Migration

One of the first questions customers ask is how to integrated ACI with existing brownfield classical ethernet networks. ACI is different in how it handles learning and forwarding compared to traditional classic ethernet switches that each treat the learning individually instead of as a whole.

Due to this specific configurations are required to achieve the goal of connecting the two separate domains. There are two unique characteristics that have to be configured in a bridge domain so that it will work properly.

• ARP Flooding: This parameters forces the ACI fabric to flood to all ports in the same Layer2 domain any ARP requests that are received. In this way any broadcast ARP requests that arrive from an external Layer 2 are forwarded to all endpoints in the same layer 2 domain or to the exterior from inside the ACI fabric
• Unknown unicast forwarding: This parameter also changes the behavior of the ACI fabric to flood any unknown unicast entries to the ports of the same layer 2 domain.

With these two parameter configuration changes completed the bridge domain will behave the same way that a classical ethernet network behaves and makes it possible to join a fabric layer 2 domain with a external layer 2 domain for migration purposes. If you look at the following diagram you will see this concept.

These servers in VLAN 200 inside the fabric can talk to the other servers in the Layer2 domain outside. With these parameter definitions the ACI fabric now modifies the fabric behavior so that these devices can function properly.

You may be asking how does Layer 3 default gateway work. In the same fashion the layer 3 default gateway lives outside the ACI fabric and the devices in the fabric will use that as the default gateway.

With this setup it becomes simple to migrate workloads into the ACI fabric. When all the endpoints of the layer2 domain have been migrated into the ACI fabric, the bridge domain can be reconfigured on the fly to include the default gateway and integrate with a fabric layer 3 external to continue operating 100% inside the fabric.

For the lab you will be configuring specific parameters of this Layer 2 bridge domain to observe the functionality. The following tables expands on the concepts we have shown.

Property	Description
Forwarding	Custom Custom Reveals the Unicast and ARP selections for custom configuration.
Unicast Routing	Enables/Disbles Unicast Routing on the BD
L2 Unknown Unicast	Flood Enables the BD to flood L2 Unknown traffic.
ARP Flooding	Allows the BD to Flood ARP within the BD

Step 1 - Navigate to Bridge Domains in the Tenants Tab under Networking

Navigate to Networking to create a Bridge Domain by clicking:

1. Click on Tenant
2. Expand Networking
3. Right-click on Bridge Domains
4. Click Create Bridge Domains

Step 2 - Create Layer 2 Bridge Domain (BD)

1. Name the aci_p21_bd_l2
2. Select the VRF aci_p21_vrf
3. Forwarding = Custom
4. L2 Unknown Unicast = Flood
5. ARP Flooding = Checked
6. Click Next

7. Uncheck Unicast Routing
8. Click Next

9. Click Finish

Please verify your work before proceeding.