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  1. I have configured spanning tree on switch A.
  2. I can show the spanning tree and it is running on switch A
  3. The 2 interface does not form an etherchannel, they are individual port
  4. There are no broadcast storm, they work just fine
  5. The root is on switch A
  6. There is no manual blocking on any of the interface
  7. The router module is 4221,module card is NIM-ES2-4

Why there are no spanning tree on the router module ? Is the spanning tree protocol running on the interface ? Why is it not looping without spanning tree ?

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    Which router and module? It likely is running stp, but not necessarily telling you it is. My 2951 with various modules is running spanning-tree, 'tho there's nothing really to configure. (I'm not digging out the 1720 to see what the WIC-4ESW says.)
    – Ricky
    Commented Oct 10 at 1:32
  • Please add the make, models and configurations of your devices (in text format, using the editor's {} preformat option). Note that for a device to be on topic here the vendor needs to offer optional, paid support, see the help center.
    – Zac67 ♦
    Commented Oct 10 at 9:31
  • The switch modules in routers are limited in features. For example, you cannot do port channels.
    – Ron Maupin ♦
    Commented Oct 13 at 15:42
  • How do you know that STP is not running on the 4221? To what VLAN are the module's interfaces 1 and 2 assigned to? To what VLAN are the two switch's interfaces assigned to?
    – Angelos Vassiliou
    Commented Oct 16 at 12:42

1 Answer 1

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From the Cisco 4-Port and 8-Port Layer 2 Gigabit EtherSwitch Network Interface Module Configuration Guide for Cisco 4000 Series ISR (emphasis mine):

Cisco 4-Ports and 8-Ports Layer 2 Gigabit EtherSwitch Network Interface Modules use STP (the IEEE 802.1D bridge protocol) on all VLANs. By default, a single instance of STP runs on each configured VLAN (provided you do not manually disable STP). You can enable and disable STP on a per-VLAN basis.

Cisco 4-Ports and 8-Ports Layer 2 Gigabit EtherSwitch Network Interface Modules support the following three STP:

  • Multiple Spanning Tree protocol
  • Per-VLAN Spanning Tree+
  • Rapid Per-VLAN Spanning Tree+

https://www.cisco.com/c/en/us/td/docs/routers/access/interfaces/NIM/software/configuration/guide/4_8PortGENIM.html#pgfId-1097411

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