9.2.1. Constructing MAC/IP Address Advertisement
9.2.1. Constructing MAC/IP Address Advertisement
BGP is extended to advertise these MAC addresses using the MAC/IP Advertisement route type in the EVPN NLRI.
The RD MUST be set per Section 7.9.
The Ethernet Segment Identifier is set to the 10-octet ESI described in Section 5 ("Ethernet Segment").
The Ethernet Tag ID may be zero or may represent a valid Ethernet Tag ID. This field may be non-zero when there are multiple bridge tables in the MAC-VRF (i.e., the PE needs to support VLAN-aware bundle service for that EVI).
When the Ethernet Tag ID in the NLRI is set to a non-zero value for a particular broadcast domain, then this Ethernet Tag ID may be either the CE's Ethernet tag value (e.g., CE VLAN ID) or the EVPN provider's Ethernet tag value (e.g., provider VLAN ID). The latter would be the case if the CE Ethernet tags (e.g., CE VLAN ID) for a particular broadcast domain are different on different CEs.
The MAC Address Length field is in bits, and it is set to 48. MAC address length values other than 48 bits are outside the scope of this document. The encoding of a MAC address MUST be the 6-octet MAC address specified by [802.1Q] and [802.1D-REV].
The IP Address field is optional. By default, the IP Address Length field is set to 0, and the IP Address field is omitted from the route. When a valid IP address needs to be advertised, it is then encoded in this route. When an IP address is present, the IP Address Length field is in bits, and it is set to 32 or 128 bits. Other IP Address Length values are outside the scope of this document. The encoding of an IP address MUST be either 4 octets for IPv4 or 16 octets for IPv6. The Length field of the EVPN NLRI (which is in octets and is described in Section 7) is sufficient to determine whether an IP address is encoded in this route and, if so, whether the encoded IP address is IPv4 or IPv6.
The MPLS Label1 field is encoded as 3 octets, where the high-order 20 bits contain the label value. The MPLS Label1 MUST be downstream assigned, and it is associated with the MAC address being advertised by the advertising PE. The advertising PE uses this label when it receives an MPLS-encapsulated packet to perform forwarding based on the destination MAC address toward the CE. The forwarding procedures are specified in Sections 13 and 14.
A PE may advertise the same single EVPN label for all MAC addresses in a given MAC-VRF. This label assignment is referred to as a per MAC-VRF label assignment. Alternatively, a PE may advertise a unique EVPN label per <MAC-VRF, Ethernet tag> combination. This label assignment is referred to as a per <MAC-VRF, Ethernet tag> label assignment. As a third option, a PE may advertise a unique EVPN label per <ESI, Ethernet tag> combination. This label assignment is referred to as a per <ESI, Ethernet tag> label assignment. As a fourth option, a PE may advertise a unique EVPN label per MAC address. This label assignment is referred to as a per MAC label assignment. All of these label assignment methods have their trade-offs. The choice of a particular label assignment methodology is purely local to the PE that originates the route.
An assignment per MAC-VRF label requires the least number of EVPN labels but requires a MAC lookup in addition to an MPLS lookup on an egress PE for forwarding. On the other hand, a unique label per <ESI, Ethernet tag> or a unique label per MAC allows an egress PE to forward a packet that it receives from another PE, to the connected CE, after looking up only the MPLS labels without having to perform a MAC lookup. This includes the capability to perform appropriate VLAN ID translation on egress to the CE.
The MPLS Label2 field is an optional field. If it is present, then it is encoded as 3 octets, where the high-order 20 bits contain the label value.
The Next Hop field of the MP_REACH_NLRI attribute of the route MUST be set to the IPv4 or IPv6 address of the advertising PE.
The BGP advertisement for the MAC/IP Advertisement route MUST also carry one or more Route Target (RT) attributes. RTs may be configured (as in IP VPNs) or may be derived automatically from the Ethernet Tag ID, in the Unique VLAN case, as described in Section 7.10.1.
It is to be noted that this document does not require PEs to create forwarding state for remote MACs when they are learned in the control plane. When this forwarding state is actually created is a local implementation matter.