Unicast routing in Cisco ACI

One of the critical features of Cisco ACI is its unicast routing capabilities, which ensure efficient and reliable delivery of data packets within a network. This article explores the principles, configuration, and benefits of unicast routing in Cisco ACI.

If Unicast Routing is disabled,

• IP Learning is disabled on BD
• BD SVI is disabled

Unicast routing is achieved through a combination of Layer 2 and Layer 3 mechanisms, ensuring that data packets can traverse the network efficiently and securely. The following steps outline the unicast routing process:

1. Endpoint Learning: When an endpoint (e.g., a virtual machine) sends a packet, the local leaf switch learns the endpoint's IP and MAC addresses, associating them with the corresponding EPG.
2. Route Distribution: The learned endpoints are propagated throughout the ACI fabric. Spine switches maintain a global endpoint table, allowing any leaf switch to know the location of any endpoint in the fabric.
3. Policy Enforcement: Based on the policies defined in the APIC, the leaf switches enforce access control lists (ACLs) and other security policies to ensure that only authorized traffic is allowed.
4. Packet Forwarding: When a leaf switch receives a packet destined for an endpoint in a different EPG, it consults its routing table, forwards the packet to the appropriate spine switch, which then routes it to the destination leaf switch. The destination leaf switch finally delivers the packet to the endpoint.

Process of Unicast Routing

Endpoint A on Leaf Switch 1 communicates with Endpoint B on Leaf Switch 2:

• Step 1: Endpoint A sends a packet to Endpoint B.
• Step 2: Leaf Switch 1 learns Endpoint A’s details and checks its database for Endpoint B.
• Step 3: Leaf Switch 1 forwards the packet to a spine switch since Endpoint B is on a different leaf switch.
• Step 4: The spine switch uses the APIC-distributed endpoint information to determine that Endpoint B is on Leaf Switch 2.
• Step 5: The spine switch forwards the packet to Leaf Switch 2.
• Step 6: Leaf Switch 2 forwards the packet to Endpoint B.

Conclusion

In a Cisco ACI fabric, the spine-leaf architecture efficiently handles endpoint learning and unicast routing through a combination of centralized policy management and distributed execution. This approach ensures that endpoints are correctly identified and traffic is efficiently routed, optimizing performance and scalability in modern data center environments. By leveraging the strengths of both spine and leaf switches, ACI provides a robust and scalable networking solution that meets the demands of contemporary IT infrastructure.