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As we know, RSTP merges "disabled", "blocking", "listening" states of STP into one "discarding" state. However, when a port is linked down, let's say no physical link on the port, is there still a state inside it? If it has, is it in "disabled" state or "discarding" state?

Because in some switch's configuration page, I can only see ports in "disabled" state when they are linked down, so I'm confused with its state.

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  • Depends on the switch. Adtran/Cisco doesn't show down ports, bay/nortel/avaya/hp show "disabled"...
    – Ricky
    Mar 14, 2016 at 1:59

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A port which is down, either manually disabled, or down due to no link protocol, doesn't play a part in STP or RSTP, so there is no STP or RSTP state. When a port comes up is when STP or RSTP will take notice of it and perform the proper STP calculations on it, assigning a state to the port.

Some documentation assigns such a port a state of "Disabled," but that is a port role rather than an STP state. Cisco claims that RSTP rolls this into the Discarding state, but since the switch will never try to send any frames to the port, it isn't really Discarding.

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  • 1. WRT the port role, there is no definition about "disabled port" in STP/RSTP neither, is it a terminology for explanation convenience? 2. So is "discarding state includes disabled state" only a Cisco declaration, but not the IEEE802.1w definition?
    – bamb00dark
    Mar 14, 2016 at 2:45
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    Ports have roles, e.g. Root, Designated, etc. I think Disabled fits a role better than a state. A state implies that it matters to STP, but a Disabled port plays no STP role since frames will never be sent to, or received from, a disabled port.
    – Ron Maupin ♦
    Mar 14, 2016 at 2:50
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    Juniper also calls a Disabled port Discarding for RSTP. I simply think it is a moot point since a disabled port will never actually discard frames since it will never receive frames.
    – Ron Maupin ♦
    Mar 14, 2016 at 2:52

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