Cisco ACI: Multi-Site Stretched Layer 2 (No-Flooding)

Cisco ACI(Cisco Application Centric Infrastructure)
Multi-Site Stretched Layer 2 (No-Flooding)

What is Cisco ACI ?
Cisco Application Centric Infrastructure (ACI) is a Next generation SDN solution and is designed for data centers spine-leaf architecture for the policy-driven solution. Cisco ACI provides application agility and data center automation with simplified operations.

What is Cisco ACI Multisite ?
Cisco ACI Multisite that enables several data centers to be linked together and managed as a single entity. Administrators are able to set up, monitor, and control network rules, application profiles, and other network resources across numerous sites using Cisco ACI Multisite from a single central console. 

In managing multi-site network settings, this offers a high degree of consistency, automation, and orchestration, assisting enterprises in enhancing network agility, lowering operational complexity, and boosting efficiency.

Fig 1.1-Multi-Site Stretched Layer 2 (No-Flooding)

Multi-Site Stretched Layer 2 (No-Flooding)
Here all the objects (tenants, VRF, bridge domains, and EPGs) to stretch across the sites. However, in the specific bridge domain configuration, no broadcast, unknown unicast, or multicast (BUM) flooding is allowed across the sites. 

The relocated endpoint must be allowed to communicate with endpoints that are part of the same IP subnet that may still be connected in the original site. 

To address this requirement, when the relocated endpoint sends an Address Resolution Protocol (ARP) request for a device in its same IP subnet, the Multi-Site architecture must deliver that ARP request in unicast mode (by performing VXLAN encapsulation directly to the leaf node at the original site in which the destination endpoint is still connected). 

This communication is possible because of the information provided across sites by the MP-BGP EVPN control plane, so the assumption is that the migrated endpoints have been discovered in the new location (a cold migration scenario). 

Traffic originating from the external Layer 3 domain must be delivered to the relocated endpoint. Traditional L3out on border leaf nodes are used. 

Because the same IP subnet is deployed in both sites, usually the same IP prefix information is sent to the WAN from the two sites. This behavior implies that by default incoming traffic may be delivered indifferently to Site 1 or Site 2. 

However, commonly one of the two sites is nominated as the home site for that specific IP subnet (the site at which, at steady state, most of endpoints for that subnet are connected). In this case, routing updates sent to the WAN are properly tuned to help ensure that all incoming traffic is steered toward the home site. 

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