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c3550 MAC broadcast is forwarded

Hi All,

I have a L3 LAN network based on Cat3550 switches. The switches

seem to process MAC broadcast packets up to L3, and route them. E.g.

there is a MAC broadcast to an IP directed broadcast IP address (eg. As on the MAC layer it is a broadcast

(FF-FF-FF-FF-FF-FF), it should not be processed (I think). It is not

processed by a 801, but is processed by the 3550. I tried all available

software releases on the 3550.

The actual scenario is: Cisco 3550 in subnet A, PC in subnet B, 4500 in

both subnet A and B. A and B subnets are in the same VLAN. When the PC

sends an IP bradcast (B.255) the 3550 processes it and sends out a unicast

to the 4500 (as the 3550 has a route to B via the 4500). The broadcast

sent by the PC has broadcast MAC destination.

I know it is recommended to have every router an IP interface in every

subnet on a given VLAN, but I wonder why does the 3550 and 801 (and I

think other traditional routers) behave differently.

Any ideas appreciated.




Re: c3550 MAC broadcast is forwarded

Often times complex troubleshooting issues are best addressed in an interactive session with one of our trained technical assistance engineers. While other forum users may be able to help, it’s often difficult to do so for this type of issue.

To utilize the resources at our Technical Assistance Center, please visit and to open a case with one of our TAC engineers, visit

If anyone else in the forum has some advice, please reply to this thread.

Thank you for posting.

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