Overview
How the DAG-bridged overlay networks works and the different components in DAG-bridged overlay networks.
Based on business requirements, enterprise networks have the flexibility to support unique per-VLAN basis overlay functions. Routed overlay is proven and recommended for the large-scale overlay networks, DAG-routed overlay provides flexible Layer 2 flood-free IP subnet stretch for selective networks, and the DAG bridge overlay addresses non-routable legacy endpoints and applications by extending the Layer 2 network across the fabric.
Cisco Catalyst 9000 series switches support the coexistence of routed, Distributed Anycast Gateway-routed, and Distributed Anycast Gateway-bridged overlay networking options within the same macro-segmented IP VRF and the micro-segmented VXLAN GPO-enabled environment.
The following figure illustrates the characteristics of an EVPN multihoming network with DAG bridged overlay fabric.
Hierarchical BGP control plane
The recommended deployment consideration to support controlled route management between EVPN multihoming peers and spine switches in large-scale campus networks is the two-tier hierarchical BGP control plane. Each BGP peering layer manages domain-specific EVPN routes to enable downstream non-blocking and all-active Layer 2 multipath networks, and manages the advertising of fabric network prefixes to enable secure global network access connectivity in core networks.
For more information, refer to the Hierarchical BGP Sessions chapter.
EVPN multihoming
Layer 2 networks with EVPN multihoming functions independent of the fabric core EVPN fabric network. A pair of Cisco Catalyst 9000 series switches in each distribution layer builds and manages EVPN multihoming in each redundancy group.
The unified Layer 2 Ethernet Segment (ES) EtherChannel connection to downstream network device can carry VLANs that are mapped to routed, selective-stretched subnets with DAG-routed overlay and advertised in the fabric core or can continue to be traditional underlay IP networks.
Layer 2 broadcast network boundary
The DAG-bridged overlay networks provide unified Layer 2 or Layer 3 network boundary as the routed overlay and the traditional underlay campus network. The Layer 2 bridge domain and the blast radius remain contained between Layer 2 access and EVPN multihoming-enabled distribution layer systems.
A pair of Cisco Catalyst 9000 series switches in the distribution layer network dynamically builds the Layer 2 VXLAN tunnel using ingress replication mode to extend Broadcast, Unknown Unicast and Multicast (BUM) traffic. With built-in loop-detection and prevention techniques, EVPN multihoming-enabled networks support a secure and scalable Layer 2 network over traditional STP based networks.
Stretched IP subnet and gateway redundancy
The DAG routed overlay network enables stretching an IP subnet between targeted EVPN multihoming network devices that support distributed connected IP or non-IP reachability across fabric networks. The selective group of Cisco Catalyst 9000 series switches in EVPN multihoming mode shares common IPv4 or IPv6 subnet function to support intra-subnet host communication based on the fabric host-routing or MAC address tables while providing load-sharing and gateway redundancy with Anycast gateway as a routed overlay.
Spine route policy
The IPv4 or IPv6 subnets and Layer 2 flood domains stretch across multiple IP gateway systems and demand a non-condensed host-level MAC-only or MAC-IP prefix to support seamless data communication across fabrics and beyond.
Large scale EVPN multihoming-enabled fabric networks benefit from the enforcement of the route policy on leaf switches towards the spine switches.
The iBGP or eBGP prefix advertisement to spine switches in the fabric core is limited to the advertising of RT-2 host prefixes and RT-5 network prefixes along with EAD per-ES RT-1 routes. Cisco Catalyst 9000 series switches in EVPN multihoming mode retain unfiltered routing with an iBGP peering session with remote ES switches for non-blocking Layer 2 multipath EtherChannel.
Routed overlay networks are recommended for better scale, performance, and resiliency.