ACI Multi-Site Control Plane

In order to understand the Cisco ACI Control plane, we have to understand, how Control plane work on underlay and as well as overlay. We will how control plane is works on both scenarios.

Cisco ACI Multi-Site Underlay Control plane

Cisco ACI Multisite underlay control plane supports OSPF protocol for establishing a Neighbourship between spines nodes deployed in each site.

Below figure describes the OSPF peering between spine nodes in each site and ISN.

OSPF control plane is used to exchange site route information for specific IP address defined on Spine nodes.

• BGP-EVPN Router-ID (EVON-ID) is unique IP address configured on each Spine which are part of ACI fabric, and is used to establish MP-BGP EVPN and VPNv4 adjacencies with sine node on remote site.
• Overlay Unicast TEP (O-UTEP): This is Anycast address, shared between all spines in same pod, and is used to source and receive unicast VXLAN data Traffic.
• Overlay Multicast TEP (O-METP): This is Anycast address, shared between all spines in same pod, and is used to perform head end replication of BUM Traffic.

Here in this underlay control plane, EVPN-RID, O-UTEP, O-MTEP are the only prefixes that must be advertised and exchanged across sites, so that ACI fabric can be enable intersite EVPN control plane and VXLAN data plane.

Also these are the only prefix that should be learned in ISN routing domain.  The TEP pool prefixes used within each site do not need to be exchanged across sites to allow intersite communication.

However, the TEP pool summary prefix is always sent from the spines toward the ISN, because this is required for the integration of Cisco ACI Multi-Pod and Multi-Site architectures. It is therefore best practice to ensure that those TEP pool prefixes are filtered on the first ISN device so that they are not injected into the ISN network (as they may overlap with the address space already deployed in the backbone of the network or in remote fabrics).

Cisco ACI Multi-Site Overlay Control Plane

In Cisco ACI Multisite, even though endpoints connected to EPG are stretched across sites, or Endpoint connected to non-stretched EPGs (with defined contracts to allow communication), as soon as host is discovered on fabric , Its Host information ( MAC / IP ) is shared or exchanged between spine  nodes of different sites via MP-BGP-EVPN.