MAC address translation (MAT) is an optional HLE feature that translates the access or network host’s MAC address into a single BD MAC address. This feature decreases the number of BGP EVPN MAC routes-to-advertise per subscriber to one and eliminates the BGP update when hosts are created and removed which increases BGP’s stability.
MAT is performed on traffic in both directions:
traffic from access to network
The system changes the source MAC address to BD MAC address before forwarding the packet to the network direction and change the destination MAC address into a real MAC address of the network host based on a ARP table lookup by using the destination IP address as key.
traffic from network to access
The system changes the destination MAC address (BD MAC) to the real host MAC address based on the ARP table or neighbor table lookup using the IP address as key, and change the source MAC address to a BD MAC address before forwarding the packet to the access direction.
With MAT enabled, upon receiving an ARP or neighbor request, the system performs a lookup in the ARP or neighbor table with the target IP address which determines the next action:
When the target IP address is known, the system responds with a BD MAC address.
When the target IP address is unknown, the request is flooded and the ARP or neighbor table is populated with the response.
Because of the ARP processing, the destination MAC of the received unicast packets would be BD MAC. The system only advertises BD MAC in EVPN when MAT is enabled. For unicast data packets received, if the destination MAC address is a BD MAC address, the ISA performs a lookup in the ARP or neighbor table by using the target IP address as key.
If there is a match, the destination MAC address is changed to the MAC address that resulted from the ARP or neighbor table lookup and change the source MAC address to the BD MAC.
If there is no match if it is unicast ARP/ND packet, then it is flooded, otherwise, it is forwarded to the BD’s ESM SAP.
If the destination MAC address is a unicast MAC address, but not a BD MAC address, the destination MAC address is forwarded based on the MAC table lookup.
If the destination MAC address is a broadcast or multicast MAC address, then the packet is flooded.
MAT requires assistive address resolution to be enabled and with MAT enabled, the system also changes the source and target hardware addresses in ARP/ND requests and replies.