EVPN VPWS as VRF Attachment Circuit

1.1. Integrated Routing and Cross-connecting

When an IP-VRF instance and an EVPN VPWS instance are connected by an virtual-interface, We call such scenarios as Integrated Routing and Cross-connecting (IRC) use-case, and the virtual-interface connecting EVPN VPWS and IP-VRF is called as IRC interface, because that the packets received from the virtual-interface is routed in the IP-VRF and the data packets sent to the virtual-interface is cross-connected to the remote AC of that EVPN VPWS.¶

The IRC use case are illustrated by the following figure:¶

                    PE1
                  +---------------------+
                  |      IRC1=10.1      |
                  |  +-----+   +------+ |.
                 .|  |VPWS1|---|IPVRF1| |  .
               .  |  +-----+   +------+ |    .
    PE4      .    |                     |      .   PE3
 +--------+.      +---------------------+       +---------+
 |        |                    |                |         |
 |+-----+ |                    | RT-2           |+------+ |
 ||VPWS1| |                    | <10.2, N1>     ||IPVRF1| |
 |+-----+ |                    | label2=IPVRF1  |+------+ |
 |   |    |                    | label1=VPWS1   |   |     |
 +---|----+.                   | RT=VPWS1      .+---|-----+
     |       .      PE2        V             .      |
     |         .  +---------------------+  .        |
     |           .|      IRC2=10.1      |.          |
   N1=10.2        |  +-----+   +------+ |         N2=20.2
                  |  |VPWS1|---|IPVRF1| |
 Behind N1:       |  +-----+   +------+ |
 60.0/24          |                     |
 70.0/24          +---------------------+

There are four PE nodes named PE1/PE2/PE3/PE4 in the above network. PE4 is a pure EVPN VPWS PE, there may be no IP-VRFs on it. PE3 is a pure L3 EVPN PE, there may be no VPWSes or MAC-VRFs on it. PE1 and PE2 are the border of the EVPN VPWS domain and the L3 EVPN domain, so they are both EVPN VPWS PE and L3 EVPN PE, there will be both EVPN IP-VRFs and EVPN VPWSes on them.¶

N1/N2/N3/N4 may be a host or an IP router. N1/N3/N4 is in the subnet 10.0/24. N2 is in the subnet 20.0/24. When N1/N2/N3/N4 is a host, it is also called H1/H2/H3/H4 in this document. When N1/N2/N3/N4 is a router, it is also called R1/R2/R3/R4 in this document. N1/N2/N3/N4's MAC address is M1/M2/M3/M4 respectively.¶

When N1 is a Router, there are two subnets behind N1, these subnets are 60.0/24 and 70.0/24.¶

Note that there may be L2 switches between N1/N2/N3/N4 and their PEs. These switches are not shown in Figure 1.¶

Note that the IRC interfaces are considered as AC interfaces in EVPN VPWS instances. At the same time, they are considered as VRF-ACs in IP-VRF instances.¶

When N1 sends an ARP Request REQ_P1, then REQ_P1 will be forwarded by PE4 to either PE1 or PE2, not to the both. Both the IRC1 on PE1 and IRC2 on PE2 are N1's subnet-gateway(SNGW). But when N2 send an ARP Reply REP_P2 to N1, then PE3 may load-balance REP_P2 to either PE1 or PE2, not to the both.¶

When REQ_P1 is load-balanced to PE1, not to PE2, but PE3 load-balance REP_P2 to PE2, The ARP entry of N1 will not be prepared on PE2 for REP_P2. So the fowarding of REP_P2 will be delayed due to ARP missing.¶

We use RT-2 routes to advertise the ARP entry of N1 from PE2 to PE3. But there SHOULD be no RT-2 advertisement in EVPN VPWS according to [RFC8214]. So the RT-2 routes from PE2 to PE3 SHOULD not carry any export-RTs of VPWS1, and the MPLS label1 field of these RT-2 routes should be set to NULL, not VPWS1.¶

Note that an ESI may be assigned to IRC1 and IRC2, But it is not necessary to advertise that ESI in the L3 EVPN domain. The ESI may be advertised in the EVPN VPWS domain only.¶

1.2. Terminology

Most of the terminology used in this documents comes from [RFC7432] and [I-D.ietf-bess-evpn-prefix-advertisement] except for the following:¶

• VRF AC: VRF Attachment Circuit, An Attachment Circuit (AC) that attaches a CE to an IP-VRF. It is defined in [RFC4364].¶

  IRC: Integrated Routing and Cross-connecting, thus a IRC interface is the virtual interface connecting an IP-VRF and an EVPN VPWS.¶

  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.¶

  IP-AD/EVI: Ethernet Auto-Discovery route per EVI, and the EVI here is an IP-VRF.¶

  IP-AD/ES: Ethernet Auto-Discovery route per ES, and the EVI for one of its route targets is an IP-VRF.¶

  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-2E: A MAC/IP Advertisement Route with a non-reserved ESI.¶

  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.¶

2.1. Constructing MAC/IP Advertisement Route

This draft introduces a new usage/construction of MAC/IP Advertisement route to enable ARP/ND synching for IP addresses in EVPN IRC use-cases. The usage/construction of this route remains similar to that described in RFC 7432 with a few notable exceptions as below.¶

• The Route-Distinguisher should be set to the corresponding EVPN-VPWS context.¶
• The Ethernet Tag should be set to the VPWS Service Instance Identifier of the IRC interface.¶
• The MAC/IP Advertisement SHOULD carry one EVI-RT (for the EVPN VPWS instance) and one ES-Import RT (for the ESI of the IRC interface).¶

• The ESI can be set to the ESI of the IRC interface.¶

  Note that the receiver use the ESI and Ethernet Tag ID to determine the VPWS Service Instance whose IRC interface is the interface that the synced ARP entry will be installed to.¶

• The MPLS Label1 should be set to the label of the <ESI,VPWS service instance identifier>.¶

• The MPLS Label2 is optional. When it is used, it should be set to IPVRF1.¶

• If the MPLS Label2 is used, the RMAC Extended Community attribute SHOULD be carried in VXLAN EVPN.¶

2.2. Constructing Ethernet A-D Route

The ESI of the IRC interface is mainly used in the EVPN VPWS domain. That ESI typically has nothing to do with the fundamental function of the L3 EVPN domain.¶

Note that PE3 or PE4 will not import the RT-2 route with an ES-import RT it doesn't recognize.¶

Note that the Ethernet A-D route advertisement in the EVPN VPWS domain still follows [RFC8214]. The IRC interface is considered as an ordinary AC in the EVPN VPWS domain.¶

2.3. Constructing IP Prefix Advertisement Route

There may be two types of IP prefixes on PE1/PE2. The first type is the prefix of the IRC interface itself. The second type is the prefixes behind N1 (especially when N1 is a router).¶

Given that PE1/PE2 can install synced ARP entries to its proper IRC interface benefitting from the RT-2 route of Section 2. This ensures that both PE1 and PE2 will know all hosts of the IRC interface's own subnet. So it is not necessary for PE1/PE2 to advertise per-host IP prefixes of that subnet to PE3 by RT-2 routes. It is recommended that PE1/PE2 advertise a single RT-5 route of that subnet to PE3 instead. The ESI of these RT-5 routes can be simply set to zero, because when PE3 receives such RT-5 routes from both PE1 and PE2, PE3 can consider them as ECMP or FRR even when their ESI is zero.¶

Note that N1 may be a host or a router, when it is a router, there may be some prefixes behind N1 on PE1. Those prefixes will be learnt via a PE-CE route protocol. N1's IP address may be considered as the overlay nexthop of those prefixes. The overlay nexthop of those prefixes will be carried in the RT-5 route's GW-IP field. Those RT-5 routes are called as RT-5G routes because their Overlay Indexes are their GW-IPs (and their ESI and label are zero).¶

Note that those RT-5G routes are advertised by PE1 to both PE2 and PE3. If the IRC1 interface fails, the prefixes of the second type will achieve more faster convergency on PE3 by the withdraw (from PE1) of the corresponding prefix of the first type.¶