I have a question regarding server to switch connection. I know in case of two switches connected together like below ,STP will work to block 3 out of 4 cables to prevent the layer2 loops, otherwise we need to bundle the ports together and form an etherchannel. etherchannel

Now my question is what is the case when the connection is between a switch and a server?

enter image description here

Will STP block one of the ports if they are not bundled in an etherchannel? Will a loop acure if they are not bundled?


  • Keep in mind that in the switch-to-switch connection that blocking occurs in one direction only. So, for example, if the top switch is the STP root then the bottom switch might be in a blocking state on 3 out of 4 of its links but the top switch would still be in a forwarding state on all 4 (..albeit likely not learning any addresses on the other three).
    – rnxrx
    Jan 8, 2018 at 19:08

2 Answers 2


Without further information, the encircled links indicate channel bonding/trunking. Bonded channels (or LAG trunks, Etherchannel) are regarded as a single, logical link. STP doesn't block any of the physical links.

Even if the physical links to a server are not bonded, there's only half a bridge loop: the switch forwards frames solely based on the destination MAC address and which port this MAC is - currently - located on from the switch's perspective. The server is not likely to speak STP, so no ports are blocked.

When the ports are bonded on the server side, the switch may see the MACs flapping back and forth. Due to the nature of broadcasts, the server will also receive its own broadcasts back on the other port.

Only static trunks can be active on one side without the other. LACP protocol trunks always come up on both sides or not at all (STP will block the spare links between switches) - they should be preferred.

However, if there's some kind of switching software active on the server it behaves like a switch. With neither bonding nor STP active on the dual link you might have a bridge loop, depending on the nature of the switch software. For instance, an ESXi vSwitch never forwards from one physical link to another physical link, so it'll never cause a bridge loop.

An alternative way to use multiple links between a server and a switch is to use them as separate links. This may accomplish better load balancing but doesn't provide full redundancy without further measures.


MPIO, Multipath IO, protocol may be used in such configurations. In such circumstances, no configurations on switch side is needed, and server's software, mpio driver, uses all links in various ways: round robin, least used, manual weight, active/passive, depending on configuration. If you intend to use such configurations with Ms Windows, mpio is probably the best choice.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.