what happens to trunks if you put an unmanaged switch between

[carl_townshend]

Hi all

If I want to connect 2 switches with a trunk, and inbetween the link I put an unmanaged switch, what will happen to the 802.1q frames ?

cheers

Carl

[Peter Paluch]

Hello Carl,

The tagged frames will be able to pass over the unmanaged switch and their 802.1Q tag will not be modified. There are two risks, however:

1. The unmanaged switch will deliver these frames only according to their Destination MAC address. It will not honor the 802.1Q tag as it knows nothing about VLANs. Another device connected to this switch may therefore get access to the tagged traffic as a result, or inject arbitrary frames to any VLAN including the native VLAN.
2. Because the 802.1Q tag is 4B long, the maximum size of frames increases from 1518B to 1522B. Frame sizes over 1518B may be considered as oversized by the unmanaged switch and dropped or corrupted - this has to be experimentally verified. Most unmanaged switches today actually allow for slightly oversized frames but I can not give any guarantees for that.

Best regards,

Peter

[ameya_oke]

Hi Peter,

Nicely explained.

Thanks.

Ameya

[Peter Paluch]

Hello Ameya,

Thank you! I appreciate it very much.

Best regards,

Peter