how to VLAN traffic from RSTP single instance to MST Instance 1

[m.z]

Hello,

I am using Cisco 3650 as L2/3 and another vendor Switch connected to it as Access Device with two parallel ports on different Vlans.

Cat 3650: Using MST. Having vlan1 in MST instance 0 and Vlans (2900, 2901) in MST instance 1. Two DHCP Pools 10.0.1.0/24 and 10.0.2.0/24 for both vlans.

Other Vendor: Using RSTP. It has a default Vlan 1 and two user Vlans (2900, 2901).

Requirement: I want my Other Vendor Vlans (2900, 2901) to talk to Cat 3650 but only with its Instance 1 which is having Vlans 2900 and 2901. But other vendor switch is creating a loop and blocking its port because RSTP puts all Vlans in single instance and when it sends traffic to 3650 it communicates only with MST instance 0 which in turn makes a loop. It doesn't communicate with to MST 1 which is having Vlans 2900 and 2901.

* Picture attached.

Question is: What/How should I configure my Switches so both the VLANs communicates with corresponding VLANs on other side (RSTP <---> MST instance 1)?

[pieterh · Accepted solution]

on both Cisco and foreign switch you need to limit the "allowed vlan" to 2900, 2901 on the trunk port connecting both switches

for Cisco:

- interface Gixx/yy

-   switchport mode trunk

-   switchport allowed vlan 2900, 2901
this way no MST0 vlan reaches the foreign switch and vica versa.
if the foreign switch NEEDS to communicate over vlan 1 then it's more complicated
if the same config as for Cisco is valid on your foreign switch, then you're done.