RFC 8660 | Segment Routing with the MPLS Data Plane | December 2019 |
Bashandy, et al. | Standards Track | [Page] |
Segment Routing (SR) leverages the source-routing paradigm. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. In the MPLS data plane, the SR header is instantiated through a label stack. This document specifies the forwarding behavior to allow instantiating SR over the MPLS data plane (SR-MPLS).¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8660.¶
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
The Segment Routing architecture [RFC8402] can be directly applied to the MPLS architecture with no change in the MPLS forwarding plane. This document specifies forwarding-plane behavior to allow Segment Routing to operate on top of the MPLS data plane (SR-MPLS). This document does not address control-plane behavior. Control-plane behavior is specified in other documents such as [RFC8665], [RFC8666], and [RFC8667].¶
The Segment Routing problem statement is described in [RFC7855].¶
Coexistence of SR over the MPLS forwarding plane with LDP [RFC5036] is specified in [RFC8661].¶
Policy routing and traffic engineering using Segment Routing can be found in [ROUTING-POLICY].¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
MPLS instantiation of Segment Routing fits in the MPLS architecture as defined in [RFC3031] from both a control-plane and forwarding-plane perspective:¶
We call the "MPLS Control Plane Client (MCC)" any control-plane entity installing forwarding entries in the MPLS data plane. Local configuration and policies applied on a router are examples of MCCs.¶
In order to have a node segment reach the node, a network operator SHOULD configure at least one node segment per routing instance, topology, or algorithm. Otherwise, the node is not reachable within the routing instance, within the topology, or along the routing algorithm, which restricts its ability to be used by an SR Policy and as a Topology Independent Loop-Free Alternate (TI-LFA).¶
The SR architecture does not prohibit having more than one SID for the same prefix. In fact, by allowing multiple SIDs for the same prefix, it is possible to have different forwarding behaviors (such as different paths, different ECMP and Unequal-Cost Multipath (UCMP) behaviors, etc.) for the same destination.¶
Instantiating Segment Routing over the MPLS forwarding plane fits seamlessly with this principle. An operator may assign multiple MPLS labels or indices to the same prefix and assign different forwarding behaviors to each label/SID. The MCC in the network downloads different MPLS labels/SIDs to the FIB for different forwarding behaviors. The MCC at the entry of an SR domain or at any point in the domain can choose to apply a particular forwarding behavior to a particular packet by applying the PUSH action to that packet using the corresponding SID.¶
When instantiating SR over the MPLS forwarding plane, a SID is represented by an MPLS label or an index [RFC8402].¶
A global SID is a label, or an index that may be mapped to an MPLS label within the Segment Routing Global Block (SRGB), of the node that installs a global SID in its FIB and receives the labeled packet. Section 2.4 specifies the procedure to map a global segment represented by an index to an MPLS label within the SRGB.¶
The MCC MUST ensure that any label value corresponding to any SID it installs in the forwarding plane follows the rules below:¶
Labels allocated in this document are considered per-platform downstream allocated labels [RFC3031].¶
The concepts of SRGB and global SID are explained in [RFC8402]. In general, the SRGB need not be a contiguous range of labels.¶
For the rest of this document, the SRGB is specified by the list of MPLS label ranges [Ll(1),Lh(1)], [Ll(2),Lh(2)],..., [Ll(k),Lh(k)] where Ll(i) =< Lh(i).¶
The following rules apply to the list of MPLS ranges representing the SRGB:¶
A local segment MAY be allocated from the Segment Routing Local Block (SRLB) [RFC8402] or from any unused label as long as it does not use a special-purpose label. The SRLB consists of the range of local labels reserved by the node for certain local segments. In a controller-driven network, some controllers or applications MAY use the control plane to discover the available set of Local SIDs on a particular router [ROUTING-POLICY]. The rules applicable to the SRGB are also applicable to the SRLB, except the SRGB MUST only be used to instantiate global SIDs into the MPLS forwarding plane. The recommended, minimum, or maximum size of the SRGB and/or SRLB is a matter of future study.¶
This subsection specifies how the MPLS label value is calculated given the index of a SID. The value of the index is determined by an MCC such as IS-IS [RFC8667] or OSPF [RFC8665]. This section only specifies how to map the index to an MPLS label. The calculated MPLS label is downloaded to the FIB, sent out with a forwarded packet, or both.¶
Consider a SID represented by the index "I". Consider an SRGB as specified in Section 2.3. The total size of the SRGB, represented by the variable "Size", is calculated according to the formula:¶
size = Lh(1)- Ll(1) + 1 + Lh(2)- Ll(2) + 1 + ... + Lh(k)- Ll(k) + 1¶
The following rules MUST be applied by the MCC when calculating the MPLS label value corresponding to the SID index value "I".¶
The label value corresponding to the SID index "I" is calculated as follows:¶
An example for how a router calculates labels and forwards traffic based on the procedure described in this section can be found in Appendix A.1.¶
The MPLS Architecture [RFC3031] defines the term Forwarding Equivalence Class (FEC) as the set of packets with similar and/or identical characteristics that are forwarded the same way and are bound to the same MPLS incoming (local) label. In Segment Routing MPLS, a local label serves as the SID for a given FEC.¶
We define SR FEC [RFC8402] as one of the following:¶
This section covers the RECOMMENDED procedure for handling the scenario where, because of an error/misconfiguration, more than one SR FEC as defined in this section maps to the same incoming MPLS label. Examples illustrating the behavior specified in this section can be found in Appendix A.2.¶
An incoming label collision occurs if the SIDs of the set of FECs {FEC1, FEC2, ..., FECk} map to the same incoming SR MPLS label "L1".¶
Suppose an anycast prefix is advertised with a Prefix-SID by some, but not all, of the nodes that advertise that prefix. If the Prefix-SID sub-TLVs result in mapping that anycast prefix to the same incoming label, then the advertisement of the Prefix-SID by some, but not all, of the advertising nodes MUST NOT be treated as a label collision.¶
An implementation MUST NOT allow the MCCs belonging to the same router to assign the same incoming label to more than one SR FEC.¶
The objective of the following steps is to deterministically install in the MPLS Incoming Label Map, also known as label FIB, a single FEC with the incoming label "L1". By "deterministically install", we mean if the set of FECs {FEC1, FEC2,..., FECk} map to the same incoming SR MPLS label "L1", then the steps below assign the same FEC to the label "L1" irrespective of the order by which the mappings of this set of FECs to the label "L1" are received. For example, first- come, first-served tiebreaking is not allowed. The remaining FECs may be installed in the IP FIB without an incoming label.¶
The procedure in this section relies completely on the local FEC and label database within a given router.¶
The collision resolution procedure is as follows:¶
Within an MCC, apply tiebreaking rules to select one FEC only, and assign the label to it. The losing FECs are handled as if no labels are attached to them. The losing FECs with algorithms other than the shortest path first [RFC8402] are not installed in the FIB.¶
The default tiebreaking rules are specified as follows:¶
These rules deterministically select which FEC to install in the MPLS forwarding plane for the given incoming label.¶
This document defines the default tiebreaking rules that SHOULD be implemented. An implementation MAY choose to support different tiebreaking rules and MAY use one of these instead of the default tiebreaking rules. To maximize MPLS forwarding consistency in case of a SID configuration error, the network operator MUST deploy, within an IGP flooding area, routers implementing the same tiebreaking rules.¶
Each FEC is assigned an administrative distance. The FEC administrative distance is encoded as an 8-bit value. The lower the value, the better the administrative distance.¶
The default FEC administrative distance order starting from the lowest value SHOULD be:¶
Explicit SID assignment to a FEC that maps to a label outside the SRGB irrespective of the owner MCC. An explicit SID assignment is a static assignment of a label to a FEC such that the assignment survives a router reboot.¶
Dynamic SID assignment:¶
To maximize MPLS forwarding consistency, if the same FEC is advertised in more than one protocol, a user MUST ensure that the administrative distance preference between protocols is the same on all routers of the IGP flooding domain. Note that this is not really new as this already applies to IP forwarding.¶
The numerical sort across FECs SHOULD be performed as follows:¶
Each FEC is assigned a FEC type encoded in 8 bits. The type codepoints for each SR FEC defined at the beginning of this section are as follows:¶
The numerical values above are mentioned to guide implementation. If other numerical values are used, then the numerical values must maintain the same greater-than ordering of the numbers mentioned here.¶
The fields of each FEC are encoded as follows:¶
All addresses are represented in 128 bits as follows:¶
All prefixes are represented by (8 + 128) bits.¶
Encode the remaining set of FECs as follows:¶
The numerical values mentioned in this section are for guidance only. If other numerical values are used, then the other numerical values MUST maintain the same numerical ordering among different SR FECs.¶
The following rule SHOULD be applied when redistributing SIDs with prefixes between routing protocol instances:¶
If the SRGB of the receiving instance is the same as the SRGB of the origin instance, then:¶
Else,¶
It is outside the scope of this document to define local node behaviors that would allow the mapping of the original index into a new index in the receiving instance via the addition of an offset or other policy means.¶
A----IS-IS----B---OSPF----C-192.0.2.1/32 (20001)¶
Consider the simple topology above, where:¶
In this case, A learns 192.0.2.1/32 as an IP leaf connected to B, which is usual for IP prefix redistribution¶
However, according to the redistribution rule above, B decides not to advertise any index with 192.0.2.1/32 into IS-IS because the SRGB is not the same.¶
Consider the example in the illustration described in Section 2.5.2.1.¶
When router B redistributes the prefix 192.0.2.1/32, router B decides to allocate and advertise the same index 1 with the prefix 192.0.2.1/32.¶
Within the SRGB of the IS-IS domain, index 1 corresponds to the local label 16001. Hence, according to the redistribution rule above, router B programs the incoming label 16001 in its FIB to match traffic arriving from the IS-IS domain destined to the prefix 192.0.2.1/32.¶
When determining what outgoing label to use, the ingress node that pushes new segments, and hence a stack of MPLS labels, MUST use, for a given FEC, the label that has been selected by the node receiving the packet with that label exposed as the top label. So in case of incoming label collision on this receiving node, the ingress node MUST resolve this collision by using this same "Incoming Label Collision resolution procedure" and by using the data of the receiving node.¶
In the general case, the ingress node may not have the exact same data as the receiving node, so the result may be different. This is under the responsibility of the network operator. But in a typical case, e.g., where a centralized node or a distributed link-state IGP is used, all nodes would have the same database. However, to minimize the chance of misforwarding, a FEC that loses its incoming label to the tiebreaking rules specified in Section 2.5 MUST NOT be installed in FIB with an outgoing Segment Routing label based on the SID corresponding to the lost incoming label.¶
Examples for the behavior specified in this section can be found in Appendix A.3.¶
PUSH, NEXT, and CONTINUE are operations applied by the forwarding plane. The specifications of these operations can be found in [RFC8402]. This subsection specifies how to implement each of these operations in the MPLS forwarding plane.¶
As described in [RFC8402], PUSH corresponds to pushing one or more labels on top of an incoming packet then sending it out of a particular physical interface or virtual interface, such as a UDP tunnel [RFC7510] or the Layer 2 Tunneling Protocol version 3 (L2TPv3) [RFC4817], towards a particular next hop. When pushing labels onto a packet's label stack, the Time-to-Live (TTL) field [RFC3032] [RFC3443] and the Traffic Class (TC) field [RFC3032] [RFC5462] of each label stack entry must, of course, be set. This document does not specify any set of rules for setting these fields; that is a matter of local policy. Sections 2.10 and 2.11 specify additional details about forwarding behavior.¶
As described in [RFC8402], the CONTINUE operation corresponds to swapping the incoming label with an outgoing label. The value of the outgoing label is calculated as specified in Sections 2.10 and 2.11.¶
As described in [RFC8402], NEXT corresponds to popping the topmost label. The action before and/or after the popping depends on the instruction associated with the active SID on the received packet prior to the popping. For example, suppose the active SID in the received packet was an Adj-SID [RFC8402]; on receiving the packet, the node applies the NEXT operation, which corresponds to popping the topmost label, and then sends the packet out of the physical or virtual interface (e.g., the UDP tunnel [RFC7510] or L2TPv3 tunnel [RFC4817]) towards the next hop corresponding to the Adj-SID.¶
If the active SID in the received packet was a Mirror SID (see [RFC8402], Section 5.1) allocated by the receiving router, the receiving router applies the NEXT operation, which corresponds to popping the topmost label, and then performs a lookup using the contents of the packet after popping the outermost label in the mirrored forwarding table. The method by which the lookup is made, and/or the actions applied to the packet after the lookup in the mirror table, depends on the contents of the packet and the mirror table. Note that the packet exposed after popping the topmost label may or may not be an MPLS packet. A Mirror SID can be viewed as a generalization of the context label in [RFC5331] because a Mirror SID does not make any assumptions about the packet underneath the top label.¶
The label corresponding to the global SID "Si", which is represented by the global index "I" and downloaded to the FIB, is used to match packets whose active segment (and hence topmost label) is "Si". The value of this label is calculated as specified in Section 2.4.¶
For Local SIDs, the MCC is responsible for downloading the correct label value to the FIB. For example, an IGP with SR extensions [RFC8667] [RFC8665] downloads the MPLS label corresponding to an Adj-SID [RFC8402].¶
When instantiated in the MPLS domain, the active segment on a packet corresponds to the topmost label and is calculated according to the procedure specified in Sections 2.10 and 2.11. When arriving at a node, the topmost label corresponding to the active SID matches the MPLS label downloaded to the FIB as specified in Section 2.4.¶
This section specifies the forwarding behavior, including the calculation of outgoing labels, that corresponds to a global SID when applying the PUSH, CONTINUE, and NEXT operations in the MPLS forwarding plane.¶
This document covers the calculation of the outgoing label for the top label only. The case where the outgoing label is not the top label and is part of a stack of labels that instantiates a routing policy or a traffic-engineering tunnel is outside the scope of this document and may be covered in other documents such as [ROUTING-POLICY].¶
Suppose an MCC on router "R0" determines that, before sending the packet towards a neighbor "N", the PUSH or CONTINUE operation is to be applied to an incoming packet related to the global SID "Si". SID "Si" is represented by the global index "I" and owned by the router Ri. Neighbor "N" may be directly connected to "R0" through either a physical or a virtual interface (e.g., UDP tunnel [RFC7510] or L2TPv3 tunnel [RFC4817]).¶
The method by which the MCC on router "R0" determines that the PUSH or CONTINUE operation must be applied using the SID "Si" is beyond the scope of this document. An example of a method to determine the SID "Si" for the PUSH operation is the case where IS-IS [RFC8667] receives the Prefix-SID "Si" sub-TLV advertised with the prefix "P/m" in TLV 135, and the prefix "P/m" is the longest matching network prefix for the incoming IPv4 packet.¶
For the CONTINUE operation, an example of a method used to determine the SID "Si" is the case where IS-IS [RFC8667] receives the Prefix-SID "Si" sub-TLV advertised with prefix "P" in TLV 135, and the top label of the incoming packet matches the MPLS label in the FIB corresponding to the SID "Si" on router "R0".¶
The forwarding behavior for PUSH and CONTINUE corresponding to the SID "Si" is as follows:¶
If neighbor "N" does not support SR or advertises an invalid SRGB or a SRGB that is too small for the SID "Si", then:¶
Else,¶
Determine the outgoing label stack¶
As specified in Section 2.7.3, the NEXT operation corresponds to popping the topmost label. The forwarding behavior is as follows:¶
The action on the packet after popping the topmost label depends on the instruction associated with the incoming label as well as the contents of the packet right underneath the top label that was popped. Examples of the NEXT operation are described in Appendix A.1¶
This section specifies the forwarding behavior for Local SIDs when SR is instantiated over the MPLS forwarding plane.¶
Suppose an MCC on router "R0" determines that the PUSH operation is to be applied to an incoming packet using the Local SID "Si" before sending the packet towards neighbor "N", which is directly connected to R0 through a physical or virtual interface such as a UDP tunnel [RFC7510] or L2TPv3 tunnel [RFC4817].¶
An example of such a Local SID is an Adj-SID allocated and advertised by IS-IS [RFC8667]. The method by which the MCC on "R0" determines that the PUSH operation is to be applied to the incoming packet is beyond the scope of this document. An example of such a method is the backup path used to protect against a failure using TI-LFA [FAST-REROUTE].¶
As mentioned in [RFC8402], a Local SID is specified by an MPLS label. Hence, the PUSH operation for a Local SID is identical to the label push operation using any MPLS label [RFC3031]. The forwarding action after pushing the MPLS label corresponding to the Local SID is also determined by the MCC. For example, if the PUSH operation was done to forward a packet over a backup path calculated using TI-LFA, then the forwarding action may be sending the packet to a certain neighbor that will in turn continue to forward the packet along the backup path.¶
A Local SID on router "R0" corresponds to a local label. In such a scenario, the outgoing label towards next hop "N" is determined by the MCC running on the router "R0", and the forwarding behavior for the CONTINUE operation is identical to the swap operation on an MPLS label [RFC3031].¶
The NEXT operation for Local SIDs is identical to the NEXT operation for global SIDs as specified in Section 2.10.2.¶
This document has no IANA actions.¶
This document describes the applicability of Segment Routing over the MPLS data plane. Segment Routing does not introduce any change in the MPLS data plane. Manageability considerations described in [RFC8402] apply to the MPLS data plane when used with Segment Routing. SR Operations, Administration, and Maintenance (OAM) use cases for the MPLS data plane are defined in [RFC8403]. SR OAM procedures for the MPLS data plane are defined in [RFC8287].¶
This document does not introduce additional security requirements and mechanisms other than the ones described in [RFC8402].¶
Consider the network diagram of Figure 1 and the IP addresses and IGP segment allocations of Figure 2. Assume that the network is running IS-IS with SR extensions [RFC8667], and all links have the same metric. The following examples can be constructed.¶
Suppose R1 wants to send IPv4 packet P1 to R8. In this case, R1 needs to apply the PUSH operation to the IPv4 packet.¶
Remember that the SID index "8" is a global IGP segment attached to the IP prefix 192.0.2.8/32. Its semantic is global within the IGP domain: any router forwards a packet received with active segment 8 to the next hop along the ECMP-aware shortest path to the related prefix.¶
R2 is the next hop along the shortest path towards R8. By applying the steps in Section 2.8, the outgoing label downloaded to R1's FIB corresponding to the global SID index "8" is 1008 because the SRGB of R2 = [1000,5000] as shown in Figure 2.¶
Because the packet is IPv4, R1 applies the PUSH operation using the label value 1008 as specified in Section 2.10.1. The resulting MPLS header will have the "S" bit [RFC3032] set because it is followed directly by an IPv4 packet.¶
The packet arrives at router R2. Because top label 1008 corresponds to the IGP SID index "8", which is the Prefix-SID attached to the prefix 192.0.2.8/32 owned by Node R8, the instruction associated with the SID is "forward the packet using one of the ECMP interfaces or next hops along the shortest path(s) towards R8". Because R2 is not the penultimate hop, R2 applies the CONTINUE operation to the packet and sends it to R3 using one of the two links connected to R3 with top label 1008 as specified in Section 2.10.1.¶
R3 receives the packet with top label 1008. Because top label 1008 corresponds to the IGP SID index "8", which is the Prefix-SID attached to the prefix 192.0.2.8/32 owned by Node R8, the instruction associated with the SID is "send the packet using one of the ECMP interfaces and next hops along the shortest path towards R8". Because R3 is the penultimate hop, we assume that R3 performs penultimate hop popping, which corresponds to the NEXT operation; the packet is then sent to R8. The NEXT operation results in popping the outer label and sending the packet as a pure IPv4 packet to R8.¶
In conclusion, the path followed by P1 is R1-R2--R3-R8. The ECMP awareness ensures that the traffic is load-shared between any ECMP path; in this case, it's the two links between R2 and R3.¶
This section outlines several examples to illustrate the handling of label collision described in Section 2.5.¶
For the examples in this section, we assume that Node A has the following:¶
The following example illustrates incoming label collision resolution for the same FEC type using MCC administrative distance.¶
FEC1:¶
Node A receives an OSPF Prefix-SID Advertisement from Node B for 198.51.100.5/32 with index=5. Assuming that OSPF SRGB on Node A = [1000,1999], the incoming label is 1005.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.105/32 with index=5. Assuming that IS-IS SRGB on Node A = [1000,1999], the incoming label is 1005.¶
FEC1 and FEC2 both use dynamic SID assignment. Since neither of the FECs are of type 'SR Policy', we use the default admin distances of 50 and 60 to break the tie. So FEC1 wins.¶
The following example Illustrates incoming label collision resolution for different FEC types using the MCC administrative distance.¶
FEC1:¶
Node A receives an OSPF Prefix-SID Advertisement from Node B for 198.51.100.6/32 with index=6. Assuming that OSPF SRGB on Node A = [1000,1999], the incoming label on Node A corresponding to 198.51.100.6/32 is 1006.¶
FEC2:¶
IS-IS on Node A assigns label 1006 to the globally significant Adj-SID (i.e., when advertised, the L-Flag is clear in the Adj-SID sub-TLV as described in [RFC8667]). Hence, the incoming label corresponding to this Adj-SID is 1006. Assume Node A allocates this Adj-SID dynamically, and it may differ across router reboots.¶
FEC1 and FEC2 both use dynamic SID assignment. Since neither of the FECs are of type 'SR Policy', we use the default admin distances of 50 and 60 to break the tie. So FEC1 wins.¶
The following example illustrates incoming label collision resolution based on preferring static over dynamic SID assignment.¶
FEC1:¶
OSPF on Node A receives a Prefix-SID Advertisement from Node B for 198.51.100.7/32 with index=7. Assuming that the OSPF SRGB on Node A = [1000,1999], the incoming label corresponding to 198.51.100.7/32 is 1007.¶
FEC2:¶
The operator on Node A configures IS-IS on Node A to assign label 1007 to the globally significant Adj-SID (i.e., when advertised, the L-Flag is clear in the Adj-SID sub-TLV as described in [RFC8667]).¶
Node A assigns this Adj-SID explicitly via configuration, so the Adj-SID survives router reboots.¶
FEC1 uses dynamic SID assignment, while FEC2 uses explicit SID assignment. So FEC2 wins.¶
The following example illustrates incoming label collision resolution using FEC type default administrative distance.¶
FEC1:¶
OSPF on Node A receives a Prefix-SID Advertisement from Node B for 198.51.100.8/32 with index=8. Assuming that OSPF SRGB on Node A = [1000,1999], the incoming label corresponding to 198.51.100.8/32 is 1008.¶
FEC2:¶
Suppose the SR Policy Advertisement from the controller to Node A for the policy identified by (Endpoint = 192.0.2.208, color = 100) that consists of SID-List=<S1, S2> assigns the globally significant Binding-SID label 1008.¶
From the point of view of Node A, FEC1 and FEC2 both use dynamic SID assignment. Based on the default administrative distance outlined in Section 2.5.1, the Binding SID has a higher administrative distance than the Prefix-SID; hence, FEC1 wins.¶
The following example illustrates incoming label collision resolution based on FEC type preference.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.110/32 with index=10. Assuming that the IS-IS SRGB on Node A = [1000,1999], the incoming label corresponding to 203.0.113.110/32 is 1010.¶
FEC2:¶
IS-IS on Node A assigns label 1010 to the globally significant Adj-SID (i.e., when advertised, the L-Flag is clear in the Adj-SID sub-TLV as described in [RFC8667]).¶
Node A allocates this Adj-SID dynamically, and it may differ across router reboots. Hence, both FEC1 and FEC2 both use dynamic SID assignment.¶
Since both FECs are from the same MCC, they have the same default admin distance. So we compare the FEC type codepoints. FEC1 has FEC type codepoint=120, while FEC2 has FEC type codepoint=130. Therefore, FEC1 wins.¶
The following example illustrates incoming label collision resolution based on address family preference.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.111/32 with index=11. Assuming that the IS-IS SRGB on Node A = [1000,1999], the incoming label on Node A for 203.0.113.111/32 is 1011.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 2001:DB8:1000::11/128 with index=11. Assuming that the IS-IS SRGB on Node A = [1000,1999], the incoming label on Node A for 2001:DB8:1000::11/128 is 1011.¶
FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are from the same MCC, they have the same default admin distance. So we compare the FEC type codepoints. Both FECs have FEC type codepoint=120. So we compare the address family. Since IPv4 is preferred over IPv6, FEC1 wins.¶
The following example illustrates incoming label collision resolution based on prefix length.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.112/32 with index=12. Assuming that IS-IS SRGB on Node A = [1000,1999], the incoming label for 203.0.113.112/32 on Node A is 1012.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.128/30 with index=12. Assuming that the IS-IS SRGB on Node A = [1000,1999], the incoming label for 203.0.113.128/30 on Node A is 1012.¶
FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are from the same MCC, they have the same default admin distance. So we compare the FEC type codepoints. Both FECs have FEC type codepoint=120. So we compare the address family. Both are a part of the IPv4 address family, so we compare the prefix length. FEC1 has prefix length=32, and FEC2 has prefix length=30, so FEC2 wins.¶
The following example illustrates incoming label collision resolution based on the numerical value of the FECs.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.113/32 with index=13. Assuming that IS-IS SRGB on Node A = [1000,1999], the incoming label for 203.0.113.113/32 on Node A is 1013.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.213/32 with index=13. Assuming that IS-IS SRGB on Node A = [1000,1999], the incoming label for 203.0.113.213/32 on Node A is 1013.¶
FEC1 and FEC2 both use dynamic SID assignment. Since both FECs are from the same MCC, they have the same default admin distance. So we compare the FEC type codepoints. Both FECs have FEC type codepoint=120. So we compare the address family. Both are a part of the IPv4 address family, so we compare the prefix length. Prefix lengths are the same, so we compare the prefix. FEC1 has the lower prefix, so FEC1 wins.¶
The following example illustrates incoming label collision resolution based on the Routing Instance ID.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.114/32 with index=14. Assume that this IS-IS instance on Node A has Routing Instance ID = 1000 and SRGB = [1000,1999]. Hence, the incoming label for 203.0.113.114/32 on Node A is 1014.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.114/32 with index=14. Assume that this is another instance of IS-IS on Node A but Routing Instance ID = 2000 is different and SRGB = [1000,1999] is the same. Hence, the incoming label for 203.0.113.114/32 on Node A is 1014.¶
These two FECs match all the way through the prefix length and prefix. So the Routing Instance ID breaks the tie, and FEC1 wins.¶
The following example illustrates incoming label collision resolution based on the topology ID.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.115/32 with index=15. Assume that this IS-IS instance on Node A has Routing Instance ID = 1000. Assume that the prefix advertisement of 203.0.113.115/32 was received in the IS-IS Multi-topology advertisement with ID = 50. If the IS-IS SRGB for this routing instance on Node A = [1000,1999], then the incoming label of 203.0.113.115/32 for topology 50 on Node A is 1015.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.115/32 with index=15. Assume that it has the same Routing Instance ID = 1000, but 203.0.113.115/32 was advertised with IS-IS Multi-topology ID = 40, which is different. If the IS-IS SRGB on Node A = [1000,1999], then the incoming label of 203.0.113.115/32 for topology 40 on Node A is also 1015.¶
Since these two FECs match all the way through the prefix length, prefix, and Routing Instance ID, we compare the IS-IS Multi-topology ID, so FEC2 wins.¶
The following example illustrates incoming label collision for resolution based on the algorithm ID.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.116/32 with index=16. Assume that IS-IS on Node A has Routing Instance ID = 1000. Assume that Node B advertised 203.0.113.116/32 with IS-IS Multi-topology ID = 50 and SR algorithm = 0. Assume that the IS-IS SRGB on Node A = [1000,1999]. Hence, the incoming label corresponding to this advertisement of 203.0.113.116/32 is 1016.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.116/32 with index=16. Assume that it is the same IS-IS instance on Node A with Routing Instance ID = 1000. Also assume that Node C advertised 203.0.113.116/32 with IS-IS Multi-topology ID = 50 but with SR algorithm = 22. Since it is the same routing instance, the SRGB on Node A = [1000,1999]. Hence, the incoming label corresponding to this advertisement of 203.0.113.116/32 by Node C is also 1016.¶
Since these two FECs match all the way through in terms of the prefix length, prefix, Routing Instance ID, and Multi-topology ID, we compare the SR algorithm IDs, so FEC1 wins.¶
The following example illustrates incoming label collision resolution based on the FEC numerical value, independent of how the SID is assigned to the colliding FECs.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.117/32 with index=17. Assume that the IS-IS SRGB on Node A = [1000,1999]; thus, the incoming label is 1017.¶
FEC2:¶
Suppose there is an IS-IS Mapping Server Advertisement (SID / Label Binding TLV) from Node D that has range = 100 and prefix = 203.0.113.1/32. Suppose this Mapping Server Advertisement generates 100 mappings, one of which maps 203.0.113.17/32 to index=17. Assuming that it is the same IS-IS instance, the SRGB = [1000,1999] and hence the incoming label for 1017.¶
Even though FEC1 comes from a normal Prefix-SID Advertisement and FEC2 is generated from a Mapping Server Advertisement, it is not used as a tiebreaking parameter. Both FECs use dynamic SID assignment, are from the same MCC, and have the same FEC type codepoint=120. Their prefix lengths are the same as well. FEC2 wins based on its lower numerical prefix value, since 203.0.113.17 is less than 203.0.113.117.¶
The following example illustrates incoming label collision resolution based on address family preference.¶
FEC1:¶
SR Policy Advertisement from the controller to Node A. Endpoint address=2001:DB8:3000::100, color=100, SID-List=<S1, S2>, and the Binding-SID label=1020.¶
FEC2:¶
SR Policy Advertisement from controller to Node A. Endpoint address=192.0.2.60, color=100, SID-List=<S3, S4>, and the Binding-SID label=1020.¶
The FEC tiebreakers match, and they have the same FEC type codepoint=140. Thus, FEC2 wins based on the IPv4 address family being preferred over IPv6.¶
The following example illustrates incoming label resolution based on the numerical value of the policy endpoint.¶
FEC1:¶
SR Policy Advertisement from the controller to Node A. Endpoint address=192.0.2.70, color=100, SID-List=<S1, S2>, and Binding-SID label=1021.¶
FEC2:¶
SR Policy Advertisement from the controller to Node A. Endpoint address=192.0.2.71, color=100, SID-List=<S3, S4>, and Binding-SID label=1021.¶
The FEC tiebreakers match, and they have the same address family. Thus, FEC1 wins by having the lower numerical endpoint address value.¶
This section presents examples to illustrate the effect of incoming label collision on the selection of the outgoing label as described in Section 2.6.¶
The following example illustrates the effect of incoming label resolution on the outgoing label.¶
FEC1:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node B for 203.0.113.122/32 with index=22. Assuming that the IS-IS SRGB on Node A = [1000,1999], the corresponding incoming label is 1022.¶
FEC2:¶
IS-IS on Node A receives a Prefix-SID Advertisement from Node C for 203.0.113.222/32 with index=22. Assuming that the IS-IS SRGB on Node A = [1000,1999], the corresponding incoming label is 1022.¶
FEC1 wins based on the lowest numerical prefix value. This means that Node A installs a transit MPLS forwarding entry to swap incoming label 1022 with outgoing label N and to use outgoing interface I. N is determined by the index associated with FEC1 (index=22) and the SRGB advertised by the next-hop node on the shortest path to reach 203.0.113.122/32.¶
Node A will generally also install an imposition MPLS forwarding entry corresponding to FEC1 for incoming prefix=203.0.113.122/32 pushing outgoing label N, and using outgoing interface I.¶
The rule in Section 2.6 means Node A MUST NOT install an ingress MPLS forwarding entry corresponding to FEC2 (the losing FEC, which would be for prefix 203.0.113.222/32).¶
The following example illustrates the effect of incoming label collision resolution on outgoing label programming on Node A.¶
FEC1:¶
SR Policy Advertisement from the controller to Node A. Endpoint address=192.0.2.80, color=100, SID-List=<S1, S2>, and Binding-SID label=1023.¶
FEC2:¶
SR Policy Advertisement from controller to Node A. Endpoint address=192.0.2.81, color=100, SID-List=<S3, S4>, and Binding-SID label=1023.¶
FEC1 wins by having the lower numerical endpoint address value. This means that Node A installs a transit MPLS forwarding entry to swap incoming label=1023 with outgoing labels, and the outgoing interface is determined by the SID-List for FEC1.¶
In this example, we assume that Node A receives two BGP/VPN routes:¶
We also assume that Node A has a BGP policy that matches color=100 and allows its usage as Service Level Agreement (SLA) steering information. In this case, Node A will install a VPN route with label stack = <S1,S2,V1> (corresponding to FEC1).¶
The rule described in Section 2.6 means that Node A MUST NOT install a VPN route with label stack = <S3,S4,V1> (corresponding to FEC2.)¶
The authors would like to thank Les Ginsberg, Chris Bowers, Himanshu Shah, Adrian Farrel, Alexander Vainshtein, Przemyslaw Krol, Darren Dukes, Zafar Ali, and Martin Vigoureux for their valuable comments on this document.¶
The following contributors have substantially helped the definition and editing of the content of this document:¶
Martin Horneffer Deutsche Telekom Email: Martin.Horneffer@telekom.de¶
Wim Henderickx Nokia Email: wim.henderickx@nokia.com¶
Jeff Tantsura Email: jefftant@gmail.com¶
Edward Crabbe Email: edward.crabbe@gmail.com¶
Igor Milojevic Email: milojevicigor@gmail.com¶
Saku Ytti Email: saku@ytti.fi¶