what is switchport trunk allowed means

[muzazamubika12]

anyone now what exactly this command is for ?

[George Stefanick · Accepted solution]

When you configure a truck all vlans are allowed. If you want to shape what vlans are allowed over, say just vlan 10 and 20. You can use the allowed command..

Make sense ?

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[muzazamubika12]

yes now i know.

but if we want to allow specific ip address can or not?

[Leo Laohoo]

No it can't.  "allowed" VLAN is Layer 2 while IP Addressing is Layer 3.

You want to allow/deny a specific IP Address or a block of IP Addresses then you are talking about Access Control List (aka ACL).

There are two types:  Standard ACL and Extended ACL.

[Leo Laohoo · Accepted solution]

By default, when you have a trunk link, ALL VLANs are allowed to pass through.

"switchport trunk allow vlan" gives you the control by specifying which VLANs you want to allow to go through the trunk link.  This is very useful when you want to, say, all server to stay inside this switch and will not be able to go "out" to the next switch. 

[Joseph W. Doherty]

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BTW, the common reason for restricting VLANs down trunks is to "prune" the VLAN(s) from where it's not needed.  This avoids forwarding unneeded VLAN broadcast traffic down the trunk and it also helps reduce the STP topology.

Also BTW, if you're using Cisco switches, and using VTP, you can enable automatic pruning, where VTP will block VLAN traffic going down trunks when it's not needed.