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bk.
Asked: 2014-04-01 09:08:24 +0800 CST

Why would a traffic flow be on a VLAN in one direction, but not the other?

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I've seen PCAPS recently from three separate companies networks where two devices are configured to transmit traffic to each other over UDP, where one side sends a VLAN header on each packet, and the other side does not.For example, two phones sending RTP to each other. X->Y is on VLAN 1, Y->X is not on a VLAN.

I'd considered this to be a misconfiguration, but since I've now seen it three times, and it appears to work fine, is there an advantage doing this? Why/When should this be done?

networking

2 Answers

  1. Best Answer

    VLAN encapsulation is determined by the edge device. If the network configuration is not entirely symmetric, one party may be talking to a raw port, and the other to a tagged port on a machine with multiple segments. If this is a client-server application, then this is not unlikely.

    Also, if there is an intervening router, it may be stripping the header, and if you are monitoring one end of the conversation, you will necessarily see data already processed in one direction.

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  2. If you're using an HP Procurve, this is the default behaviour for a monitor port. If you think about how an Access port works (tag outbound traffic, and un-tag inbound traffic), this sort of makes sense.

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