16 group IDs in the allowed group ID range (0-255). Only 16 group IDs, however, can be used from that range.
Q. What is the implication of using the same HSRP group ID on multiple interfaces?

A. When you define the same HSRP group ID on multiple interfaces, they share the same HSRP virtual MAC address. In most modern LAN switches, there would be no issues because they maintain a per-vlan MAC address table. However, if your network contains any third party switches, which maintain a system wide MAC-address table regardless of VLAN, you may experience problems.
Q. I was able to configure more than 16 HSRP groups on my Catalyst 3550. I upgraded the switch from 12.1(11)EA1 to 12.1(13)EA1 and now I cannot. What has changed?

A. As having more than 16 HSRP groups could trigger connectivity issues, such a configuration is not possible anymore starting 12.1(12c)EA1.

Updated: Apr 27, 2005 Document ID: 27880