[MLS Switch - L3 vlan int] Forwarding or stop broadcast?

[Jeetiz95110]

Hi everyone (sorry for my english),

I study for the CCNP Switch, and I'm asking myself about the following architecture:

(all link are trunk except for hosts)

Considering that broadcast is stopped by routers, if an host in vlan 2 send a broadcast, do my active HSRP 3750:
1) will forwarding via the trunk (no restriction on trunk) or
2) stop the broadcast via the vlan layer 3 interface

Thanks for your advices

[Antonio Knox · Accepted solution]

1) The 3750 will only forward broadcasts within vlan 2 via Layer 2.  Therefore the broadcast WILL be forwarded through the trunk, to all active interfaces in vlan 2.  The broadcast will NOT go over trunks where vlan 2 may be pruned (assuming you are pruning).

2) You are correct, the router does not forward broadcasts, therefore your active HSRP vlan interface (not the switch as a whole) will NOT forward broadcasts via Layer 3.

Please rate helpful posts.

[Jeetiz95110]

Thanks for you answer!

OK so, if my distribution layer is connect to core layer via physical layer 3 interface (no switchport), broadcast won't be propagated, I suppose?

[Antonio Knox · Accepted solution]

Correct, the broadcast is NOT propogated outside of the vlan in which it originated. 

It's very important to understand that the broadcast, with no pruning enabled, could be seen on all devices in your diagram, but ONLY to ports on the switches that are associated with that vlan in which the broadcast originates.

So, if the concern here is to keep broadcast traffic down, be sure to configure ports within a vlan ONLY on switches where needed, and perhaps employ VTP pruning, so that switches that aware of the vlan, if they have no ports associated with that vlan, will not receive broadcast traffic in that vlan.

VTP Pruning

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml#vtp_pruning

I hope this makes sense.