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diff --git a/doc/rfc/rfc1364.txt b/doc/rfc/rfc1364.txt new file mode 100644 index 0000000..b01f639 --- /dev/null +++ b/doc/rfc/rfc1364.txt @@ -0,0 +1,787 @@ + + + + + + +Network Working Group K. Varadhan +Request for Comments: 1364 OARnet + September 1992 + + + BGP OSPF Interaction + +Status of this Memo + + This RFC specifies an IAB standards track protocol for the Internet + community, and requests discussion and suggestions for improvements. + Please refer to the current edition of the "IAB Official Protocol + Standards" for the standardization state and status of this protocol. + Distribution of this memo is unlimited. + +Abstract + + This memo defines the various criteria to be used when designing + Autonomous System Border Routers (ASBR) that will run BGP with other + ASBRs external to the AS and OSPF as its IGP. + +Table of Contents + + 1. Introduction ................................................. 2 + 2. Route Exchange ............................................... 2 + 2.1. Exporting OSPF routes into BGP ............................. 3 + 2.2. Importing BGP routes into OSPF ............................. 4 + 3. BGP Identifier and OSPF router ID ............................ 5 + 4. Setting OSPF tags, BGP ORIGIN and AS_PATH attributes ......... 5 + 4.1. Semantics of the characteristics bits ...................... 7 + 4.2. Configuration parameters for setting the OSPF tag .......... 8 + 4.3. Manually configured tags ................................... 9 + 4.4. Automatically generated tags ................................ 9 + 4.4.1. Routes with incomplete path information, pl = 0 ........... 9 + 4.4.2. Routes with incomplete path information, pl = 1 ........... 9 + 4.4.3. Routes with incomplete path information, pl >= 1 ..........10 + 4.4.4. Routes with complete path information, pl = 0 .............10 + 4.4.5. Routes with complete path information, pl = 1 .............11 + 4.4.6. Routes with complete path information, pl >= 1 ............11 + 4.5. Miscellaneous tag settings ..................................12 + 4.6. Summary of the TagType field setting ........................12 + 5. Setting OSPF Forwarding Address and BGP NEXT_HOP attribute ....12 + 6. Security Considerations .......................................13 + 7. Acknowledgements ..............................................13 + 8. Bibliography ..................................................14 + 9. Author's Address ..............................................14 + + + + + +Varadhan [Page 1] + +RFC 1364 BGP OSPF Interaction September 1992 + + +1. Introduction + + This document defines the various criteria to be used when designing + Autonomous System Border Routers (ASBR) that will run BGP [RFC1267] + with other ASBRs external to the AS, and OSPF [RFC1247] as its IGP. + + This document defines how the following fields in OSPF and attributes + in BGP are to be set when interfacing between BGP and OSPF at an + ASBR: + + OSPF cost and type vs. BGP INTER-AS METRIC + OSPF tag vs. BGP ORIGIN and AS_PATH + OSPF Forwarding Address vs. BGP NEXT_HOP + + For a more general treatise on routing and route exchange problems, + please refer to [ROUTE-LEAKING] and [NEXT-HOP] by Philip Almquist. + + This document uses the two terms "Autonomous System" and "Routing + Domain". The definitions for the two are below: + + The term Autonomous System is the same as is used in the BGP-3 RFC + [RFC1267], given below: + + "The use of the term Autonomous System here stresses the fact + that, even when multiple IGPs and metrics are used, the + administration of an AS appears to other ASs to have a single + coherent interior routing plan and presents a consistent picture + of what networks are reachable through it. From the standpoint of + exterior routing, an AS can be viewed as monolithic: reachability + to networks directly connected to the AS must be equivalent from + all border gateways of the AS." + + The term Routing Domain was first used in [ROUTE-LEAKING] and is + given below: + + "A Routing Domain is a collection of routers which coordinate + their routing knowledge using a single (instance of) a routing + protocol." + +2. Route Exchange + + This section discusses the constraints that must be met to exchange + routes between an external BGP session with a peer from another AS + and internal OSPF routes. + + BGP does not carry subnet information in routing updates. Therefore, + when referring to a subnetted network in the OSPF routing domain, we + consider the equivalent network route in the context of BGP. + + + +Varadhan [Page 2] + +RFC 1364 BGP OSPF Interaction September 1992 + + + Multiple subnet routes for a subnetted network in OSPF are collapsed + into one network route when exported into BGP. + +2.1. Exporting OSPF routes into BGP + + 1. The administrator must be able to selectively export routes + into BGP via an appropriate filter mechanism. + + This filter mechanism must support such control with the + granularity of a single network. + + Additionally, the administrator must be able to filter based + on the OSPF tag and the various sub-fields of the OSPF tag. + The settings of the tag and the sub-fields are defined in + section 4 in more detail. + + o By default, no routes must be exported from OSPF into + BGP. A single mechanism must permit all OSPF inter-area + and intra-area routes to be exported into BGP. + + OSPF external routes of type 1 and type 2 must never be + exported into BGP unless they are explicitly configured. + + 2. When configured to export a network, the ASBR must advertise + a network route for a subnetted network, as long as at least + one subnet in the subnetted network is reachable via OSPF. + + 3. The network administrator must be able to statically + configure the BGP attribute INTER-AS METRIC to be used for + any network route. + + o By default, the INTER_AS METRIC must default to 1. + + Explanatory text: The OSPF cost and the BGP INTER-AS METRIC + are of different widths. The OSPF cost is a two level + metric. The BGP INTER-AS METRIC is only an optional non- + transitive attribute. Hence, a more complex BGP INTER-AS + METRIC-OSPF cost mapping scheme is not necessary. + + 4. When an ASBR is advertising an OSPF route to network Y to + external BGP neighbours and learns that the route has become + unreachable, the ASBR must immediately propogate this + information to the external BGP neighbours. + + 5. An implementation of BGP and OSPF on an ASBR must have a + mechanism to set up a minimum amount of time that must elapse + between the learning of a new route via OSPF and subsequent + advertisement of the route via BGP to the external + + + +Varadhan [Page 3] + +RFC 1364 BGP OSPF Interaction September 1992 + + + neighbours. + + o The default value for this setting must be 0, indicating + that the route is to be advertised to the neighbour BGP + peers instantly. + + Note that [RFC1267] mandates a mechanism to dampen the + inbound advertisements from adjacent neighbours. + +2.2. Importing BGP routes into OSPF + + 1. BGP implementations should allow an AS to control + announcements of BGP-learned routes into OSPF. + Implementations should support such control with the + granularity of a single network. Implementations should also + support such control with the granularity of an autonomous + system, where the autonomous system may be either the + autonomous system that originated the route or the autonomous + system that advertised the route to the local system + (adjacent autonomous system). + + o By default, no routes must be imported from BGP into + OSPF. Administrators must configure every route they + wish to import. + + A mechanism may allow an administrator to configure an + ASBR to import all the BGP routes into the OSPF routing + domain. + + 2. The administrator must be able to configure the OSPF cost and + the OSPF metric type of every route imported into OSPF. + + o The OSPF cost must default to 1; the OSPF metric type + must default to type 2. + + 3. Routes learned via IBGP must not be imported into OSPF. + + 4. The ASBR must never generate a default route into the OSPF + routing domain unless explicitly configured to do so. + + A possible criterion for generating default into an IGP is to + allow the administrator to specify a set of (network route, + AS_PATH, default route cost, default route type) tuples. If + the ASBR learns of the network route for an element of the + set, with the corresponding AS_PATH, then it generates a + default route into the OSPF routing domain, with cost + "default route cost" and type, "default route type". The + lowest cost default route will then be injected into the OSPF + + + +Varadhan [Page 4] + +RFC 1364 BGP OSPF Interaction September 1992 + + + routing domain. + + This is the recommended method for originating default routes + in the OSPF routing domain. + +3. BGP Identifier and OSPF router ID + + The BGP identifier must be the same as the OSPF router id at all + times that the router is up. + + This characteristic is required for two reasons. + + i. Consider the scenario in which 3 routers, RT1, RT2, and RT3, + belong to the same autonomous system. + + +-----+ + | RT3 | + +-----+ + | + + Autonomous System running OSPF + + / \ + +-----+ +-----+ + | RT1 | | RT2 | + +-----+ +-----+ + + Both RT1 and RT2 have routes to an external network X and import it + into the OSPF routing domain. RT3 is advertising the route to + network X to other external BGP speakers. RT3 must use the OSPF + router ID to determine whether it is using RT1 or RT2 to forward + packets to network X and hence build the correct AS_PATH to advertise + to other external speakers. + + More precisely, RT3 must use the AS_PATH of the route announced by + the ASBR, whose BGP Identifier is the same as the OSPF routerID + corresponding to its route for network X. + + ii. It will be convenient for the network administrator looking at + an ASBR to correlate different BGP and OSPF routes based on + the identifier. + +4. Setting OSPF tags, BGP ORIGIN and AS_PATH attributes + + The OSPF external route tag is a "32-bit field attached to each + external route . . . It may be used to communicate information + between AS boundary routers; the precise nature of such information + is outside the scope of [the] specification." [RFC1247] + + + +Varadhan [Page 5] + +RFC 1364 BGP OSPF Interaction September 1992 + + + OSPF imports information from various routing protocols at all its + ASBRs. In some instances, it is possible to use protocols other than + EGP or BGP across autonomous systems. It is important, in BGP, to + differentiate between routes that are external to the OSPF routing + domain but must be considered internal to the AS, as opposed to + routes that are external to the AS. + + Routes that are internal to the AS and that may or may not be + external to the OSPF routing domain will not come to the various BGP + speakers via IBGP. Therefore, ASBRs running BGP must have knowledge + of this class of routes so that they can advertise these routes to + the various external AS without waiting for IBGP updates about these + routes. + + Additionally, in the specific instance of an AS intermixing routers + running EGP and BGP as external gateway routing protocols, using OSPF + as an IGP, the network administrator does not have to configure IBGP + on every ASBR running EGP and not running BGP, if this information + can be carried in the OSPF tag field. + + We use the external route tag field in OSPF to intelligently set the + ORIGIN and AS_PATH attributes in BGP. Both the ORIGIN and AS_PATH + attributes are well-known, mandatory attributes in BGP. The exact + mechanism for setting the tags is defined below. + + The tag is broken up into sub-fields shown below. The various sub- + fields specify the characteristics of the route imported into the + OSPF routing domain. + + The high bit of the OSPF tag is known as the "Automatic" bit. When + this bit is set to 1, the following sub-fields apply: + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |a|c|p l| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + a is 1 bit called the Automatic bit, indicating that the + Completeness and PathLength bits have been generated + automatically by a router. The meaning of this characteristic + and its setting are defined below. + + c is 1 bit of Completeness information. The meaning of this + characteristic and its settings are defined below. + + pl are 2 bits of PathLength information. The meaning of this + characteristic and its setting are defined below. + + + +Varadhan [Page 6] + +RFC 1364 BGP OSPF Interaction September 1992 + + + ArbitraryTag (or "at") + is 12 bits of tag information, which defaults to 0 but can be + configured to anything else. + + AutonomousSystem (or "as") + is 16 bits, indicating the AS number corresponding to the + route, 0 if the route is to be considered as part of the local + AS. + + When the Automatic bit is set to 0, the following sub-fields apply: + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |a| LocalInfo | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + a is 1 bit called the Automatic bit, set to 0. + + LocalInfo (or "li") + is 31 bits of an arbitrary value, manually configured by the + network administrator. + + The format of the tag for various values of the characteristics bits + is defined below. + +4.1. Semantics of the characteristics bits + + The Completeness and PathLength characteristics bits define the + characteristic of the route imported into OSPF from other ASBRs in + the autonomous system. This setting is then used to set the ORIGIN + and NEXT_HOP attributes when re-exporting these routes to an external + BGP speaker. + + o The "a" bit or the Automatic characteristic bit is set when + the Completeness and PathLength characteristics bits are + automatically set by a border router. + + For backward compatibility, the Automatic bit must default to + 0 and the network administrator must have a mechanism to + enable automatic tag generation. Nothing must be inferred + about the characteristics of the OSPF route from the tag + bits, unless the tag has been automatically generated. + + o The "c" bit of the Completeness characteristic bit is set + when the source of the incoming route is known precisely, for + instance, from an IGP within the local autonomous system or + EGP at one of the autonomous system's boundaries. It refers + + + +Varadhan [Page 7] + +RFC 1364 BGP OSPF Interaction September 1992 + + + to the status of the path information carried by the routing + protocol. + + o The "pl" or the PathLength characteristic sub-field is set + depending on the length of the AS_PATH that the protocol + could have carried when importing the route into the OSPF + routing domain. The length bits will indicate whether the + AS_PATH attribute for the length is zero, one, or greater + than one. + + Routes imported from an IGP will usually have an AS_PATH of + length of 0, routes imported from an EGP will have an AS_PATH + of length 1, BGP and routing protocols that support complete + path information, either as AS_PATHs or routing domain paths, + will indicate a path greater than 1. + + The OSPF tag is not wide enough to carry path information + about routes that have an associated PathLength greater than + one. Path information about these routes will have to be + carried via IBGP. Such routes must not be exported from OSPF + into BGP. + + For brevity in the following sections, the keywords O and P refer to + the BGP ORIGIN and AS_PATH attributes respectively. Likewise, we use + the abbreviations , "l" and "nh" for the local_AS and next_hop_AS + respectively in the following sections. + +4.2. Configuration parameters for setting the OSPF tag + + o There must be a mechanism to enable automatic generation of + the tag characteristic bits. + + o Configuration of an ASBR running OSPF must include the + capability to associate a tag value, for the ArbitraryTag, or + LocalInfo sub-field of the OSPF tag, with each instance of a + routing protocol. + + o Configuration of an ASBR running OSPF must include the + capability to associate an AS number with each instance of a + routing protocol. + + Associating an AS number with an instance of an IGP is + equivalent to flagging those set of routes imported from the + IGP to be external routes outside the local autonomous + system. + + Specifically, when the IGP is RIP [RFC1058], it should be + possible to associate a tag and/or an AS number with every + + + +Varadhan [Page 8] + +RFC 1364 BGP OSPF Interaction September 1992 + + + interface running RIP on the ASBR. + +4.3. Manually configured tags + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |0| LocalInfo | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + This tag setting corresponds to the administrator manually setting + the tag bits. Nothing shall be inferred about the characteristics of + the route corresponding to this tag setting. + + For backward compatibility with existing implementations of OSPF + currently deployed in the field, this must be the default setting for + importing routes into the OSPF routing domain. There must be a + mechanism to enable automatic tag generation for imported routes. + + The OSPF tag to BGP attribute mappings for these routes must be + a=0, li=Arbitrary_Value => O=<INCOMPLETE>, P=<l> + +4.4. Automatically generated tags + + 4.4.1. Routes with incomplete path information, pl = 0. + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|0|0|0| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + These are routes imported from routing protocols with incomplete + path information and cannot or may not carry the neighbour AS or + AS path as part of the routing information. + + The OSPF tag to BGP attribute mappings for these routes must be + a=1,c=0,pl=00,as=0 => O=<EGP>, P=<l> + + 4.4.2 Routes with incomplete path information, pl = 1. + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|0|0|1| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + These are routes imported from routing protocols with incomplete + + + +Varadhan [Page 9] + +RFC 1364 BGP OSPF Interaction September 1992 + + + path information and carry the neighbour AS as part of the routing + information. + + The OSPF tag to BGP attribute mappings for these routes must be + a=1,c=0,pl=01,as=nh => O=<EGP>, P=<l, nh> + + This setting should be used for importing EGP routes into the OSPF + routing domain. This setting can also be used when importing BGP + routes whose origin=<EGP> and AS_PATH=<nh>; if the BGP learned + route has no other transitive attributes, then its propogation via + IBGP can be suppressed. + + 4.4.3. Routes with incomplete path information, pl >= 1. + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|0|1|0| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + These are routes imported from routing protocols with truncated + path information. + + The OSPF tag to BGP attribute mappings for these routes must be + a=1,c=0,pl=10,as=don't care + + These are imported by a border router, which is running BGP to a + stub domain, and not running IBGP to other ASBRs. This causes a + truncation of the AS_PATH. These routes must not be re-exported + into BGP at another ASBR. + + 4.4.4. Routes with complete path information, pl = 0. + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|1|0|0| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + These are routes imported from routing protocols with either + complete path information or are known to be complete through + means other than that carried by the routing protocol. + + The OSPF tag to BGP attribute mappings for these routes must be + a=1,c=1,pl=00,as=0 => O=<IGP>, P=<l> + + This should be used for importing routes into OSPF from an IGP. + + + + +Varadhan [Page 10] + +RFC 1364 BGP OSPF Interaction September 1992 + + + 4.4.5. Routes with complete path information, pl = 1. + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|1|0|1| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + These are routes imported from routing protocols with either + complete path information, or are known to be complete through + means other than that carried by the routing protocol. The + routing protocol also has additional information about the + neighbour AS of the route. + + The OSPF tag to BGP attribute mappings for these routes must be + a=1,c=1,pl=01,as=nh => O=<IGP>, P=<l, nh> + + This setting should be used when the administrator explicitly + associates an AS number with an instance of an IGP. This setting + can also be used when importing BGP routes whose origin=<IGP> and + AS_PATH=<nh>; if the BGP learned route has no other transitive + attributes, then its propogation via IBGP can be suppressed. + + 4.4.6. Routes with complete path information, pl >= 1. + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|1|1|0| ArbitraryTag | AutonomousSystem | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + These are routes imported from routing protocols with complete + path information and carry the AS path information as part of the + routing information. + + The OSPF tag must be set to + a=1,c=1,pl=10,as=don't care + + These routes must not be exported into BGP because these routes + are already imported from BGP into the OSPF RD. Hence, it is + assumed that the BGP speaker will convey this information to other + BGP speakers via IBGP. + + Note that an implementation may import BGP routes with a path + length of 1 and no other transitive attributes directly into OSPF + and not send these routes via IBGP. In this situation, it must + use tag settings corresponding to 4.1.2.2, or 4.1.2.5. + + + + +Varadhan [Page 11] + +RFC 1364 BGP OSPF Interaction September 1992 + + +4.5. Miscellaneous tag settings + + 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + |1|x|1|1| Reserved for future use | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + The value of pl = 3 is reserved during automatic tag generation. + Routers must not generate such a tag when importing routes into the + OSPF routing domain. ASBRs must ignore tags which indicate a pl = 3. + +4.6. Summary of the tag sub-field setting + + The following table summarises the various combinations of automatic + tag settings for the Completeness and PathLength sub-field of the + OSPF tag and the default behaviour permitted for each setting. + + Completeness := 0 | 1 ; + PathLength := 00 | 01 | 10 | 11; + ORIGIN := <INCOMPLETE> | <IGP> | <EGP>; + + AS_PATH := valid AS path settings as defined in BGP. + + pl = 00 pl = 01 pl = 10 pl = 11 + +---------------------------------------------------------------- + | +c = 0 | <EGP><l> <EGP><l,nh> never export reserved +c = 1 | <IGP><l> <IGP><l,nh> out of band reserved + | + + The "out of band" in the table above implies that OSPF will not be + able to carry everything that BGP needs in its routing + information. Therefore, some other means must be found to carry + this information. In BGP, this is done via IBGP. + +5. Setting OSPF Forwarding Address and BGP NEXT_HOP attribute + + Forwarding addresses are used to avoid extra hops between multiple + routers that share a common network and that speak different routing + protocols with each other. + + Both BGP and OSPF have equivalents of forwarding addresses. In BGP, + the NEXT_HOP attribute is a well-known, mandatory attribute. OSPF + has a Forwarding address field. We will discuss how these are to be + filled in various situations. + + Consider the 4 router situation below: + + + +Varadhan [Page 12] + +RFC 1364 BGP OSPF Interaction September 1992 + + + RT1 and RT2 are in one autonomous system, RT3 and RT4 are in another. + RT1 and RT3 are talking BGP with each other. + RT3 and RT4 are talking OSPF with each other. + + +-----+ +-----+ + | RT1 | | RT2 | + +-----+ +-----+ + | | common network + ---+-----------------------+-------------------------- + <BGP> | | + +-----+ <OSPF> +-----+ + | RT3 | | RT4 | + +-----+ +-----+ + + + - Importing network X to OSPF: + + Consider an external network X, learnt via BGP from RT1. + + RT3 must always fill the OSPF Forwarding Address with the BGP + + NEXT_HOP attribute for the route to network X. + + - Exporting network Y to BGP: + + Consider a network Y, internal to the OSPF routing domain, + RT3's route to network Y is via RT4, and network Y is to be + exported via BGP to RT1. + + If network Y is not a subnetted network, RT3 must fill the + NEXT_HOP attribute for network Y with the address of RT4. + This is to avoid requiring packets to take an extra hop + through RT3 when traversing the AS boundary. This is similar + to the concept of indirect neighbour support in EGP [RFC888, + RFC827]. + +6. Security Considerations + + Security considerations are not discussed in this memo. + +7. Acknowledgements + + I would like to thank Yakov Rekhter, Jeff Honig, John Moy, Tony Li, + and Dennis Ferguson for their help and suggestions in writing this + document, without which I could not have written this document. I + would also like to thank them for giving me the opportunity to write + this document, and putting up with my muddlements through various + phases of this document. + + + +Varadhan [Page 13] + +RFC 1364 BGP OSPF Interaction September 1992 + + + I would also like to thank the countless number of people from the + OSPF and BGP working groups who have offered numerous suggestions and + comments on the different stages of this document. + +8. Bibliography + + [RFC827] Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827, + BBN, October 1982. + + [RFC888] Seamonson, L., and E. Rosen, "STUB Exterior Gateway + Protocol", RFC 888, BBN, January 1984. + + [RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058, + Rutgers University, June 1988. + + [RFC1267] Lougheed, K., and Y. Rekhter, "A Border Gateway + Protocol 3 (BGP-3)", RFC 1267, cisco Systems, + T.J. Watson Research Center, IBM Corp., October 1991. + + [RFC1268] Rekhter, Y., and P. Gross, Editors, "Application of the + Border Gateway Protocol in the Internet", RFC 1268, + T.J. Watson Research Center, IBM Corp., ANS, October 1991. + + [RFC1247] Moy, J., "The OSPF Specification - Version 2:", RFC 1247, + Proteon, January 1991. + + [ROUTE-LEAKING] Almquist, P., "Ruminations on Route Leaking", + Work in Progress. + + [NEXT-HOP] Almquist, P., "Ruminations on the Next Hop", + Work in Progress. + +9. Author's Address + + Kannan Varadhan + Internet Engineer, OARnet + 1224 Kinnear Road + Columbus, OH 43212-1136 + + EMail: kannan@oar.net + + + + + + + + + + + +Varadhan [Page 14] +
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