Summary

Controller Bridge (CB) and Port Extender (PE) is an IEEE 802.1BR standard implementation in RouterOS for CRS3xx series switches. It allows virtually extending the CB ports with a PE device and manage these extended interfaces from a single controlling device. Such configuration provides a simplified network topology, flexibility, increased port density and ease manageability. An example of Controller Bridge and Port Extender topology can be seen below.

The Controller Bridge establishes communication with the Port Extender through a cascade port. Similarly, the Port Extender will communicate with the Controller Bridge only through an upstream port. On a PE device, control ports must be configured and only one port (closest to the CB) will act as an upstream port, other control ports can act as a backup for upstream port or even cascade port for switches connected in series (e.g. Port Extender 2 and 3 in the image above). Cascade and upstream ports are used to transmit and receive control and network traffic. Extended ports are interfaces that are controlled by the CB device and they are typically connected to the end hosts. Extended ports only transmit and receive network traffic.

See supported features for each switch model below.

Quick setup

In this example, we will create a Controlling Bridge (e.g. a CRS317-1G-16S+ switch) that will connect to a single Port Extender (e.g. a CRS326-24G-2S+ switch) through an SFP+1 interface.

First, configure a bridge with enabled VLAN filtering on a CB device:

/interface bridge
add name=bridge1 vlan-filtering=yes

On the same device, configure a port that is connected to the PE device and will act as cascade port:

/interface bridge port-controller
set bridge=bridge1 cascade-ports=sfp-sfpplus1 switch=switch1

Last, on a PE device, simply configure a control port, which will be selected as an upstream port:

/interface bridge port-extender
set control-ports=sfp-sfpplus1 switch=switch1

Once PE and CB devices are connected, all interfaces that are on the same switch group (except for control ports) will be extended and can be further configured on a CB device. An automatic bridge port configuration will be applied on the CB device which adds all extended ports in a single bridge, this configuration can be modified afterward.

Discovery and control protocols

Before frame forwarding on extended ports is possible, Controlling Bridge and Port Extender must discover each other and exchange with essential information.

CB and PE enabled devices are using a neighbor discovery protocol LLDP with specific Port Extension TLV. This allows CB and PE devices to advertise their support on cascade and control ports.

Once LLDP messages are exchanged between CB and PE, a Control and Status Protocol (CSP) over an Edge Control Protocol (ECP) will initiate. The CSP is used between CB and PE to assert control and receive status information from the associated PE - it assigns unique IDs for extended ports, controls data-path settings (e.g. port VLAN membership) and sends port status information (e.g. interface stats, PoE-out monitoring). The ECP provides a reliable and sequenced frame delivery (encoded with EtherType 0x8940).

Packet flow

To better understand the underlying principles of Controlling Bridge and Port Extender, a packet walkthrough is provided below:

1. An L2 packet is received on the extended port;
2. The Port Extender encapsulates the packet with an E-TAG header (EtherType 0x893F) and forwards it through an upstream port, towards the Controller Bridge. An E-TAG packet contains information regarding the PE source port ID;
3. The Controller Bridge receives the E-TAG packet and knows exactly which extended interface received it. The CB then internally decapsulates the packet and proceed it through a regular switching decision (host learning, destination address lookup, VLAN filtering, etc.);
4. Once a switching decision is made, the CB will again encapsulate the original packet with an E-TAG and send it through a cascade port, towards Port Extender;
   1. For a single destination packet (unicast), the CB will include the PE destination port ID in the E-TAG and send it through a correct cascade port;
   2. For a multi-destination packet (broadcast, multicast or unknown-unicast), the CB will include a target group mark and source port ID in the E-TAG and send a single packet replica per every cascade port;
5. Once a PE device receives an E-TAG packet on the upstream port, PE decapsulates it and sends the original L2 packet through the extended port;
   1. For a single destination packet (unicast), the PE will send the packet only to the correct extended port;
   2. For a multi-destination packet (broadcast, multicast or unknown-unicast), the PE will send a single packet replica per every extended port (except for the source port where the packet was received).

Controller Bridge settings and monitoring

This section describes the Controller Bridge settings and monitoring options. 

Sub-menu: /interface bridge port-controller

After CB and PE devices are configured and connected, each PE device will be automatically visible on the device menu, use print and monitor commands to see more details.

[admin@Controller] > interface bridge port-controller device print
Flags: I - inactive 
 0   name="pe1" pe-mac=64:D1:54:EB:AE:BC descr="MikroTik RouterOS 6.48beta35 (testing) CRS328-24P-4S+" control-ports=pe1-sfpplus1,pe1-sfpplus2 

 1   name="pe2" pe-mac=64:D1:54:C7:3A:58 descr="MikroTik RouterOS 6.48beta35 (testing) CRS326-24G-2S+" control-ports=pe2-sfpplus1
[admin@Controller] > interface bridge port-controller device monitor pe2
                 name: pe2
               status: active
  connected-via-ports: sfp-sfpplus1==pe1-sfpplus1,pe1-sfpplus2==pe2-sfpplus1
   connected-via-devs: controller,pe1

Sub-menu: /interface bridge port-controller device

Additionally, each PE device interface can be monitored on the port menu, use print and monitor commands to see more details.

[admin@Controller] > interface bridge port-controller port print where !disabled
Flags: I - inactive, X - disabled, R - running, U - upstream-port, C - cascade-port 
 #    NAME                                   DEVICE
 0 I  pe1-ether1                             pe1
 1 R  pe1-ether2                             pe1
 2 R  pe1-ether3                             pe1
 3 R  pe1-ether4                             pe1
 4  U pe1-sfpplus1                           pe1
 5 RC pe1-sfpplus2                           pe1
 6 I  pe2-ether1                             pe2
 7 R  pe2-ether2                             pe2
 8 R  pe2-ether3                             pe2
 9 R  pe2-ether4                             pe2
10  U pe2-sfpplus1                           pe2
[admin@Controller] > interface bridge port-controller port monitor [find where !disabled]
           name: pe1-ether1 pe1-ether2 pe1-ether3 pe1-ether4 pe1-sfpplus1 pe1-sfpplus2 pe2-ether1 pe2-ether2 pe2-ether3 pe2-ether4 pe2-sfpplus1
         status: unknown    link-ok    link-ok    link-ok    no-link      link-ok      unknown    link-ok    link-ok    link-ok    no-link
           rate:            1Gbps      1Gbps      1Gbps      10Gbps       10Gbps                  1Gbps      1Gbps      1Gbps      10Gbps
    port-status: not-added  ok         ok         ok         ok           ok           not-added  ok         ok         ok         ok
           pcid:            457        458        459        480          481                     509        510        511        532

Sub-menu: /interface bridge port-controller port

The Controller Bridge can monitor the PoE-out related information from Port Extenders on the port poe menu, use print and monitor commands to see more details. For more information regarding PoE-out, please visit the PoE-out manual.

[admin@Controller] > interface bridge port-controller port poe print
 # NAME                                    DEVICE
 0 pe1-ether1                              pe1
 1 pe1-ether2                              pe1
 2 pe1-ether3                              pe1
 3 pe1-ether4                              pe1
 4 pe1-ether5                              pe1
 5 pe1-ether6                              pe1
 6 pe1-ether7                              pe1
...
[admin@Controller] > interface bridge port-controller port poe monitor pe1-ether2,pe1-ether3
               name: pe1-ether2 pe1-ether3
     poe-out-status: powered-on powered-on
    poe-out-voltage: 52.8V      52.9V
    poe-out-current: 123mA      95mA
      poe-out-power: 6.4W       5W

Port Extender settings

This section describes the Port Extender settings.

Sub-menu: /interface bridge port-extender