Centerlized EVPN Prefix Advertisement for Common Prefixes behind Different CEs

1.1. ECMP for Centerlized RT-5 Advertisement

R1 and R2 both establish a single CE-BGP session with DGW1. These CE-BGP session can be called the centerlized CE-BGP session.¶

SN21 is a common prefix of R1 and R2, when R4 send traffic to a host inside subnet SN21, DGW1' should load-balance that traffic between PE1, PE2 and PE3.¶

         +--------------------->----------------------+
         |                   CE-BGP                   |
         |              PE3                           |
         |             +----------+                   |
         |             | +------+ | ------>           | CE-BGP
      R2 |             | |      | | RT-2R             | SN21,SN2
     +-------+         | | VPNx | | 20.2(MAC34)       | NH=20.2
     |       |  P3.1   | |      | | MAC34             |
     | ...................(20.9)| |                   |
     | .     |         | +------+ |                   |  DGW1'
SN2--| .     |         +----------+               +---|---------+
     | .     |            ^           <---------- |   V         |
     | .     |            | RT-2       RT-5       | +---------+ |
     |(20.2) |            | 20.2       SN21       | |  VPNx   | |
     | .     |            | ESI34      GW-IP=20.2 | |         |....R4
 +---| .     |            |                       | |(z.z.z.z)| |
 |   | .     |         +----------+ ------>       | |         | |
 |   | .     |         | +------+ | RT-2R         | +---------+ |
 |   | ...................(20.9)| | 20.2          |             |
 |   |       |  P4.1   | |      | | MAC34         +-------------+
 |   +-------+         | |      | |
SN21               PE1 | | VPNx | | ------>
 |    R1               | |      | | RT-2R
 |   +-------+         | |      | | 10.2
 |   |       |  P1.1   | |      | | MAC21
 |   | ...................(10.9)| |                      DGW1
 |   | .     |         | +------+ |               +-------------+
 |   | .     |         +----------+               |             |
 +---| .     |            ^           <---------- | +---------+ |
     | .     |            | RT-2       RT-5       | |         | |
     |(10.2) |            | 10.2       SN21       | |  VPNx   | |
     | .     |            | ESI21      GW-IP=10.2 | |         |....R3
     | .     |            |                       | |(z.z.z.z)| |
SN1--| .     |         +----------+ ------>       | +---------+ |
     | .     |         | +------+ | RT-2R         |   ^         |
     | ...................(10.9)| | 10.2          |   |         |
     |       |  P2.1   | |      | | MAC21         +---|---------+
     +-------+         | | VPNx | |                   |
         |             | |      | |                   | CE-BGP
         |             | +------+ |                   | SN21,SN1
         |             +----------+                   | NH=10.2
         |              PE2                           |
         |                   CE-BGP                   |
         +--------------------->----------------------+

Note that we just use centerlized CE-BGP session to discover CE-prefixes, but we still expect a distributed Layer 3 forwarding framework.¶

1.2. Terminology

Most of the terminology used in this documents comes from [RFC7432] and [RFC9136] except for the following:¶

* L3 EVI:

An EVPN instance spanning the Provider Edge (PE) devices participating in that EVPN which contains VRF ACs and maybe contains IRB interfaces or IRC interfaces.¶

* CE-BGP:

The BGP session between PE and CE. Note that CE-BGP route doesn't have a RD or Route-Target.¶

* RMAC:

Router's MAC, which is signaled in the Router's MAC extended community.¶

* RT-2R:

When a MAC/IP Advertisement Route is used in the context of an IP-VRF, it is called as a RT-2R in this draft.¶

* RT-5E:

An EVPN Prefix Advertisement Route with a non-reserved ESI.¶

* RT-5G:

An EVPN Prefix Advertisement Route with a zero ESI and a non-zero GW-IP.¶

* RT-5L:

An EVPN Prefix Advertisement Route with both zero ESI and zero GW-IP, but a valid MPLS label.¶

* Internal Remote PE:

When PEx is called as an EVPN route ERy's internal remote PE, that is saying that, PEx is on the ES which is identified by ERy's ESI field. When ERy's SOI is not zero, that is aslo saying that PEx has been attached to the ethernet tag which is identified by the <ESI, SOI>.¶

* External Remote PE:

When PEx is called as an EVPN route ERy's external remote PE, that is saying that, PEx is not on the ES which is identified by ERy's ESI field. When ERy's SOI is not zero, PEx may aslo be a PE which has not been attached to the ethernet tag which is identified by the <ESI, SOI>.¶

* CE-Prefix:

When an IP prefix can be reached through CEx from PEy, that IP prefix is called as PEy's CE-prefix behind CEx in this draft. PEy's CE-prefix behind CEx is also called as PEy's CE-prefix for short in this draft.¶

* Common CE-Prefix:

When an CE-Prefix can be reached through either CEy or CEz from PEy, in this draft, it is called as a common CE-Prefix of CEy and CEz,from the viewpoint of PEy.¶

* Exclusive CE-Prefix:

When an CE-Prefix of PEy can be reached through CEy, and it can't be reached through other CEs of PEy, it is called as an exlusive CE-Prefix of CEy, from the viewpoint of PEy.¶

* SNGW:

Sub-Net-specific Gate Way IP address, the SNGW of a subnet is an IP address which is used by the hosts of that subnet to be the nexthop of the default route of these host.¶

* Overlay nexthop :

The CE-Prefix's nexthop IP address which is in the address-space of the L3 EVI.¶

* Original Overlay nexthop :

The overlay nexthop which is advertised by the CE through a PE-CE route protocol.¶

3.1. Basic Control Plane Procedures

        *  Based on the BD-10 Route Target in DGW1 and DGW2, the IP
           Prefix route is also imported, and SN1/24 is added to the IP-
           VRF with Overlay Index IP2 pointing at the local BD-10.  In
           this example, it is assumed that the RT-5 from NVE2 is
           preferred over the RT-5 from NVE3.  If both routes were
           equally preferable and ECMP enabled, SN1/24 would also be
           added to the routing table with Overlay Index IP3.