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Andrey128
Beginner

How does an unmanaged switch affect the spanning tree network

Hello,

I did not find that information anywhere.

Give me some links or explain here pelase.

3 REPLIES 3
Giuseppe Larosa
Hall of Fame Master

Hello Andrey128,

the unmanaged switch may be running a flavor of STP or not.

If the unmanaged switch runs a form of STP it will take part in STP. Being unmanaged it will have default settings like Bridge priority 32768. It is likely that it will run 802.1D that is a single STP instance for all Vlans so interaction with Cisco switches will happen only in Vlan 1.

To avoid that the unmanaged switch becomes the root bridge or the secondary root bridge you have to configure manually the root bridge and the secondary root bridge so that they will have lower then default priorities.

On user ports where you don't expect a switch to be connected you can deploy bpduguard so that the port is put in errordisable when an STP BPDU is detected. This allows to prevent use of unmanaged switches plugged in by end users.

For unmanaged switches not running any form of STP STP bpduguard can detect when two ports from your network devices are connected to it  by recieving each other STP BPDUs so do not use bpdu filter on end user ports.

Hope to help

Giuseppe

ok,

thank you!

Peter Paluch
Hall of Fame Cisco Employee

Hello Andrey,

In addition to Giuseppe's great answer, let me add a few points.

Assuming that the unmanaged switch does not run STP at all, some precautions should be taken:

  1. If other switches run RSTP or RPVST+ and multiple switches are connected to this unmanaged switch, then on the managed switches, the ports towards the unmanaged switch should be manually configured as link-type shared. Cisco switches try to guess the RSTP link type based on the duplex setting. However, this guess will fail them in this case, as the unmanaged switch will negotiate full duplex link (that defaults to point-to-point link type on Cisco switches), however, this interconnection is not truly point-to-point, as there are many RSTP switches interconnected by the same segment.
  2. The STP ran by other switches will not prevent loops that occur on the unmanaged switch itself (e.g. looping two or more its ports).

Having managed and unmanaged switches in a network is therefore not recommended, even though I understand that in many cases, it is not easy to perform such a costly upgrade. The network will work in this mixed mode but it is not protected as well as it could be if all switches supported some STP version.

Best regards,

Peter