SR-MPLS / SRv6 Transport Interworking

Segment Routing (SR) allows source nodes to steer packets through SR paths. It can be implemented over IPv6 or MPLS . When SR is implemented over IPv6, it is called SRv6. When SR is implemented over MPLS, it is called SR-MPLS. This document describes procedures for interworking between an SR-MPLS transit domain and an SRv6 transit domain. Each domain contains Provider Edge (PE) and Provider (P) routers. Area Border Routers (ABR) provide connectivity between domains. The procedures described in this document require the ABR to carry a route to each PE router. However, they do not required the ABR to carry service (i.e., customer) routes. In that respect, these procedures resemble L3VPN Interprovider Option C . Procedures described in this document support interworking for global IPv4 and IPv6 service prefixes. They do not support interworking for VPN services prefixes where the SR-MPLS domain uses MPLS service labels.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

<---------------------- SR Path 2 -------------------------- ------ ------ ------ ------ ------ | PE | | P | | ABR | | P | | PE | |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5| ------ ------ ------ ------ ------ Seg. A Seg. B Seg. C Seg. D Seg. E <---------- SRv6 Domain ----------> <--------- SR-MPLS Domain ---------> ]]> depicts interworking between an SR-MPLS domain and an SRv6 domain. The SRv6 domain contains PE Node 1 and P Node 2. The SR-MPLS domain contains P Node 4 and PE node 5. Both domains contain ABR Node 3. Nodes 1 and 2 MUST support SRv6 but are NOT REQUIRED to support SR-MPLS. Nodes 4 and 5 MUST support SR-MPLS but are NOT required to support SRv6. Node 3 MUST support both SRv6 and SR-MPLS. It must also support interworking procedures. Network operators configure a loopback interface on Nodes 1 through 5. These are called Loopback1 through Loopback5. They also configure 2 additional loopback interfaces on PE Node 5. These are called Loopback5.IPv4 and Loopback5.IPv6. Each node instantiates an SR Segment (i.e., Segment A through Segment E). SR Path 1 begins on PE Node 1 and ends on PE Node 5. It visits Nodes 2, 3, 4, and 5, executing the instructions associated with Segments B, C, and D. SR Path 2 begins on PE Node 5 and ends on PE Node 1. It visits Nodes 4, 3, 2, and 1, executing the instructions associated with Segments D, C, B and A.

------ ------ ------ ------ ------ | PE | | P | | ABR | | P | | PE | |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5| ------ ------ ------ ------ ------ Seg. A Seg. B Seg. C Seg. D Seg. E IPv6: IPv6: SR-MPLS: SR-MPLS: SA: Node 1 SA: Node 1 Seg. D Exp. Null DA: Seg. B DA: Seg. C Exp. Null Payload SRH: SRH: Payload SL: 1 SL: 0 SID: Seg. C SID: Seg. C Payload Payload ]]> depicts the forwarding plane as a packet traverses SR Path 1, from Node 1 to Node 5. In this example, PE Node 1 receives an IPv4 packet. PE Node 1 encapsulates the IPv4 packet in an SRv6 header. The SRv6 header contains an IPv6 header and a Segment Routing Header (SRH). The Destination Address in the IPv6 header is a Segment Identifier (SID) that represents Segment B. Segment B is an END instruction instantiated on P Node 2. The SRH contains a Segments Left field and one SID. The Segments Left field is equal to 1 and the SID represents Segment C, and END.DM instruction instantiated on ABR Node 3. PE Node 1 forwards the packet to P Node 2. When P Node 2 receives the packet, it processed the END instruction. It decrements the Segments Left field in the SRH and copies the SID from the SRH to the Destination Address field of the IPv6 header. It then forwards the packet to ABR Node 3. When ABR Node 3 receives the packet, it processes the END.DM instruction. It removes the SRv6 header and replaces it with an SR-MPLS label stack that contains two entries. The top entry represents a prefix SID instantiated on P Node 4. The bottom entry is an Explicit Null instruction (i.e., MPLS label 0), instantiated on PE Node 5. ABR Node 3 then forwards the packet to P Node 4. P Node 4 processes the prefix SID, removing the top entry from the SR-MPLS label stack and forwarding the packet to PE Node 5. PE Node 5 processes the Explicit Null instruction, removing the remaining SR-MPLS label stack entry and processing the payload.

depicts the forwarding plane as a packet traverses SR Path 2, from Node 5 to Node 1. In this example, PE Node 5 receives an IPv4 packet. PE Node 5 encapsulates the IPv4 packet in an SR-MPLS label stack that contains two entries. The top entry represents a prefix SID instantiated on P Node 4. The bottom entry is a binding SID instantiated on ABR Node 3. PE Node 5 then forwards the packet to P Node 4. P Node 4 processes the prefix SID, removing the top entry from the SR-MPLS label stack and forwarding the packet to ABR Node 3. ABR Node 3 processes binding SID, removing the remaining SR-MPLS label stack entry and replacing it with an SRv6 header. The SRv6 header contains an IPv6 header and an SRH. The Destination Address in the IPv6 header is a Segment Identifier (SID) that represents Segment B. Segment B is an END instruction instantiated on P Node 2. The SRH contains a Segments Left field and one SID. The Segments Left field is equal to 1 and the SID represents Segment A, an END.DT46 instruction instantiated on PE Node 1. That instruction causes the packet to be forwarded using the main IP forwarding table, not a VPN forwarding table. ABR Node 3 forwards the packet to P Node 2. When P Node 2 receives the packet, it processed the END instruction. It decrements the Segments Left field in the SRH and copies the SID from the SRH to the Destination Address field of the IPv6 header. It then forwards the packet to PE Node 1. PE Node 1 processes its END.DT46 instruction, removing the SRv6 header and processing the payload.

The End.DM SID MUST be the last segment in a SR Policy. Its arguments are associated with an SR-MPLS label stack. When Node N receives a packet destined to S and S is a locally instantiated End.DM SID, Node N executes the following procedure:

<-- PE and ABR Routes (iBGP) -> <-- PE and ABR Routes(BGP-LU) -> ------ ------ ------ ------ ------ | PE | | P | | ABR | | P | | PE | |Node 1| --- |Node 2| --- |Node 3| --- |Node 4| --- |Node 5| ------ ------ ------ ------ ------ <---------- SRv6 Domain ----------> <--------- SR-MPLS Domain ---------> ]]> In the , PE Node 1 and PE Node 5 exchange customer Network Layer Reachability Information (NLRI) using either a direct BGP session or a route reflector. All customer routes exchanged between PE Node 1 and PE Node 5 belong to the general routing instance. They cannot belong to a VPN. PE Node 1 exchanges loopback routes with ABR Node 3, using either a direct BGP session or a route reflector. Likewise, ABR Node 3 exchanges loopback with PE Node 5, using either a direct BGP session or a route reflector. PE Node 1 and ABR Node 3 bind SIDs to the loopback routes that they exchange, as described in . PE Node 5 and ABR Node 3 bind labels to the loopback routes that they exchange, as described in . Both domains use an IGP to distribute link state information and establish connectivity within the domain.

PE Node 5 advertises an IPv4 customer route to PE Node 1 using BGP as follows: IPv4 Prefix Next-hop: Loopback5.IPv4 This causes PE Node 1 to resolve the customer route through its route to Loopback5.IPv4. The following paragraphs describe how PE Node 1 acquires a route to Loopback5.IPv4. PE Node 5 advertises Loopback5.IPv4 to ABR Node 3 using BGP Labeled Unicast (BGP-LU) as follows: Prefix: Loopback5.IPv4 Next-hop: Loopback5 Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 5 MPLS Label: Explicit Null (0) Now, ABR Node 3 resolves its route to Loopback5.IPv4 through its IGP route to Loopback5. Therefore, when forwarding traffic bound for Loopback5.IPv4, it imposes: An SR-MPLS label stack associated with the IGP route to Loopback5 An additional Explicit Null label ABR Node 3 advertises Loopback5.IPv4 to PE Node 1 using BGP as follows: Prefix: Loopback5.IPv4 Next-hop: Loopback3 Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 1 SID: SID C (i.e., an END.DM SID instantiated on ABR Node 3) Now, PE Node 1 resolves its route to Loopback5.IPv4 through its IGP route to Loopback3. Therefore, when forwarding traffic bound for Loopback5.IPv4, it imposes an SRv6 header that includes the following SIDs: SIDS associated with the IGP route to Loopback3 SID C

PE Node 1 advertises an IPv4 customer route to PE Node 5 using BGP as follows: IPv4 Prefix Next-hop: Loopback1 This causes PE Node 5 to resolve the customer route through its route to Loopback1. The following paragraphs describe how PE Node 5 acquires a route to Loopback1. PE Node 1 advertises Loopback1 to ABR Node 3 using BGP as follows: Prefix: Loopback1 Next-hop: Loopback1 Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 1 SID: SID A (i.e., An END.DT46 SID instantiation on PE Node 1. This instruction causes a packet to be forwarded using the main IP forwarding table, not a VPN forwarding table.) Now, ABR Node 3 resolves its route to Loopback1 through its IGP route to Loopback1. Therefore, when forwarding traffic bound for Loopback1, it imposes an SRv6 header that includes: SIDS associated with the IGP route to Loopback1 SID A ABR Node 3 advertises Loopback1 to PE Node 5 using BGP-LU as follows: Prefix: Loopback1 Next-hop: Loopback3 Color Community: Color to distinguish between paths between ABR Node 3 and PE Node 5 MPLS Label: A binding label that represents the SRv6 path between ABR Node 3 and PE Node 5 Now, PE Node 5 resolves its route to Loopback1 through its IGP route to Loopback3. Therefore, when forwarding traffic bound for Loopback1, it imposes: An SR-MPLS label stack associated with the IGP route to ABR3 An additional label representing a binding SID. The binding SID maps to the SRv6 path between ABR Node 3 and PE Node 5

The authors will request an early allocation from the "SRv6 Endpoint Behaviors" sub-registry of the "Segment Routing Parameters" registry.

Because SR inter-working requires co-operation between inter-working domains, this document introduces no security consideration beyond those addressed in , and .

1.Ketan Talaulikar Cisco Systems ketant.ietf@gmail.com

Thanks to Melchior Aelmans, Takuya Miyasaka and Jeff Tantsura for their comments.