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Network Working Group                                     J. Parker, Ed.Request for Comments: 3787                             Axiowave NetworksCategory: Informational                                         May 2004Recommendations for Interoperable IP Networksusing Intermediate System to Intermediate System (IS-IS)Status of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2004).  All Rights Reserved.Abstract   This document discusses a number of differences between the   Intermediate System to Intermediate System (IS-IS) protocol used to   route IP traffic as described inRFC 1195 and the protocol as it is   deployed today.  These differences are discussed as a service to   those implementing, testing, and deploying the IS-IS Protocol to   route IP traffic.  A companion document describes the differences   between the protocol described in ISO 10589 and current practice.Table of Contents1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .22.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .23.  Unused Features . . . . . . . . . . . . . . . . . . . . . . .24.  Overload Bit. . . . . . . . . . . . . . . . . . . . . . . . .35.  Migration from Narrow Metrics to Wide . . . . . . . . . . . .46.  Intermediate System Hello (ISH) PDU . . . . . . . . . . . . .67.  Attached Bit. . . . . . . . . . . . . . . . . . . . . . . . .78.  Default Route . . . . . . . . . . . . . . . . . . . . . . . .89.  Non-homogeneous Protocol Networks . . . . . . . . . . . . . .810.  Adjacency Creation and IP Interface Addressing. . . . . . . .911.  Security Considerations . . . . . . . . . . . . . . . . . . .912.  References. . . . . . . . . . . . . . . . . . . . . . . . . .1012.1. Normative References. . . . . . . . . . . . . . . . . .1012.2. Informative References. . . . . . . . . . . . . . . . .1013.  Author's Address. . . . . . . . . . . . . . . . . . . . . . .1014.  Full Copyright Statement. . . . . . . . . . . . . . . . . . .11Parker                       Informational                      [Page 1]

RFC 3787         Interoperable IP Networks using IS-IS          May 20041.  Introduction   Interior Gateway Protocols such as IS-IS are designed to provide   timely information about the best routes in a routing domain.  The   original design of IS-IS, as described in ISO 10589 [1] has proved to   be quite durable.  However, a number of original design choices have   been modified.  This document describes some of the differences   between the protocol as described inRFC 1195 [2] and the protocol   that can be observed on the wire today.  A companion document   describes the differences between the protocol described in ISO 10589   and current practice [8].   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in   this document are to be interpreted as described inRFC 2119 [3].2.  Acknowledgments   This document is the work of many people, and is the distillation of   over a thousand mail messages.  Thanks to Vishwas Manral, who pushed   to create such a document.  Thanks to Danny McPherson, the original   editor, for kicking things off.  Thanks to Mike Shand, for his work   in creating the protocol, and his uncanny ability to remember what   everything is for.  Thanks to Micah Bartell and Philip Christian, who   showed us how to document difference without displaying discord.   Thanks to Les Ginsberg, Neal Castagnoli, Jeff Learman, and Dave Katz,   who spent many hours educating the editor.  Thanks to Radia Perlman,   who is always ready to explain anything.  Thanks to Satish Dattatri,   who was tenacious in seeing things written up correctly, and to Bryan   Boulton for his work on the IP adjacency issue.  Thanks to Russ   White, whose writing improved the treatment of every topic he   touched.  Thanks to Shankar Vemulapalli, who read several drafts with   close attention.  Thanks to Don Goodspeed, for his close reading of   the text.  Thanks to Michael Coyle for identifying the quotation from   Jan L.A. van de Snepscheut.  Thanks for Alex Zinin's ministrations   behind the scenes.  Thanks to Tony Li and Tony Przygienda, who kept   us on track as the discussions veered into the weeds.  And thanks to   all those who have contributed, but whose names I have carelessly   left from this list.3.  Unused Features   Some features defined inRFC 1195 are not in current use.Parker                       Informational                      [Page 2]

RFC 3787         Interoperable IP Networks using IS-IS          May 20043.1.  Inter-Domain Routing Protocol Information TLV, Code 131RFC 1195 defines an Inter-Domain Routing Protocol Information TLV,   with code 131, designed to convey information transparently between   boundary routers.  TLV 131 is not used, and MUST be ignored if   received.3.2.  Authentication TLV, Code 133RFC 1195 defines an authentication TLV, code 133, which contains   information used to authenticate the PDU.  This TLV has been replaced   by TLV 10, described in "IS-IS Cryptographic Authentication" [4].   TLV 133 is not used, and MUST be ignored.4.  Overload Bit   To deal with transient problems that prevent an IS from storing all   the LSPs it receives, ISO 10589 defines an LSP Database Overload   condition insection 7.3.19.  When an IS is in Database Overload   condition, it sets a flag called the Overload Bit in the non-   pseudonode LSP number Zero that it generates.Section 7.2.8.1 of ISO   10589 instructs other systems not to use the overloaded IS as a   transit router.  Since the overloaded IS does not have complete   information, it may not be able to compute the right routes, and   routing loops could develop.  However, an overloaded router may be   used to reach End Systems directly attached to the router, as it may   provide the only path to an End System.   The ability to signal reduced knowledge is so useful that the meaning   of this flag has been overloaded.  In a Service Provider's network,   when a router running BGP and IS-IS reboots, BGP might take more time   to converge than IS-IS.  Thus the router may drop traffic for   destinations not yet learned via BGP.  It is convenient to set the   Overload Bit until BGP has converged, as described in "Intermediate   System to Intermediate System (IS-IS) Transient Blackhole Avoidance"   [6].   An implementation SHOULD use the Overload Bit to signal that it is   not ready to accept transit traffic.   An implementation SHOULD not set the Overload bit in PseudoNode LSPs   that it generates, and Overload bits seen in PseudoNode LSPs SHOULD   be ignored.  This is also discussed in the companion document on ISO   interoperability [8].RFC 1195 makes clear when describing the SPF algorithm for IP routers   in section C.1.4 that directly connected IP subnetworks are reachable   when an IS is overloaded.Parker                       Informational                      [Page 3]

RFC 3787         Interoperable IP Networks using IS-IS          May 2004      Note that the End Systems neighbors of the system P includes IP      reachable address entries included in the LSPs from system P.   When processing LSPs received from a router which has the Overload   bit set in LSP number Zero, the receiving router SHOULD treat all IP   reachability advertisements as directly connected and use them in its   SPF computation.   Since the IP prefixes that an overloaded router announces will be   treated as directly attached, an overloaded router SHOULD take care   in selecting which routes to advertise in the LSPs it generates.5.  Migration from Narrow Metrics to Wide   The IS-Neighbors TLV (TLV 2) as defined in ISO 10589 and the IP   Reachability TLV (TLV 128/TLV 130) as defined inRFC 1195 provide a 6   bit metric for the default link metric to the listed neighbor.  This   metric has proved too limited.  The Extended IS-Neighbors TLV (TLV   22) and the Extended IP Reachability TLV (TLV 135) are defined in   "IS-IS extensions for Traffic Engineering" [5].  The Extended IS-   Neighbors TLV (TLV 22) defines a 24 bit metric, and the Extended IP   Reachability TLV (TLV 135) defines a 32 bit metric for IP Networks   and Hosts.   If not all devices in the IS-IS domain support wide metrics, narrow   metrics MUST continue to be used.  Once all devices in the network   are able to support the new TLVs containing wide metrics, the network   can be migrated to the new metric style, though care must be taken to   avoid routing loops.   We make the following assumptions about the implementation:      (1)   Each system can generate and understand both narrow and wide            metrics.      (2)   The implementation can run the SPF algorithm on an LSP DB            with instances of both metric styles.      (3)   If there are two metric styles for a link or IP prefix, it            will pick one of them as the true cost for the link.   To compare the different variants of the narrow metric with wide   metrics, we need an algorithm that translates External and Internal   narrow metrics into a common integer range.  Since we have different   computations for the L1 and L2 routes, we only need to map metrics   from a single level.Parker                       Informational                      [Page 4]

RFC 3787         Interoperable IP Networks using IS-IS          May 2004   InRFC 1195 section 3.10.2, item 2c) states that the IP prefixes   located in "IP External Reachability" with internal-metric and IP   prefixes located in "IP Internal Reachability" with internal-metric   have the same preference.  As defined in "Domain-wide Prefix   Distribution with Two-Level IS-IS", the Most Significant Bit on an L1   metric tells us if the route has been leaked down, but does not   change the distance.  Thus we will ignore the MSBit.   We interpret the default metric as an 7 bit quantity.  Metrics with   the external bit set are interpreted as metrics in the range   [64..127].  Metrics with the external bit clear are interpreted as   metrics in the range [0..63].5.1.  Transition Algorithm   To facilitate a smooth transition between the use of narrow metrics   exclusively to the use of wide metrics exclusively, the following   steps must be taken, in the order below.      (1)   All routers advertise Narrow Metrics as defined in ISO            10589, and consider narrow metrics only in their SPF            computation.      (2)   Each system is configured in turn to send wide metrics as            well as narrow metrics.  The two metrics for the same link            or IP prefix SHOULD agree.      (3)   When all systems are advertising wide metrics, make any            changes necessary on each system to consider Wide Metrics            during the SPF, and change MaxPathMetric to 0xFE000000.      (4)   Each system is configured in turn to stop advertising narrow            metrics.      (5)   When the network is only using wide metrics, metrics on            individual links may be rescaled to take advantage of the            larger metric.5.2.  Dealing with Non-Equal Metrics   The algorithm above assumes that the metrics are equal, and thus   needs to make no assumption about which metric the SPF algorithm   uses.  This section describes the changes that should be made to the   SPF algorithm when both Narrow and Wide metric styles should be   considered.  Using a common algorithm allows different   implementations to compute the same distances independently, even if   the wide and narrow metrics do not agree.Parker                       Informational                      [Page 5]

RFC 3787         Interoperable IP Networks using IS-IS          May 2004   The standard SPF algorithm proceeds by comparing sums of link costs   to obtain a minimal cost path.  During transition, there will be more   than one description of the same links.  We resolve this by selecting   the minimum metric for each link.  This may give us a path with some   links chosen due to a wide metric and some links chosen due to a   narrow metric.   The description below is more complex than the implementation needs   to be: the implementation may simply select the minimal cost neighbor   in TENT, discarding paths to destinations we have already reached, as   described in ISO 10589.   The variables MaxPathMetric and MaxLinkMetric SHOULD retain the   values defined in Table 2 ofsection 8 of ISO 10589.   In C.2.5 Step 0 of the description of the SPF algorithm, section b)      d(N) = cost of the parent circuit of the adjacency N      If multiple styles of metric for the link are defined, the cost      will be the minimum available cost for the circuit.   In C.2.5 Step 0 of the description of the SPF algorithm, section i)      d(N) = metric of the circuit      If multiple styles of metric for the link are defined, the cost      will be the minimum available cost for the circuit.   In C.2.6 Step 1 of the description of the SPF algorithm, section a)      dist(P,N) = d(P) + metric(P,N)      If multiple styles of metric for the neighbor are defined, the      cost will be the minimum available cost for the circuit.6.  Intermediate System Hello (ISH) PDU   The original intent ofRFC 1195 was to provide a routing protocol   capable of handling both CLNS and IPv4 reachability information.  To   allow CLNS Endstations (ES) to know that they are attached to a   router, Intermediate Systems are required to send Intermediate System   Hello PDUs (ISH) for End Stations when a point-to-point circuit comes   up.  Furthermore, an IS is not allowed to send Intermediate System to   Intermediate System Hello PDUs (IIH) before receiving an ISH from a   peer.  This reduces routing protocol traffic on links with a single   IS.Parker                       Informational                      [Page 6]

RFC 3787         Interoperable IP Networks using IS-IS          May 2004   For this reasonsection 5.1RFC 1195 states:         "On point-to-point links, the exchange of ISO 9542 ISHs         (intermediate system Hellos) is used to initialize the link,         and to allow each router to know if there is a router on the         other end of the link, before IS-IS Hellos are exchanged.  All         routers implementing IS-IS (whether IP-only, OSI-only, or         dual), if they have any interfaces on point-to-point links,         must therefore be able to transmit ISO 9542 ISHs on their         point-to-point links."Section 5.1RFC 1195 reinforces the need to comply withsection 8.2.4   of ISO 10589.  However, in an IP Only environment, the original need   for the ISH PDU is not present.   A multi-protocol IS that supports the attachment of CLNS ESs over   Point to Point circuits must act in accordance withsection 8.2.2 ISO   10589 when CLNS functionality is enabled.   An IP only implementation SHOULD issue an ISH PDU as described insection 8.2.3 of ISO 10589.  This is to inter-operate with   implementations which require an ISH to initiate the formation of an   IS-IS adjacency.   An IP Only implementation may issue an IIH PDU when a point to point   circuit transitions into an "Up" state to initiate the formation of   an IS-IS adjacency, without sending an ISH PDU.  However, this may   not inter-operate with implementations which require an ISH for   adjacency formation.   An IS may issue an IIH PDU in response to the receipt of an IIH PDU   in accordance withsection 8.2.5.2 ISO 10589, even though it has not   received an ISH PDU.7.  The Attached Bit   Insection 7.2.9.2 of ISO 10589, an algorithm is described to   determining when the attachedFlag should be set on an intermediate   system.  Some implementations also allow the attachedFlag to be set   on Intermediate Systems routing IP traffic when there is a default   route in the local routing table, or when some other state is reached   that implies a connection to the rest of the network.Parker                       Informational                      [Page 7]

RFC 3787         Interoperable IP Networks using IS-IS          May 20048.  Default RouteRFC 1195 states insection 1.3:         Default routes are permitted only at level 2 as external routes         (i.e., included in the "IP External Reachability Information"         field, as explained in sections3 and5).  Default routes are         not permitted at level 1.   Because of the utility of the default route when dealing with other   routing protocols and the ability to influence the exit point from an   area, an implementation MAY generate default routes in Level 1.9.  Non-homogeneous Protocol NetworksRFC 1195 assumes that every deployment of IS-IS routers will support   a homogeneous set of protocols.  It anticipates OSI only, IP only, or   dual OSI and IP routers.  While it allows mixed areas with, for   example, both pure IP and Dual IP and OSI routers, it allows only IP   traffic in such domains, and OSI traffic only when pure OSI and Dual   IP and OSI routers are present.  Thus it provides only lowest common   denominator routing.RFC 1195 also requires the inclusion of the Protocol Supported TLV   with code 129 in IIH and ISH PDUs and in LSP number Zero.  IP capable   routers MUST generate a Protocol Supported TLV, and MUST include the   IP protocol as a supported protocol.  A router that does not include   the Protocols Supported TLV may be assumed to be a pure OSI router   and can be interpreted as implicitly "advertising" support for the   OSI protocol.   The requirements ofRFC 1195 are ample if networks adhere to this   restriction.  However, the behavior of mixed networks that do not   follow these guidelines is not well defined.   The ITU-T requires that SONET/SDH equipment running the IS-IS   protocol must not form an adjacency with a neighbour unless they   share at least one network layer protocol in common.  Unless this   feature is present in every IS in the SONET or SDH DCN network the   network may not function correctly.  Implementors MAY include this   feature if they wish to ensure interoperability with SONET and SDH   DCN networks.   Definition of an interoperable strategy for resolving the problems   that arise in non-homogeneous protocol networks remains incomplete.   Members of the ITU are actively working on a proposal: see   "Architecture and Specification of Data Communication Network", [7].Parker                       Informational                      [Page 8]

RFC 3787         Interoperable IP Networks using IS-IS          May 200410.  Adjacency Creation and IP Interface AddressingRFC 1195 states that adjacencies are formed without regard to IP   interface addressing.  However, many current implementations refuse   adjacencies based on interface addresses and related issues.   Insection 4.2,RFC 1195 requires routers with IP interface addresses   to advertise the addresses in an IP Interface Address TLV (132)   carried in IIH PDUs.  Some implementations will not interoperate with   a neighbor router that does not include the IP Interface Address TLV.   Further, some implementations will not form an adjacency on broadcast   interfaces with a peer who does not share an interface address in   some common IP subnetwork.   If a LAN contains a mixture of implementations, some that form   adjacencies with all neighbors and some that do not, care must be   taken when assigning IP addresses.  If not all routers in a LAN are   on the same IP subnet, it is possible that DIS election may fail,   leading to the election of multiple DISs on a LAN, or no DIS at all.   Even if DIS election succeeds, black holes can result because the   IS-IS LAN transitivity requirements ofsection 6.7.3 ISO 10589 are   not met.   Unnumbered point to point links do not have IP interface addresses,   though they may have other IP addresses assigned to the routers.  The   IP address assigned to two routers that are neighbors on an   unnumbered point to point link do not need to be related.  However,   some implementations will not form an adjacency on numbered point to   point links if the interface addresses of each endpoint are not in   the same IP subnetwork.  This means that care must be taken in   assigning IP interface addresses in all networks.   For an implementation to interoperate in a such mixed environment, it   MUST include an IP Interface address (TLV 132) in its IIH PDUs.  The   network administrator should ensure that there is a common IP subnet   assigned to links with numbered interfaces, and that all routers on   each link have a IP Interface Addresses belonging to the assigned   subnet.11.  Security Considerations   The clarifications in this document do not raise any new security   concerns, as there is no change in the underlying protocol described   in ISO 10589 [1] andRFC 1195 [2].   The document does make clear that TLV 133 has been deprecated and   replaced with TLV 10.Parker                       Informational                      [Page 9]

RFC 3787         Interoperable IP Networks using IS-IS          May 200412.  References12.1.  Normative References   [1]  ISO, "Intermediate system to Intermediate system routeing        information exchange protocol for use in conjunction with the        Protocol for providing the Connectionless-mode Network Service        (ISO 8473)," ISO/IEC 10589:2002.   [2]  Callon, R., "OSI IS-IS for IP and Dual Environment,"RFC 1195,        December 1990.   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [4]  Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication",RFC 3567, July 2003.   [5]  Smit, H. and T. Li, "Intermediate System to Intermediate System        (IS-IS) Extensions for Traffic Engineering (TE)",RFC 3784, May        2004.   [6]  McPherson, D., "Intermediate System to Intermediate System (IS-        IS) Transient Blackhole Avoidance",RFC 3277, April 2002.12.2.  Informative References   [7]  ITU, "Architecture and Specification of Data Communication        Network", ITU-T Recommendation G.7712/Y.1703, November 2001   [8]  Parker, J., Ed., "Recommendations for Interoperable Networks        using Intermediate System to Intermediate System (IS-IS)",RFC3719, February 2004.13.  Author's Address   Jeff Parker   Axiowave Networks   200 Nickerson Road   Marlborough, Mass 01752   USA   EMail: jparker@axiowave.comParker                       Informational                     [Page 10]

RFC 3787         Interoperable IP Networks using IS-IS          May 200414.  Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 78, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Parker                       Informational                     [Page 11]