Introduction

Ethernet VPN (EVPN) uses BGP as it's control protocol. MP-BGP was extended to support the additional features of EVPN. This document goes over its extensions and uses.

Overview

• The Multi-Protocol BGP (MP-BGP) was added with new extensions to support EVPN. These included the additions of new Network Layer Reachability Information (NLRI) called EVPN NLRI.
• EVPN NLRI is carried in the MP-BGP Address Family Identifier (AFI L2VPN) 25 and Subsequent Address Family Identifier (SAFI EVPN) 70. See other AFI/SAFI at BGP-Address-Family.
• For two PEs to exchange EVPN information they have to establish the BGP session with EVPN capability.
• Below is the new format:

                   +-----------------------------------+
                   |    Route Type (1 octet)           |
                   +-----------------------------------+
                   |     Length (1 octet)              |
                   +-----------------------------------+
                   | Route Type specific (variable)    |
                   +-----------------------------------+

• Route Type - there were 4 types defined for EVPN NLRI. This field species the type used in BGP messages.
• Length - length of the EVPN NLRI included, since Route Type is variable length.
• Route Type Specific - details specific to each route type.

Route Types

• There are four route types in the EVPN NLRI:
  • Ethernet Auto-Discovery (A-D) Route
  • MAC/IP advertisement Route
  • Inclusive Multicast Ethernet Tag Route
  • Ethernet Segment Route

Ethernet Auto-Discovery Route

• Ethernet Auto-Discovery Route EVPN NLRI is used in two conditions Fast Convergence and Aliasing.
• Fast convergence applies to situations where the PE loses connectivity to the CE and it need to invalidate all the MAC addresses learned from that Ethernet Segment.
• PEs will advertise a Unreachable NLRI with this type to invalidate all MAC addresses using the Ethernet Segment as the Next Hop.
• Aliasing is …………
• Use the link below to expend the format of this EVPN NLRI.

                   +---------------------------------------+
                   | Route Distinguisher (RD)   (8 octets) |
                   +---------------------------------------+
                   |Ethernet Segment Identifier (10 octets)|
                   +---------------------------------------+
                   |  Ethernet Tag ID (4 octets)           |
                   +---------------------------------------+
                   |  MPLS Label (3 octets)                |
                   +---------------------------------------+

• For more information on EVPN Fast Convergence see VPN-EVPN-Fast-Convergence
• For more information on EVPN Aliasing see VPN-EVPN-Aliasing

MAC/IP Advertisement Route

• NLRI used to advertise MAC addresses from remote PEs. In the RFC draft this is referred to as Remote Learning vs local learning between the CE and PE.
• Expend below to see the format of MAC/IP NLRI.

                   +---------------------------------------+
                   |      RD   (8 octets)                  |
                   +---------------------------------------+
                   |Ethernet Segment Identifier (10 octets)|
                   +---------------------------------------+
                   |  Ethernet Tag ID (4 octets)           |
                   +---------------------------------------+
                   |  MAC Address Length (1 octet)         |
                   +---------------------------------------+
                   |  MAC Address (6 octets)               |
                   +---------------------------------------+
                   |  IP Address Length (1 octet)          |
                   +---------------------------------------+
                   |  IP Address (0 or 4 or 16 octets)     |
                   +---------------------------------------+
                   |  MPLS Label1 (3 octets)               |
                   +---------------------------------------+
                   |  MPLS Label2 (0 or 3 octets)          |
                   +---------------------------------------+

• IP address by default is not advertised and is optional. The use of IP addresses allows for IP-to-MAC binding. The use case is if a PE receives an ARP request for a MAC it has it is binding table, instead of sending out it can just respond to it locally.

Inclusive Multicast Ethernet Tag Route

                   +---------------------------------------+
                   |      RD   (8 octets)                  |
                   +---------------------------------------+
                   |  Ethernet Tag ID (4 octets)           |
                   +---------------------------------------+
                   |  IP Address Length (1 octet)          |
                   +---------------------------------------+
                   |   Originating Router's IP Addr        |
                   |          (4 or 16 octets)             |
                   +---------------------------------------+

Ethernet Segment Route

• This special route type is used to advertise the Ethernet Segments that each PE is connected to.
• It is used for topologies where the CE has multiple connections to PEs, for each PE to find the other PEs sharing the same Ethernet Segment ID (ESIs).
• The way it works is a PE advertises the Ethernet Segment Route for the Ethernet Segment it is directly connected to. Only the PEs that have the same ESI import it, all others ignore it.
• Ethernet Segment Routes are tagged with ES-Import Route Target ext community.
• After each PE discovers other PEs sharing the same ESI, Designated Forwarder (DF) election begins.
• The elected DF will forward all BUM (broadcast, unknown unicast, multicast) frames. Election is based on lowest IP address of the originating router's IP address.

                   +---------------------------------------+
                   |      RD   (8 octets)                  |
                   +---------------------------------------+
                   |Ethernet Segment Identifier (10 octets)|
                   +---------------------------------------+
                   |  IP Address Length (1 octet)          |
                   +---------------------------------------+
                   |   Originating Router's IP Addr        |
                   |          (4 or 16 octets)             |
                   +---------------------------------------+

Additional Resources

Draft-ietf-l2vpn-evpn
IANA AFI
IANA SAFI

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