Cisco ACI Traffic Forwarding Simplified | Part I

This article is part One of a series dedicated to exploring how ACI manages various types of traffic, including Layer 2, Layer 3, and ARP traffic.

To fully understand ACI’s handling of these traffic types, it’s essential to first delve into the concept of endpoint learning, which forms the foundation of ACI’s traffic management capabilities.

What is Endpoint Learning?

In the ACI architecture, an endpoint refers to a host or server connected to an ACI leaf switch. Endpoints are identified by:

1.??? MAC Address (Mandatory): Each endpoint must have a unique MAC address.

2.??? IP Address (Optional): ACI can learn an endpoint’s IP address but may not in certain scenarios. Therefore, an endpoint is defined as having one MAC address and zero or more IP addresses.

·??????? ACI supports up to 1,024 IP addresses per endpoint, alongside a single MAC address.

Comparison: Traditional Networking Tables vs. ACI Tables

• In traditional networks, three primary tables are used for forwarding traffic:

1.??? MAC Address Table: Stores MAC addresses for Layer 2 forwarding.

2.??? Routing Information Base (RIB): Also known as the routing table, it holds IPv4 and IPv6 routes for Layer 3 forwarding.

3.??? ARP Table: Maps IP addresses to MAC addresses for Layer 2-to-Layer 3 resolution.

• In ACI:

MAC Address Table + ARP Table = Endpoint Table

o?? The Endpoint Table combines MAC and IP address information for each endpoint.

o?? ACI learns this data by inspecting the source MAC and IP addresses of incoming packets in the data plane.

o?? This eliminates the dependency on ARP for endpoint learning.

How ACI Learns Endpoints

When a packet arrives at an ACI leaf switch:

1.??? The switch inspects the source MAC address and source IP address in the packet.

2.??? This information is used to identify and record the endpoint in the Endpoint Table.

Routing in ACI: The Role of RIB and ARP Tables

ACI retains a routing table, often called:

·??????? RIB (Routing Information Base) OR

·??????? LPM Table (Longest Prefix Match Table)

• Contents of the ACI Routing Table:

1.??? Internal Routes:

o?? Bridge domain subnets configured under a VRF on the leaf switch.

2.??? External Routes:

o?? Routes received from external routing devices connected via L3Out.

ARP Table in ACI:

While ACI does not rely on ARP for endpoint learning, it maintains an ARP table for a specific purpose:

·??????? Mapping next-hop IP addresses to MAC addresses for devices connected through L3Out.

This is essential when determining the MAC address of the next-hop device for external routing.

Overview of ACI Table Lookup Process

When a packet arrives at an ACI leaf switch, the system uses a structured process to determine how to forward the packet. The process involves two main tables:

The Endpoint Table and the Routing Table, which are queried in a specific order.

Step-by-Step Lookup Order

1. First Lookup: Endpoint Table

• The lookup process always begins with the Endpoint Table.
• Layer 2 Traffic: For packets related to Layer 2 traffic, ACI searches for the destination MAC address in the Endpoint Table.
• Layer 3 Traffic: For Layer 3 traffic, the search focuses on the destination IP address in the Endpoint Table.

2. Second Lookup: Routing Table

• If the destination endpoint is not found in the Endpoint Table, and the traffic involves Layer 3 routing (IP address):

o?? ACI proceeds to search the Routing Table.

• The Routing Table lookup is only necessary if the destination IP address is not already mapped in the Endpoint Table.

Lookup Process Key Takeaways

• The Endpoint Table is always the first step in the lookup process.
• The Routing Table is consulted only when the Endpoint Table does not contain the required destination information (applicable to Layer 3 traffic).
• This efficient lookup process ensures ACI optimizes traffic forwarding by leveraging endpoint learning.

See You in the next one,

Mohammed Khalefa

30.1.2025

Next Article:

PART2 : https://www.dhirubhai.net/pulse/cisco-aci-traffic-forwarding-simplified-part-ii-mohammed-khalefa-ayndf/