• Advertisement

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

VTP, Portfast, Spanning Tree and all the other switch related stuff.

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

Postby Guest » Tue Jan 04, 2011 10:33 am

Hi everyone sorry for my english,

 

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

Question_STP_LAND.png

(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

Guest
 

Advertisement

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

Postby Guest » Tue Jan 04, 2011 11:59 am

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.

Guest
 

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

Postby Guest » Tue Jan 04, 2011 1:17 pm

Thanks for you answer!

 

OK so, if my distribution layer is connect to core layer via physical layer 3 interface (no switchport), broadcast won be propagated, I suppose?

Guest
 

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

Postby Guest » Tue Jan 04, 2011 2:28 pm

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

 

Its 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.

Guest
 



  • Advertisement


Similar topics

SRW2024 console unconnectable / unknown "switch ip"
Forum: Small Business Switches
Author: Anonymous
Replies: 5

Basic VLAN question
Forum: Small Business Switches
Author: Guest
Replies: 0

How to display switch in CCA v3.2
Forum: Anything Networking
Author: Guest
Replies: 0

Trouble uplinking to 2960 switch
Forum: Small Business Switches
Author: Anonymous
Replies: 0

Broadcast Multiple SSID
Forum: Cisco Wireless
Author: Anonymous
Replies: 0


Return to Cisco Switching

Who is online

Users browsing this forum: No registered users and 3 guests