VXLAN EVPN packet walk

IRB model for same-VRF routing

1 · Access-side frame

2 · Across the fabric

3 · Destination-side frame

Underlay headers VXLAN context Tenant Ethernet Tenant IP and payload

Header terms

Flags 0x08 (I=1)
The VXLAN I bit says the 24-bit VNI is valid. With all reserved flag bits zero, the first byte is 0x08; the first four on-wire bytes are 08 00 00 00, or 0x08000000 as one network-order 32-bit word.
RMAC
The VTEP’s overlay router/system MAC. Symmetric IRB uses ingress and egress RMACs for the inner router-to-router Ethernet header carried by an L3 VNI; it is not the destination host MAC.
IRB-MAC-20
The MAC of the IRB/SVI joining VRF_A to VLAN 20/bridge domain 20—usually VLAN 20’s anycast-gateway MAC. In asymmetric IRB it becomes the source MAC of the routed frame sent toward Server B.

Control plane: RT, RD, and EVPN route types decide what is installed, but none of them appears as a packet header.

Assumptions: remote destination leaves; direct EVPN route import for VRF_B; no default, static, policy, or service route from VRF_A to VRF_C.

Asymmetric IRB is standards-defined here for same-VRF A→B. RT leaking alone does not provide the destination MAC-VRF state needed for a generic asymmetric A→D path, so that case retains the symmetric IP-VRF path.

Server access links may be untagged. The leaf maps the local VLAN/bridge domain to a VNI. If both endpoints are on one leaf, forwarding is local and there is no VXLAN header.