RSTP Root port selection

RouterOS General
1

Hi Guys,

We have a core router CCR 1036, from which, 2 CRS328 switches , SW-1F & SW-4F, connected for LAN distribution. Both switches have LAG connectivity from the router, named “bonding-SW-1F” & “bonding-SW-4F”
The network has default VLAN 1, and the bridge is running in RSTP mode. The bridge is choosing “bonding-SW-1F” as root port every time.I wanted to make “bonding-SW-4F” as my root port since most of my connections are distributed from that switch. I’ve tried lowering path cost, setting admin-mac to that of “bonding-SW-4F”, but still, the bridge root port is choosing “bonding-SW-1F” .

Am I missing something? Please check my bridge configuration below:

/interface bridge
add admin-mac=08:55:31:EB:6C:26 auto-mac=no name=bridge
/interface bridge port
add bridge=bridge interface=bonding-SW-4F
add bridge=bridge interface=sfp-sfpplus1
add bridge=bridge interface=sfp-sfpplus2
add bridge=bridge interface=ether3
add bridge=bridge interface=ether4
add bridge=bridge interface=bonding-SW-1F



[admin-TP@Mikro > /interface bridge print 
Flags: X - disabled, R - running 
 0 R name="bridge" mtu=auto actual-mtu=1500 l2mtu=1580 arp=enabled arp-timeout=auto mac-address=08:55:31:EB:6C:26 protocol-mode=rstp fast-forward=yes igmp-snooping=no 
     auto-mac=no admin-mac=08:55:31:EB:6C:26 ageing-time=5m priority=0x8000 max-message-age=20s forward-delay=15s transmit-hold-count=6 vlan-filtering=no dhcp-snooping=no 
[admin-TP@Mikro] >

Please suggest, what I can do to change the root port? Is it necessary
Capture.PNG
scenario-01.jpg

2

You almost certainly want the CCR to be the root bridge, by setting the priority to say e.g. 0x7000 instead of 0x8000.
This will then show two designated ports, to each of your CRS access switches.
The root port will then show on each CRS. You can’t have a root port on a root bridge.
SW-1F is obviously the root bridge at the moment (check the MAC addresses of all the relevant interfaces on all devices to work out why!).

Of course it’s all a bit moot unless you are planning on connecting the two switches together.

3

It’s a misunderstanding of how the spanning tree works. If there is no ring topology, i.e. there is no other connection between the two switches but via the CCR1036, it doesn’t matter at all which of the three devices is the root bridge. If there is a ring topology, you most likely don’t want either of the switches to be a root bridge of the spanning tree - you probably want the CCR1036 itself to be the root bridge. A root port is the port facing towards the root bridge, so it doesn’t exist on the root bridge itself.

To make a bridge a root one within the network, you have to set its priority parameter to the lowest one among all the other bridges in the system, i.e. 4096 (0x1000). Google up why it is 4096 rather than 0 or 1. Among bridges with the same priority value, the one with lowest numeric value of MAC address is chosen, which explains why in your case, one of the CRS328s became the root although you’ve kept the default priority of 32768 (0x8000) on all devices.

The path cost is only used to choose among the paths towards the root bridge if more than one such path exists.

4

Thanks a lot for helping out! Since I had set all bridge priority to default, the SW-1F was selected as root because of the lower Bridge MAC address. After changing the priority of the CCR bridge to 0x7000, both SW-1F and SW-4F became designated bridges.
I’m planning on having a ring topology later on. Will tune up the configuration for STP again.

5

Thanks for the detailed information! It really helped me clear out some misconceptions as well!