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Obsoletes: H. 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. This document replaces RFC This document extends the semantics presented in RFC 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. Motivations for Domain-Wide Prefix Distribution.

Requirements Language. Inter-Operability with Older Implementations. Comparisons with Other Proposals. Security Considerations. Normative References. Informative References. This document replaces [ RFC ], which was an Informational document. This document is on the standards track. No other intentional substantive changes have been made. An IS-IS routing domain a. Within each L1 area, all routers exchange link state information.

L2 routers also exchange L2 link state information to compute routes between areas. RFC 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. These summaries are injected into L2. Motivations for Domain-Wide Prefix Distribution The mechanisms specified in RFC 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 information Li, et al. 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. Many ISP networks depend on this technique, which is known as "shortest exit routing". 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.

Current Li, et al. This tends to cause external traffic to enter the domain at the point closest to the exit router. 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. Scalability The disadvantage to performing the domain-wide prefix distribution described above is that it has an impact on the scalability of IS-IS. 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 fully Li, et al. The encoding is an extension of the encoding in RFC To some extent, in IS-IS the level 2 backbone can be seen as a separate area itself. These routes can be regarded as inter-area routes. 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. RFC 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 RFC states this quite clearly in the note in paragraph 3. 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. 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. These rules should also be applied when advertising IP routes derived from L2 routing into L1.

RFC 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. There are four variables involved. The level of the LSP in which the route is advertised. There are currently two possible values: level 1 and level 2. The route-type, which can be derived from the type of TLV in which the prefix is advertised.

The metric-type: internal or external. The fact whether this route is leaked down in the hierarchy, and thus can not be advertised back up. The following list describes the types of IP prefixes and how they are encoded.

These IP prefixes are directly connected to the advertising router. These IP prefixes are learned from other IGPs, and are usually not directly connected to the advertising router. These prefixes cannot be distinguished from L2 intra-area routes. These prefixes cannot be distinguished from L2 external routes. These prefixes can not be distinguished from L2 external routes with external metric. 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 Another reason is that this maximum value of 63 does not mean infinity e. 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.

Based on these assumptions, this document defines the following route preferences.



Updated by: Network Working Group H. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. This information describes additional details regarding the state of the network that are useful for traffic engineering computations.


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. This RFC is available in both postscript and text versions. Where possible, use of the postscript version is recommended.


IS-IS is a link-state routing protocoloperating by reliably flooding link state information throughout a network of routers. The result is that an IS-IS router is only ever a part of a single area. It accomplishes this by determining the best route for rrfc through a packet-switched network. This article is about a computer network protocol.

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