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Assume a trunked environment with two switches, S1 and S2. The swtiches are connected with a trunk port designed to pass VLAN 26. What would happen if VLAN 26 on S2 is configured as a private-vlan with the default gateway and DHCP server and default gateway as promisc ports. What if S1's VLAN 26 is configured as a standard VLAN. Would the hosts on S1 be able to communicate with the promisc ports on S2? Would they be able to communicate with the hosts on S2?

To further complicate things what if the DHCP server were to reside on S1 and I wanted S2 to have private VLANS with promisc ports as the gateway and DHCP server while still leaving S1 in a standard vlan configuration.

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1 Answer 1

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What would happen [...]

The world is not going to explode, if this is what you mean. You should have done some basic research beforehand - there even is a Wikipedia article explaining your exact scenario:

The following table shows the traffic which can flow between all these ports.   
            I-Port  P-Port  C1-Port C2-Port Uplink
I-Port      Deny    Permit  Deny    Deny    Permit
P-Port      Permit  Permit  Permit  Permit  Permit
C1-Port     Deny    Permit  Permit  Deny    Permit
C2-Port     Deny    Permit  Deny    Permit  Permit
Uplink      Per/Den Permit  Permit  Permit  Permit

Traffic from an Uplink port to an Isolated port will be denied if it is in the Isolated VLAN. Traffic from an Uplink port to an isolated port will be permitted if it is in the primary VLAN.

[...]

Would the hosts on S1 be able to communicate with the promisc ports on S2?

Obviously, as the trunk would be part of the private VLAN.

Would they be able to communicate with the hosts on S2?

Obviously not - for the same reason. Unless VLAN 26 is the default VLAN for the trunk

what if the DHCP server were to reside on S1

In this case, in order for the DHCP server to communicate with the I-Ports on S2 your trunk needs to be a promiscuous member of the PVLAN.

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Somewhat rude.... but thanks for the help. Upvote! –  user974896 Oct 18 '12 at 16:54

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