ipSpace.net » Documents » Using BGP in a Data Center Leaf-and-Spine Fabric » EVPN Route Target Considerations
Similar to MPLS/VPN, EVPN uses Route Target extended BGP community to indicate the VPN membership of individual prefixes advertised EVPN BGP updates. The Route Target community could take any of the common formats, using IP address or AS number in the high-order bits.
BGP MPLS-Based Ethernet VPN (RFC 7432) and A Network Virtualization Overlay Solution Using Ethernet VPN (RFC 8365) advocate the use of automatic Route Targets based on local AS number and VLAN (RFC 7432) or VXLAN/NVGRE/I-SID/VLAN ID (RFC 8365) in simple EVPN topologies. This approach works very well as long as all the PE-switches use the same BGP AS number, resulting in matching Route Target values on all PE-routers.
Simplest designs using EBGP as the underlay fabric routing protocol use a different AS number on every leaf switch (see Autonomous Systems and AS Numbers section), resulting in automatic Route Targets that are not comparable between PE-switches. It’s thus impossible to deploy simple EBGP-based EVPN fabric without manual Route Target configuration or modification of Route Target handling during the EVPN prefix import process.
There are several ways to make automatic EVPN Route Targets work in environments using EBGP as the underlay fabric routing protocol.
An oft-proposed approach uses the same AS number on all leaf switches. This design clearly works, but requires more complex BGP neighbor configuration using allowas-in and/or as-override.
Several vendors modified local BGP behavior to support auto-generated EVPN route targets across multiple AS numbers:
Other tweaks you might see include:
All these options are vendor-specific modifications to local BGP behavior (similar to BGP weights). While they should have no impact on other EVPN nodes in the same fabric, I’d recommend extensive testing if you want to use them in multi-vendor environment.
Summary:
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