trunking problem

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Nov 17th, 2009
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I have to connect two sites(I have connected them and everything works OK).

The problem is that I do not want that broadcasts come from one site to another.

So I have created another vlan on HQ and Remote SWITCH. I have set the port to witch the server is connected to trunk mode allowing the two VLANS.

So when I connet a computer on the remote site I cannot ping that. Any Idea

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Giuseppe Larosa Tue, 11/17/2009 - 07:24
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hello Enkli,

only a L3 routed link is a firewall for broadcasts

a L2 trunk when working does not block broadcasts.

if 802.1Q trunk verify if native mismatch at the two sides of the link.

this can be a problem.

verify using

sh interface type x/y switchport

check for native vlan line

Hope to help


LordFlasheart Tue, 11/17/2009 - 07:46
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Further to Giuseppe's answer you can remove VLANs from trunks by issuing the "switchport trunk allowed vlan remove" command.


hobbe Tue, 11/17/2009 - 08:38
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If you do not want broadcasts to come from site A to site B then you could do one of two things.

Routing, that would set the systems on the different sites in different broadcast domains.


Acces-control Lists on the outside interface of the switch where you block the inbound broadcast address.

I would go with routing.

The acls will most likely cause problems for you somewhere down the line.

Why do you not want broadcasts to go from A to B ?

Broadcasts are a quite essential part of IP if you are in the same subnet and broadcast zone.


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