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RTG Area LSR Working Group RFC This document describes the mechanism that can be used to advertise the stub link attributes within the IS-IS or OSPF domain.

Stub links are used commonly within enterprise or service provider networks. One common use case is the inter-AS routing scenario where there are no IGP adjacencies between the adjacent BGP domains, another use case is at the network boundary that the interfaces are used to connect to the application servers. For operators that have multiple ASes interconnecting with each other via the stub links, there is a requirement to obtain the inter-AS topology information as described in . To achieve such goal, it is required that the BGP-LS to be enabled on every router that has the stub links, which is challenging for the network operation. It is desirable to advertise the stub link info into the IGP to ease the deployment of BGP-LS on any router in the IGP domain. For stub links that are used to connect the servers, knowing the status of these stub links can facilitate the routers within the IGP to accomplish TE tasks in some scenarios. But OSPF and IS-IS have no capability to identify such stub links and their associated attributes now. This document defines the protocol extension for OSPFv2/v3 and IS-IS to indicate the stub links and their associated attributes.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

OSPF defines the Inter-AS-TE-v2 LSA and Inter-AS-TE-v3 LSA to carry the TE information about inter-AS links. IS-IS defines the Inter-AS Reachability TLV to carry the TE information about inter-AS links. But they are normally being used under RSVP-TE, especially inter-domain RSVP-TE scenarios. As illustrated in the potential scenarios that described in , there is still the need for a generic solution which also covers non inter-AS stub links. Then, to solve the problems that described in the applied scenarios, this document defines the Stub-Link TLV to identify the stub link and transmit the associated attributes for OSPF and IS-IS respectively.

The following sections define the protocol extension to indicate the stub link and its associated attributes in OSPFv2/v3 and IS-IS.

This document defines the Stub-Link TLV to describe stub link of a single router. This Stub-Link TLV is only applicable to the Inter- AS-TE-v2 LSA and Inter-AS-TE-v3 LSA . Inclusion in other LSAs MUST be ignored. The OSPF Stub-Link TLV which is under the IANA codepoint "Top Level Types in TE LSAs" has the following format:

Type: The TLV type. The value is 7(TBD) for OSPF Stub-Link. Length: Variable, dependent on sub-TLVs Flags: Define the type of the stub-link: U bit(bit 0): Identify the unnumbered stub link if this bit is set. bit 1-bit 15: Reserved Link Prefix Sub-TLV: The prefix of the stub-link. It's format is defined in and . Existing Sub-TLVs: Sub-TLV that defined within "Open Shortest Path First (OSPF) Traffic Engineering TLVs" for TE Link TLV(Value 2) can be included if necessary. If the stub-link is identified as unnumbered stub link (U bit is set), then the "Remote IPv4 Address sub-TLV" or "Remote Interface IPv6 Address sub-TLV", which should be set to the identifier value of remote router, SHOULD be included to facilitate the pairing of inter-AS link. If this TLV is advertised multiple times in the same Inter-AS-TE-v2/ v3 LSA, only the first instance of the TLV is used by receiving OSPFv2/v3 routers. This situation SHOULD be logged as an error. If this TLV is advertised multiple times for the same link in different Inter-AS-TE-v2/v3 LSA originated by the same OSPFrouter, the OSPFStub-Link TLV in these LSAs with the smallest Opaque ID is used by receiving OSPFrouters. This situation may be logged as a warning. It is RECOMMENDED that OSPF routers advertising OSPF Stub-Link TLVs in different OSPF Inter-AS-TE v2/v3 LSAs re-originate these LSAs in ascending order of Opaque ID to minimize the disruption. This document creates a registry for Stub-Link attributes in .

This document defines the IS-IS Stub-Link TLV to describes stub link of a single router. The IS-IS Stub-Link TLV has the following format:

Type: IS-IS TLV codepoint. Value is 151 (TBD) for stub-link TLV. Length: Variable, dependent on sub-TLVs Flags: Define the type of the stub-link: 0: U bit(bit 0): Identify the unnumbered stub link if this bit is set. bit 1-bit 15: Reserved Link Prefix Sub-TLV: The prefix of the stub-link. It's format is defined in and . Existing Sub-TLVs: Sub-TLVs that defined within "IS-IS Sub-TLVs for TLVs Advertising Neighbor Information " can be included if necessary. If the stub-link is identified as unnumbered stub link type (U bit is set), then the "IPv4 Remote ASBR ID" or "IPv6 Remote ASBR ID" sub-TLV SHOULD be included to facilitate the pairing of inter-AS link.

The IPv4 Prefix Sub-TLV has the following format:

Type: IPv4 Prefix Sub-TLV codepoint. Value is 37(TBD) for OSPF(under "Types for sub-TLVs of TE Link TLV (Value 2)"), 46(TBD) for IS-IS(under "IS-IS Sub-TLVs for TLVs Advertising Neighbor Information") Length: Netmask length value of the IPv4 Prefix. Value should be in 2-32. IPv4 Prefix: The value of 4-octet IPv4 Prefix address, the host part should be zero.

The IPv6 Prefix Sub-TLV has the following format:

Type: IPv6 Prefix Sub-TLV codepoint. Value is Value is 38(TBD) for OSPF(under "Types for sub-TLVs of TE Link TLV (Value 2)"), 47(TBD) for IS-IS(under "IS-IS Sub-TLVs for TLVs Advertising Neighbor Information") Length: Netmask length value of the IPv6 Prefix. Value should be in 2-128. IPv6 Prefix: The value of 16-octet IPv6 Prefix address, the host part should be zero.

Security concerns for IS-IS are addressed in and Security concern for OSPFv3 is addressed in Advertisement of the additional information defined in this document introduces no new security concerns.

IANA is requested to the allocation in following registries:

Thanks Ketan Talaulikar, Shunwan Zhang, Peter Psenak, Tony Li, Les Ginsberg, Dhruv Dhody, Jeff Tantsura and Robert Raszuk for their suggestions and comments on this idea.

The following sections describe the scenarios that knowing the stub link related attributes information can help solve the corresponding necessity in questions.

Figure 1 describes the scenario that the necessity of inter-AS topology recovery for Native IP point-to-point stub link scenario. R10, R11 and R12 are located in AS1. R20, R21,R22 are located in AS2. The controller runs BGP-LS with R10 in AS1 and R20 in AS2 respectively. There is one BGP session among the border router R11 and R21, which are connected by several stub links(passive interfaces) between them. The situation within the R21 and R22 are the same. Since the links between the border routers are passive, there will be no IGP neighbors between them. The BGP-LS information carried in each AS will not report these stub links,and the controller can't recovery the inter-AS topology automatically.

Figure 2 describes the similar situation but in LAN environment. The border routers of AS1, AS2 and AS3 are connected via one LAN interfaces(that is to say, the corresponding interfaces on R1, R2 and R3 are on the same subnet). There are three different BGP sessions from the loopback address of the border routers among them respectively. It is necessary to recovery the underlying inter-AS topology automatically.

Figure 3 describes the scenario that the stub link information can be used for egress engineering for Anycast servers that connected to the network. In the example, the R1, R2 and R3 are border routers which are connected directly the server S1, S2 and S3 that have the same IP address IPa. The characteristics of the stub links that connected to these Anycast servers are different. It will be help for the router R0, to know the attributes of the stub links and select the optimal Anycast server to serve the customer's application.

Figure 4 describes the scenario that the stub link information can facilitate the optimized BGP next hop selection. The router R10 and R20 which are located in different AS establish the BGP session directly, with the explicit route set on each other which point to the egress stub interface between the border routers. The attributes of the stub links among the border routers are vary. It is certainly will be helpful for the router R10 and R20 to select the optimized BGP next hop, that is via the stub links among them, to reach each other.

| | | | | +-----------------+ +-------------------+ | | | | | | | | +---+-+ +-+---+ | | | | | +--------+ | | | | +--------+R11 |--------| R21+----------+ | | | | +--------+ | | | | | +---+-+ +-+---+ | | | +--+--+ | | +--+--+ | | | | | | | | | | |R10 | | | |R20 | | | | | | | | | | | +--+--+ +---+-+ +-+---+ +--+--+ | | | | +--------+ | | | | +--------+R12 |--------| R22+----------+ | | | +--------+ | | | +---+-+ +-+---+ | | | | | +-------+AS1+-----+ +---------+AS2+-----+ Figure 4: Optimized BGP next hop selection ]]>