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  • 1.  MSTP Path Selection Issue in Ring Topology (Juniper EX2300-C & Cisco 3560-CX)

    Posted 05-13-2025 09:54

    I'm currently setting up a prototype ring (or small mesh) network using MSTP, and I'm running into an issue with path selection. The network consists of three switches:

    • Switch N: Juniper EX2300-C (serving as the core distribution switch)

    • Switch S: Juniper EX2300-C

    • Switch A: Cisco Catalyst 3560-CX

    These three switches are interconnected in a ring topology using 1G SFP modules and LC-LC fiber cables. The goal is for Switch N (the core) to have connectivity to Switch A via Switch S - i.e., we want the path N → S → A to be the primary forwarding path, and the direct link N → A to be used only as a backup or failover.

    Here's what we have configured so far:

    • All switches are running MSTP and are in the same MST region.

    • We increased the port cost and port priority on the N → A link to try to discourage its use.

    Despite this, MSTP continues to choose the direct N → A link as the active path, rather than the desired N → S → A path.

    I will attach a rough topology diagram and relevant MSTP configuration snippets for context.

    MSTP and interface configuration snippets:

    Switch N: Juniper EX2300-C

    set protocols mstp configuration-name AMBA-DIST
set protocols mstp revision-level 1
set protocols mstp bridge-priority 0
set protocols mstp interface ge-0/1/0 cost 50000
set protocols mstp interface ge-0/1/1

    Interfaces:

    set interfaces ge-0/1/0 description N-A
set interfaces ge-0/1/0 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/1/0 unit 0 family ethernet-switching vlan members all
set interfaces ge-0/1/1 description N-S
set interfaces ge-0/1/1 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/1/1 unit 0 family ethernet-switching vlan members all

    Switch S: Juniper EX2300-C

    set protocols mstp configuration-name AMBA-DIST
set protocols mstp revision-level 1
set protocols mstp bridge-priority 32k
set protocols mstp interface ge-0/1/0
set protocols mstp interface ge-0/1/1

    Interfaces:

    set interfaces ge-0/1/0 description S-N
set interfaces ge-0/1/0 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/1/0 unit 0 family ethernet-switching vlan members all
set interfaces ge-0/1/1 description S-A
set interfaces ge-0/1/1 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/1/1 unit 0 family ethernet-switching vlan members all

    Switch A: Cisco Catalyst 3560-CX

    spanning-tree mode mst
spanning-tree extend system-id
!
spanning-tree mst configuration
name AMBA-MST
revision 1
instance 1 vlan 12
!
spanning-tree mst 1 priority 24576
spanning-tree vlan 12 priority 4096

    Interfaces:

    interface GigabitEthernet1/0/15
description A-S
switchport mode trunk
spanning-tree mst 1 port-priority 64
spanning-tree mst 1 cost 20000
!
interface GigabitEthernet1/0/16
description A-N
switchport mode trunk
spanning-tree mst 1 cost 50000

    Question: Why might MSTP still be preferring the N → A path despite its higher cost and priority? What should I check or adjust to ensure that N → S → A is the preferred forwarding path?

    Any insights or suggestions would be greatly appreciated!

    Thanks in advance!



    ------------------------------
    NEEHAAR C
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  • 2.  RE: MSTP Path Selection Issue in Ring Topology (Juniper EX2300-C & Cisco 3560-CX)

    Posted 05-14-2025 03:45

    Hi,

    AFAIK mstp protocol between juniper and cisco is not compatible.

    Thanks


    Original Message


  • 3.  RE: MSTP Path Selection Issue in Ring Topology (Juniper EX2300-C & Cisco 3560-CX)

    Posted 05-14-2025 21:45

    Of course the mstp protocol is compatible between Juniper and Cisco.

    The point here is that you're NOT on the same MSTP region, because:

    • you didn't map the same vlans on the same MSTP instances on all the switches
    • your MSTP config name is not the same on all the switches

    So on all the Juniper gears you must «set protocols mstp msti 1 vlan 12», and «set protocols mstp configuration-name AMBA-MST».



    ------------------------------
    Olivier Benghozi
    ------------------------------

    Original Message