Going Large: Extended Route Targets or Extended Confusion?#

Robin Christ

• Why: A concrete protocol design proposal addressing a real gap in EVPN-VXLAN route target encoding — relevant to the PM's BGP/EVPN protocol internals interest. Signals a potential IETF work item.
• What: Identifies that 32-bit ASN + 24-bit VNI doesn't fit into the 48-bit payload of standard Route Targets (based on BGP Extended Communities). Proposes "Extended Route Targets" based on BGP Large Communities (96 bits total): reserve ASN 0xFFFFFFFF for IANA-managed Large Communities, add an 8-bit type field, leaving 56 bits — a perfect fit for 32-bit AS + 24-bit VNI.
• How: BGP Large Communities (RFC 8092) are extended from "AS-owned" semantics to include IANA-managed types. The proposal moves ASN 0xFFFFFFFF from "NOT RECOMMENDED" to "reserved for IANA-managed BGP Large Communities" with an 8-bit type field. Currently, vendors handle the overflow poorly: Cumulus doesn't auto-generate RTs, Juniper truncates to 2 least-significant ASN bytes.
• Where applicable: EVPN-VXLAN fabric designers using eBGP-only designs with 4-byte ASNs. The proposal is a discussion-starter, not yet an IETF draft. Broadly relevant to anyone who's hit the RT encoding ceiling in large-scale EVPN fabrics.