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Network Working Group                                              T. LiRequest for Comments: 5302                        Redback Networks, Inc.Obsoletes:2966                                                  H. SmitUpdates:1195Category: Standards Track                                  T. Przygienda                                                                 Z2 Sagl                                                            October 2008Domain-Wide Prefix Distribution with Two-Level IS-ISStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This document describes extensions to the Intermediate System to   Intermediate System (IS-IS) protocol to support optimal routing   within a two-level domain.  The IS-IS protocol is specified in ISO   10589, with extensions for supporting IPv4 (Internet Protocol)   specified inRFC 1195.  This document replacesRFC 2966.   This document extends the semantics presented inRFC 1195 so that a   routing domain running with both level 1 and level 2 Intermediate   Systems (IS) (routers) can distribute IP prefixes between level 1 and   level 2, and vice versa.  This distribution requires certain   restrictions to ensure that persistent forwarding loops do not form.   The goal of this domain-wide prefix distribution is to increase the   granularity of the routing information within the domain.Li, et al.                  Standards Track                     [Page 1]

RFC 5302            Domain-wide Prefix Distribution         October 2008Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Motivations for Domain-Wide Prefix Distribution  . . . . .31.2.  Scalability  . . . . . . . . . . . . . . . . . . . . . . .51.3.  Requirements Language  . . . . . . . . . . . . . . . . . .62.  Proposed Syntax and Semantics for L2->L1 Inter-Area Routes . .6     2.1.  Clarification of External Route-Type and External           Metric-Type  . . . . . . . . . . . . . . . . . . . . . . .72.2.  Definition of External IP Prefixes in Level 1 LSPs . . . .83.  Types of IP Routes in IS-IS and Their Order of Preference  . .8     3.1.  Overview of All Types of IP Prefixes in IS-IS Link           State PDUs . . . . . . . . . . . . . . . . . . . . . . . .9     3.2.  Order of Preference for all Types of IP Routes in IS-IS  . 11     3.3.  Additional Notes on What Prefixes to Accept or           Advertise  . . . . . . . . . . . . . . . . . . . . . . . .124.  Inter-Operability with Older Implementations . . . . . . . . .125.  Comparisons with Other Proposals . . . . . . . . . . . . . . .136.  Security Considerations  . . . . . . . . . . . . . . . . . . .137.  References . . . . . . . . . . . . . . . . . . . . . . . . . .147.1.  Normative References . . . . . . . . . . . . . . . . . . .147.2.  Informative References . . . . . . . . . . . . . . . . . .14Li, et al.                  Standards Track                     [Page 2]

RFC 5302            Domain-wide Prefix Distribution         October 20081.  Introduction   This document describes extensions to the Intermediate System to   Intermediate System (IS-IS) protocol to support optimal routing   within a two-level domain.  The IS-IS protocol is specified in   [ISO-10589], with extensions for supporting IPv4 (Internet Protocol)   specified in [RFC1195].   This document replaces [RFC2966], which was an Informational   document.  This document is on the standards track.  No other   intentional substantive changes have been made.   This document extends the semantics presented inRFC 1195 so that a   routing domain running with both level 1 and level 2 Intermediate   Systems (IS) (routers) can distribute IP prefixes between level 1 and   level 2, and vice versa.  This distribution requires certain   restrictions to ensure that persistent forwarding loops do not form.   The goal of this domain-wide prefix distribution is to increase the   granularity of the routing information within the domain.   An IS-IS routing domain (a.k.a. an autonomous system running IS-IS)   can be partitioned into multiple level 1 (L1) areas, and a level 2   (L2) connected subset of the topology that interconnects all of the   L1 areas.  Within each L1 area, all routers exchange link state   information.  L2 routers also exchange L2 link state information to   compute routes between areas.RFC 1195 defines the Type, Length, and Value (TLV) tuples that are   used to transport IPv4 routing information in IS-IS.RFC 1195 also   specifies the semantics and procedures for interactions between   levels.  Specifically, routers in an L1 area will exchange   information within the L1 area.  For IP destinations not found in the   prefixes in the L1 database, the L1 router should forward packets to   the nearest router that is in both L1 and L2 (i.e., an L1L2 router)   with the "attached bit" set in its L1 Link State Protocol Data Unit   (LSP).   Also perRFC 1195, an L1L2 router should be manually configured with   a set of prefixes that summarizes the IP prefixes reachable in that   L1 area.  These summaries are injected into L2.RFC 1195 specifies   no further interactions between L1 and L2 for IPv4 prefixes.1.1.  Motivations for Domain-Wide Prefix Distribution   The mechanisms specified inRFC 1195 are appropriate in many   situations and lead to excellent scalability properties.  However, in   certain circumstances, the domain administrator may wish to sacrifice   some amount of scalability and distribute more specific informationLi, et al.                  Standards Track                     [Page 3]

RFC 5302            Domain-wide Prefix Distribution         October 2008   than is described byRFC 1195.  This section discusses the various   reasons why the domain administrator may wish to make such a   tradeoff.   One major reason for distributing more prefix information is to   improve the quality of the resulting routes.  A well-known property   of prefix summarization or any abstraction mechanism is that it   necessarily results in a loss of information.  This loss of   information in turn results in the computation of a route based upon   less information, which will frequently result in routes that are not   optimal.   A simple example can serve to demonstrate this adequately.  Suppose   that an L1 area has two L1L2 routers that both advertise a single   summary of all prefixes within the L1 area.  To reach a destination   inside the L1 area, any other L2 router is going to compute the   shortest path to one of the two L1L2 routers for that area.  Suppose,   for example, that both of the L1L2 routers are equidistant from the   L2 source and that the L2 source arbitrarily selects one L1L2 router.   This router may not be the optimal router when viewed from the L1   topology.  In fact, it may be the case that the path from the   selected L1L2 router to the destination router may traverse the L1L2   router that was not selected.  If more detailed topological   information or more detailed metric information was available to the   L2 source router, it could make a more optimal route computation.   This situation is symmetric in that an L1 router has no information   about prefixes in L2 or within a different L1 area.  In using the   nearest L1L2 router, that L1L2 is effectively injecting a default   route without metric information into the L1 area.  The route   computation that the L1 router performs is similarly suboptimal.   Besides the optimality of the routes computed, there are two other   significant drivers for the domain-wide distribution of prefix   information.   When a router learns multiple possible paths to external destinations   via BGP, it will select only one of those routes to be installed in   the forwarding table.  One of the factors in the BGP route selection   is the IGP cost to the BGP next hop address.  Many ISP networks   depend on this technique, which is known as "shortest exit routing".   If a L1 router does not know the exact IGP metric to all BGP speakers   in other L1 areas, it cannot do effective shortest exit routing.   The third driver is the current practice of using the IGP (IS-IS)   metric as part of the BGP Multi-Exit Discriminator (MED).  The value   in the MED is advertised to other domains and is used to inform other   domains of the optimal entry point into the current domain.  CurrentLi, et al.                  Standards Track                     [Page 4]

RFC 5302            Domain-wide Prefix Distribution         October 2008   practice is to take the IS-IS metric and insert it as the MED value.   This tends to cause external traffic to enter the domain at the point   closest to the exit router.  Note that the receiving domain MAY,   based upon policy, choose to ignore the MED that is advertised.   However, current practice is to distribute the IGP metric in this way   in order to optimize routing wherever possible.  This is possible in   current networks that only are a single area, but becomes problematic   if hierarchy is to be installed into the network.  This is again   because the loss of end-to-end metric information means that the MED   value will not reflect the true distance across the advertising   domain.  Full distribution of prefix information within the domain   would alleviate this problem, as it would allow accurate computation   of the IS-IS metric across the domain, resulting in an accurate value   presented in the MED.1.2.  Scalability   The disadvantage to performing the domain-wide prefix distribution   described above is that it has an impact on the scalability of IS-IS.   Areas within IS-IS help scalability in that LSPs are contained within   a single area.  This limits the size of the link state database,   which in turn limits the complexity of the shortest path computation.   Further, the summarization of the prefix information aids scalability   in that the abstraction of the prefix information removes the sheer   number of data items to be transported and the number of routes to be   computed.   It should be noted quite strongly that the distribution of prefixes   on a domain-wide basis impacts the scalability of IS-IS in the second   respect.  It will increase the number of prefixes throughout the   domain.  This will result in increased memory consumption,   transmission requirements, and computation requirements throughout   the domain.   It must also be noted that the domain-wide distribution of prefixes   has no effect whatsoever on the first aspect of scalability, namely   the existence of areas and the limitation of the distribution of the   link state database.   Thus, the net result is that the introduction of domain-wide prefix   distribution into a formerly flat, single area network is a clear   benefit to the scalability of that network.  However, it is a   compromise and does not provide the maximum scalability available   with IS-IS.  Domains that choose to make use of this facility should   be aware of the tradeoff that they are making between scalability and   optimality, and provision and monitor their networks accordingly.   Normal provisioning guidelines that would apply to a fullyLi, et al.                  Standards Track                     [Page 5]

RFC 5302            Domain-wide Prefix Distribution         October 2008   hierarchical deployment of IS-IS will not apply to this type of   configuration.1.3.  Requirements Language   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 [RFC2119].2.  Proposed Syntax and Semantics for L2->L1 Inter-Area Routes   This document defines the syntax of how to advertise level 2 routes   in level 1 LSPs.  The encoding is an extension of the encoding inRFC1195.   To some extent, in IS-IS the level 2 backbone can be seen as a   separate area itself.RFC 1195 defines that L1L2 routers can   advertise IP routes that were learned via L1 routing into L2.  These   routes can be regarded as inter-area routes.RFC 1195 defines that   these L1->L2 inter-area routes must be advertised in L2 LSPs in the   "IP Internal Reachability Information" TLV (TLV 128).  Intra-area L2   routes are also advertised in L2 LSPs in an "IP Internal Reachability   Information" TLV.  Therefore, L1->L2 inter-area routes are   indistinguishable from L2 intra-area routes.RFC 1195 does not define L2->L1 inter-area routes.  A simple   extension would be to allow an L1L2 router to advertise routes   learned via L2 routing in its L1 LSP.  However, to prevent routing-   loops, L1L2 routers MUST NOT advertise L2->L1 inter-area routes that   they learn via L1 routing back into L2.  Therefore, there must be a   way to distinguish L2->L1 inter-area routes from L1 intra-area   routes.  [RFC5305] defines the "up/down bit" for this purpose in the   extended IP reachability TLV (TLV 135).RFC 1195 defines TLVs 128   and 130 to contain IP routes.  TLVs 128 and 130 have a Metric field   that consists of 4 Type of Service (TOS) metrics.  The first metric,   the so-called "default metric", has the high-order bit reserved (bit   8).  Routers must set this bit to zero on transmission, and ignore it   on receipt.   This document redefines this high-order bit in the default Metric   field in TLVs 128 and 130 to be the up/down bit.  L1L2 routers MUST   set this bit to one for prefixes that are derived from L2 routing and   are advertised into L1 LSPs.  The bit MUST be set to zero for all   other IP prefixes in L1 or L2 LSPs.  Prefixes with the up/down bit   set that are learned via L1 routing MUST NOT be advertised by L1L2   routers back into L2.Li, et al.                  Standards Track                     [Page 6]

RFC 5302            Domain-wide Prefix Distribution         October 20082.1.  Clarification of External Route-Type and External Metric-TypeRFC 1195 defines two TLVs for carrying IP prefixes.  TLV 128 is   defined as "IP Internal Reachability Information", and should be used   to carry IP prefixes that are directly connected to IS-IS routers.   TLV 130 is defined as "IP External Reachability Information", and   should be used to carry routes learned from outside the IS-IS domain.RFC 1195 documents TLV type 130 only for level 2 LSPs.RFC 1195 also defines two types of metrics.  Metrics of the internal   metric-type should be used when the metric is comparable to metrics   used to weigh links inside the IS-IS domain.  Metrics of the external   metric-type should be used if the metric of an IP prefix cannot be   directly compared to internal metrics.  The external metric-type can   only be used for external IP prefixes.  A direct result is that   metrics of the external metric-type should never be seen in TLV 128.   To prevent confusion, this document states again that when a router   computes IP routes, it MUST give the same preference to IP routes   advertised in an "IP Internal Reachability Information" TLV and IP   routes advertised in an "IP External Reachability Information" TLV.RFC 1195 states this quite clearly in the note in paragraph 3.10.2,   item 2c).  This document does not alter this rule of preference.   NOTE:  Internal routes (routes to destinations announced in the "IP      Internal Reachability Information" field) and external routes      using internal metrics (routes to destinations announced in the      "IP External Reachability Information" field, with a metric of      type "internal") are treated identically for the purpose of the      order of preference of routes, and the Dijkstra calculation.   However, IP routes advertised in "IP External Reachability   Information" with the external metric-type MUST be given less   preference than the same IP routes advertised with the internal   metric-type, regardless of the value of the metrics.   While IS-IS routers MUST NOT give different preference to IP prefixes   learned via "IP Internal Reachability Information" and "IP External   Reachability Information" when executing the Dijkstra calculation,   routers that implement multiple IGPs are free to use this distinction   between internal and external routes when comparing routes derived   from different IGPs for inclusion in their global Routing Information   Base (RIB).Li, et al.                  Standards Track                     [Page 7]

RFC 5302            Domain-wide Prefix Distribution         October 20082.2.  Definition of External IP Prefixes in Level 1 LSPsRFC 1195 does not define the "IP External Reachability Information"   TLV for L1 LSPs.  However, there is no reason why an IS-IS   implementation could not allow for redistribution of external routes   into L1.  Some IS-IS implementations already allow network   administrators to do this.  This document loosens the restrictions inRFC 1195 and allows for the inclusion of the "IP External   Reachability Information" TLV in L1 LSPs.RFC 1195 defines that IP routes learned via L1 routing must always be   advertised in L2 LSPs in an "IP Internal Reachability Information"   TLV.  Now that this document allows "IP External Reachability   Information" TLVs in L1 LSPs and allows for the advertisement of   routes learned via L2 routing into L1, the above rule needs an   extension.   When an L1L2 router advertises an L1 route into L2, where that L1   route was learned via a prefix advertised in an "IP External   Reachability Information" TLV, that L1L2 router SHOULD advertise that   prefix in its L2 LSP within an "IP External Reachability Information"   TLV.  L1 routes learned via an "IP Internal Reachability Information"   TLV SHOULD still be advertised within an "IP Internal Reachability   Information" TLV.  These rules should also be applied when   advertising IP routes derived from L2 routing into L1.  Of course in   this case, the up/down bit MUST be set also.RFC 1195 defines that if a router sees the same external prefix   advertised by two or more routers with the same external metric, it   must select the route that is advertised by the router that is   closest to itself.  It should be noted that now that external routes   can be advertised from L1 into L2, and vice versa, the router that   advertises an external prefix in its LSP might not be the router that   originally injected this prefix into the IS-IS domain.  Therefore, it   is less useful to advertise external routes with external metrics   into other levels.3.  Types of IP Routes in IS-IS and Their Order of PreferenceRFC 1195 and this document define several ways of advertising IP   routes in IS-IS.  There are four variables involved.   1.  The level of the LSP in which the route is advertised.  There are       currently two possible values: level 1 and level 2.   2.  The route-type, which can be derived from the type of TLV in       which the prefix is advertised.  Internal routes are advertised       in IP Internal Reachability Information TLVs (TLV 128), andLi, et al.                  Standards Track                     [Page 8]

RFC 5302            Domain-wide Prefix Distribution         October 2008       external routes are advertised in IP External Reachability       Information TLVs (TLV 130).   3.  The metric-type: internal or external.  The metric-type is       derived from the internal/external metric-type bit in the Metric       field (bit 7).   4.  The fact whether this route is leaked down in the hierarchy, and       thus can not be advertised back up.  This information can be       derived from the newly defined up/down bit in the default Metric       field.3.1.  Overview of All Types of IP Prefixes in IS-IS Link State PDUs   The combination IP Internal Reachability Information and external   metric-type is not allowed.  Also, the up/down bit MUST NOT be set in   L2 LSPs.  This leaves us with 8 different types of IP advertisements   in IS-IS.  However, there are more than 8 reasons for IP prefixes to   be advertised in IS-IS.  The following list describes the types of IP   prefixes and how they are encoded.   L1 intra-area routes:  These are advertised in L1 LSPs, in TLV 128.      The up/down bit is set to zero, metric-type is internal metric.      These IP prefixes are directly connected to the advertising      router.   L1 external routes:  These are advertised in L1 LSPs, in TLV 130.      The up/down bit is set to zero, metric-type is internal metric.      These IP prefixes are learned from other IGPs, and are usually not      directly connected to the advertising router.   L2 intra-area routes:  These are advertised in L2 LSPs, in TLV 128.      The up/down bit is set to zero, metric-type is internal metric.      These IP prefixes are directly connected to the advertising      router.  These prefixes cannot be distinguished from L1->L2 inter-      area routes.   L2 external routes:  These are advertised in L2 LSPs, in TLV 130.      The up/down bit is set to zero, metric-type is internal metric.      These IP prefixes are learned from other IGPs, and are usually not      directly connected to the advertising router.  These prefixes      cannot be distinguished from L1->L2 inter-area external routes.   L1->L2 inter-area routes:  These are advertised in L2 LSPs, in TLV      128.  The up/down bit is set to zero, metric-type is internal      metric.  These IP prefixes are learned via L1 routing, and were      derived during the L1 Shortest Path First (SPF) computation fromLi, et al.                  Standards Track                     [Page 9]

RFC 5302            Domain-wide Prefix Distribution         October 2008      prefixes advertised in L1 LSPs in TLV 128.  These prefixes cannot      be distinguished from L2 intra-area routes.   L1->L2 inter-area external routes:  These are advertised in L2 LSPs,      in TLV 130.  The up/down bit is set to zero, metric-type is      internal metric.  These IP prefixes are learned via L1 routing,      and were derived during the L1 SPF computation from prefixes      advertised in L1 LSPs in TLV 130.  These prefixes cannot be      distinguished from L2 external routes.   L2->L1 inter-area routes:  These are advertised in L1 LSPs, in TLV      128.  The up/down bit is set to one, metric-type is internal      metric.  These IP prefixes are learned via L2 routing, and were      derived during the L2 SPF computation from prefixes advertised in      TLV 128.   L2->L1 inter-area external routes:  These are advertised in L1 LSPs,      in TLV 130.  The up/down bit is set to one, metric-type is      internal metric.  These IP prefixes are learned via L2 routing,      and were derived during the L2 SPF computation from prefixes      advertised in L2 LSPs in TLV 130.   L1 external routes with external metric:  These are advertised in L1      LSPs, in TLV 130.  The up/down bit is set to zero, metric-type is      external metric.  These IP prefixes are learned from other IGPs,      and are usually not directly connected to the advertising router.   L2 external routes with external metric:  These are advertised in L2      LSPs, in TLV 130.  The up/down bit is set to zero, metric-type is      external metric.  These IP prefixes are learned from other IGPs,      and are usually not directly connected to the advertising router.      These prefixes cannot be distinguished from L1->L2 inter-area      external routes with external metric.   L1->L2 inter-area external routes with external metric:  These are      advertised in L2 LSPs, in TLV 130.  The up/down bit is set to      zero, metric-type is external metric.  These IP prefixes are      learned via L1 routing, and were derived during the L1 SPF      computation from prefixes advertised in L1 LSPs in TLV 130 with      external metrics.  These prefixes can not be distinguished from L2      external routes with external metric.   L2->L1 inter-area external routes with external metric:  These are      advertised in L1 LSPs, in TLV 130.  The up/down bit is set to one,      metric-type is external metric.  These IP prefixes are learned via      L2 routing, and were derived during the L1 SPF computation from      prefixes advertised in L2 LSPs in TLV 130 with external metrics.Li, et al.                  Standards Track                    [Page 10]

RFC 5302            Domain-wide Prefix Distribution         October 20083.2.  Order of Preference for all Types of IP Routes in IS-IS   Unfortunately, IS-IS cannot depend on metrics alone for route   selection.  Some types of routes must always be preferred over   others, regardless of the costs that were computed in the Dijkstra   calculation.  One of the reasons for this is that inter-area routes   can only be advertised with a maximum metric of 63.  Another reason   is that this maximum value of 63 does not mean infinity (e.g., like a   hop count of 16 in RIP denotes unreachable).  Introducing a value for   infinity cost in IS-IS inter-area routes would introduce counting-   to-infinity behavior via two or more L1L2 routers, which would have a   bad impact on network stability.   The order of preference of IP routes in IS-IS is based on a few   assumptions.   oRFC 1195 defines that routes derived from L1 routing are preferred      over routes derived from L2 routing.   o  The note inRFC 1195, paragraph 3.10.2, item 2c) defines that      internal routes with internal metric-type and external prefixes      with internal metric-type have the same preference.   oRFC 1195 defines that external routes with internal metric-type      are preferred over external routes with external metric-type.   o  Routes derived from L2 routing are preferred over L2->L1 routes      derived from L1 routing.   Based on these assumptions, this document defines the following route   preferences.   1.  L1 intra-area routes with internal metric; L1 external routes       with internal metric   2.  L2 intra-area routes with internal metric; L2 external routes       with internal metric; L1->L2 inter-area routes with internal       metric; L1->L2 inter-area external routes with internal metric   3.  L2->L1 inter-area routes with internal metric; L2->L1 inter-area       external routes with internal metric   4.  L1 external routes with external metric   5.  L2 external routes with external metric; L1->L2 inter-area       external routes with external metricLi, et al.                  Standards Track                    [Page 11]

RFC 5302            Domain-wide Prefix Distribution         October 2008   6.  L2->L1 inter-area external routes with external metric3.3.  Additional Notes on What Prefixes to Accept or AdvertiseSection 3.1 enumerates all used IP route-types in IS-IS.  Besides   these defined route-types, the encoding used would allow for a few   more potential combinations.  One of them is the combination of "IP   Internal Reachability Information" and external metric-type.  This   combination SHOULD NOT be used when building an LSP.  Upon receipt of   an IP prefix with this combination, routers MUST ignore this prefix.   Another issue would be the usage of the up/down bit in L2 LSPs.   Because IS-IS is currently defined with two levels of hierarchy,   there should never be a need to set the up/down bit in L2 LSPs.   However, if IS-IS would ever be extended with more than two levels of   hierarchy, L2-only (or L1L2) routers will need to be able to accept   L2 IP routes with the up/down bit set.  Therefore, it is RECOMMENDED   that implementations ignore the up/down bit in L2 LSPs, and accept   the prefixes in L2 LSPs regardless of whether the up/down bit is set.   This will allow for simpler migration once more than two levels of   hierarchy are defined.   Another detail that implementors should be aware of is the fact that   L1L2 routers SHOULD only advertise in their L2 LSP those L1 routes   that they use for forwarding themselves.  They SHOULD NOT   unconditionally advertise into L2 all prefixes from LSPs in the L1   database.   Not all prefixes need to be advertised up or down the hierarchy.   Implementations might allow for additional manual filtering or   summarization to further bring down the number of inter-area prefixes   they advertise in their LSPs.  It is also RECOMMENDED that the   default configuration of L1L2 routers not advertise any L2 routes   into L1 (see alsoSection 4).4.  Inter-Operability with Older Implementations   The solution in this document is not fully compatible withRFC 1195.   It is an extension toRFC 1195.  If routers do not use the new   functionality of external L1 routes or L2->L1 inter-area routes,   older implementations that strictly followRFC 1195 will be   compatible with newer implementations that follow this document.   Implementations that do not accept the "IP External Reachability   Information" TLV in L1 LSPs will not be able to compute external L1   routes.  This could cause routing loops between L1-only routers that   do understand external L1 routes for a particular destination, and   L1-only routers that use the default route pointing to the closest   attached L1L2 router for that destination.Li, et al.                  Standards Track                    [Page 12]

RFC 5302            Domain-wide Prefix Distribution         October 2008   Implementations that followRFC 1195 SHOULD ignore bit 8 in the   default Metric field when computing routes.  Therefore, even older   implementations that do not know of the up/down bit should be able to   accept the new L2->L1 inter-area routes.  These older implementations   will install the new L2->L1 inter-area routes as L1 intra-area   routes, but that in itself does not cause routing loops among L1-only   routers.   However, it is vital that the up/down bit is recognized by L1L2   routers.  As has been stated before, L1L2 routers MUST NOT advertise   L2->L1 inter-area routes back into L2.  Therefore, if L2 routes are   advertised down into an L1 area, it is required that all L1L2 routers   in that area run software that understands the new up/down bit.   Older implementations that followRFC 1195 and do not understand the   new up/down bit will treat the L2->L1 inter-area routes as L1 intra-   area routes, and they will advertise these routes back into L2.  This   can cause routing loops, sub-optimal routing, or extra routing   instability.  For this reason, it is RECOMMENDED that implementations   by default not advertise any L2 routes into L1.  Implementations   SHOULD force the network administrator to manually configure L1L2   routers to advertise any L2 routes into L1.5.  Comparisons with Other Proposals   In [RFC5305], a new TLV is defined to transport IP prefix   information.  This TLV format also defines an up/down bit to allow   for L2->L1 inter-area routes.RFC 5305 also defines a new TLV to   describe links.  Both TLVs have wider metric space and have the   possibility to define sub-TLVs to advertise extra information   belonging to the link or prefix.  The wider metric space in IP prefix   TLVs allows for more granular metric information about inter-area   path costs.  To make full use of the wider metric space, network   administrators must deploy both new TLVs at the same time.   Deployment ofRFC 5305 requires an upgrade of all routers in the   network and a transition to the new TLVs.  Such a network-wide   upgrade and transition might not be an easy task.  In this case, the   solution defined in this document, which requires only an upgrade of   L1L2 routers in selected areas, might be a good alternative to the   solution defined in 5305.6.  Security Considerations   This document raises no new security issues for IS-IS; for general   security considerations for IS-IS see [RFC5304].Li, et al.                  Standards Track                    [Page 13]

RFC 5302            Domain-wide Prefix Distribution         October 20087.  References7.1.  Normative References   [ISO-10589]  ISO, "Intermediate System to Intermediate System                intra-domain routeing information exchange protocol for                use in conjunction with the protocol for providing the                connectionless-mode network service (ISO 8473)",                International Standard 10589:2002, Second Edition, 2002.   [RFC1195]    Callon, R., "Use of OSI IS-IS for routing in TCP/IP and                dual environments",RFC 1195, December 1990.   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.7.2.  Informative References   [RFC2966]    Li, T., Przygienda, T., and H. Smit, "Domain-wide Prefix                Distribution with Two-Level IS-IS",RFC 2966,                October 2000.   [RFC5304]    Li, T. and R. Atkinson, "IS-IS Cryptographic                Authentication",RFC 5304, October 2008.   [RFC5305]    Li, T. and H. Smit, "IS-IS Extensions for Traffic                Engineering",RFC 5305, October 2008.Li, et al.                  Standards Track                    [Page 14]

RFC 5302            Domain-wide Prefix Distribution         October 2008Authors' Addresses   Tony Li   Redback Networks, Inc.   300 Holger Way   San Jose, CA  95134   USA   Phone: +1 408 750 5160   EMail: tony.li@tony.li   Henk Smit   EMail: hhw.smit@xs4all.nl   Tony Przygienda   Z2 Sagl   Via Tersaggio 20   CH-6949 Comano   Switzerland   EMail: prz@net4u.chLi, et al.                  Standards Track                    [Page 15]

RFC 5302            Domain-wide Prefix Distribution         October 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   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, THE IETF TRUST 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.Li, et al.                  Standards Track                    [Page 16]