Bridge Domain

A “bridge domain” is a set of logical ports that share the same flooding or broadcast characteristics.

In this example, we are going to pass a L2 VLAN through a Juniper MX Series Router (model MX2008) using a “Bridge-Domain”. You can configure one or more bridge domains on MX Series routers to perform Layer 2 bridging. The Layer 2 bridging functions of the MX Series routers include integrated routing and bridging (IRB) for support for Layer 2 bridging and Layer 3 IP routing on the same interface.

In a typical Service Provider environment, most connectivity requests are delivered as L3 P2P connections. But in some rear cases you will be tasked to deliver a purely L2 circuit, for example if a client wishes to connect offices in separate geographical locations. This wouldn’t be a problem if your edge device towards your upstream transit provider is a switch. But what if your edge device is a router?

Routers are L3 devices, they segment networks as opposed to switches (L2 devices) which bridge networks. For this example, we shall refer to the scenario below:

In the scenario above, your upstream transit provider hands over to you L2 VLAN coming from your client’s “Location A” at your boarder router, and you are supposed to pass it through the router as L2 downstream to your client’s “Location B”.

Unfortunately, simply tagging the upstream and downstream interfaces with the VLAN will not do the trick, in this case we use a technique known as “Bridge-Domain”.

Example VLAN ID: 101

Example Client Name: BANK-X

Configure the physical interface facing the upstream transit services

set interfaces xe-1/0/0 description “UPSTREAM-TRANSIT-PROVIDERS”

set interfaces xe-1/0/0 flexible-vlan-tagging

set interfaces xe-1/0/0 encapsulation flexible-ethernet-services

Configure the interface logical unit VLAN 101 facing upstream location

set interfaces xe-1/0/0 unit 101 description “TO-UPSTREAM-CLIENT-BANK-X-LOCATION-A”

set interfaces xe-1/0/0 unit 101 encapsulation vlan-bridge

set interfaces xe-1/0/0 unit 101 family bridge interface-mode trunk

set interfaces xe-1/0/0 unit 101 family bridge vlan-id-list 101

Configure the physical interface facing downstream (Location B)

set interfaces xe-1/0/1 description “DOWNSTREAM-TO-CLIENTS”

set interfaces xe-1/0/1 flexible-vlan-tagging

set interfaces xe-1/0/1 encapsulation flexible-ethernet-services

Configure the interface logical unit VLAN 101 facing downstream (Location B)

set interfaces xe-1/0/1 unit 101 description “TO-DOWNSTREAM-CLIENT-BANK-X-LOCATION-B”

set interfaces xe-1/0/1 unit 101 encapsulation vlan-bridge

set interfaces xe-1/0/1 unit 101 family bridge interface-mode trunk

set interfaces xe-1/0/1 unit 101 family bridge vlan-id-list 101

Configure the bridge-domain to pass the VLAN-ID (101)

set bridge-domains Bank_X_Bridge domain-type bridge

set bridge-domains Bank_X_Bridge vlan-id 101

Now you can “commit check” to validate your configuration and “commit” to save your configuration if the validation passes.

If your configuration is successful, you should be able to learn the MAC addresses of the client interfaces/devices connected at either end of the circuit.

Here are some helpful commands you can use to check the success of your configuration: