Limitation of HSRP on vlans Catalyst 3750

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May 11th, 2007
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Hi


Can somebody explain me why there can only be HSRP configured on a maximum of 32 vlans on a catalyst 3750?


So, I have 40 vlans and I can not configure HSRP on each vlan.


Thanks

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Danilo Dy Fri, 05/11/2007 - 06:05
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Hi,


Its platform design limitation. Even big switches have limitations of 16, 32, and 256 depending on the platform.


3750-E theoritically support up to 32000 HSRP as you can re-use the Standby Group. See the Q&A here http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1dde1bfa


HSRP use for gateway redundancy between two L3 device. If your two 3750 switches are in close proximity, you can replace HSRP with STACK instead. STACK two 3750, create SVI for each VLAN as gateway, when the master switch that curently hold the SVI fails, it will be taken over by the member switch that is elected as master - the MAC address change though.


Dandy

bbaillie Fri, 05/11/2007 - 09:54
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The limitation is due to the assignment of the Virtual MAC address for the HSRP group.

System assigned as: 0000.0c07.acXX, where XX is the HSRP group number

This means only two bytes can be used as the MAC unique identifier, or 32 unique combinations of the available HEX number set. Many different virtual interfaces can belong to the same group so you can still use HSRP to your advantage by putting more than one SVI into the same HSRP group.


Cheers,


Brian

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