rfc1195.txt

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Callon                                                         [Page 15]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   the correct next-hop router). This information is made available by
   inclusion of the IP interface address in the IS-IS Hello packets.
   Specifically, each IS-IS Hello packet contains the IP address(es) of
   the interface over which the Hello is transmitted. The IS-IS allows
   multiple IP addresses to be assigned to each physical interface.

   In some cases, it will be useful for IP-capable routers to be able to
   determine an IP address(es) of all other routers at their level
   (i.e., for level 1 routers: all other routers in their area; for
   level 2 routers: all other level 2 routers in the routing domain).
   This is useful whenever an IP packet is to be sent to a router, such
   as for encapsulation or for transmission of network management
   packets. This information is made available by inclusion of IP
   address in LSPs. Specifically, each IS-IS LSP includes one or more IP
   addresses of the router which transmits the LSP. An IP-capable router
   is required to include at least one of its IP addresses in its LSPs,
   and may optionally include several or all of its IP addresses. Where
   a single router operates as both a level 1 and a level 2 router, it
   is required to include the same IP address(es) in its level 1 and
   level 2 LSPs.

   IP-capable routers need to know, for any given IP destination
   address, the correct route to that destination. Specifically, level 1
   routers need to know what IP addresses are reachable from each level
   1 router in their area. In addition, level 1 routers need to find
   level 2 routers (for traffic to IP addresses outside of their area).
   Level 2 routers need to know what IP addresses are reachable
   internally (either directly, or via level 1 routing) from other level
   2 routers, and what addresses are reachable externally from other
   level 2 routers. All of this information is made available by
   inclusion of IP reachable address information in the Link State
   Packets.

   Internal (within the routing domain) and external (outside the
   domain) reachability information is announced separately in level 2
   LSPs. Reachable IP addresses include a default metric, and may
   include multiple TOS-specific metrics. In general, for external
   routes, metrics may be of type "internal" (i.e., directly comparable
   with internal metrics) or of type "external" (i.e., not comparable
   with the internal metric). A route using internal metrics (i.e.,
   either announced as "IP internal reachability information", or
   announced as "IP external reachability information" with an internal
   metric) is always preferred to a route using external metrics (i.e.,
   announced as "IP external reachability information", with an external
   metric).

   The detailed encoding of the IP-specific information included in
   routing packets is provided in section 5 (Structure and Encoding of



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   PDUs).

3.2 Hierarchical Abbreviation of IP Reachability Information

   Level 2 routers include in their level 2 LSPs a list of all [IP
   address, subnet mask, metric] combinations reachable in their area.
   In general, this information may be determined from the level 1 LSPs
   from all routers in the area. If we ignore resource constraints, then
   it would be permissible for a level 2 router to simply duplicate all
   [IP address, subnet mask, metric] entries from all level 1 routers in
   its area (with appropriate metric adjustment), for inclusion in its
   level 2 LSP. However, in order for hierarchical routing to scale to
   large routing domain sizes, it is highly desired to abbreviate the
   reachable address information.

   This is accomplished by manual configuration of summary addresses.
   Each level 2 router may be configured with one or more [IP address,
   subnet mask, metric] entries for announcement in their level 2 LSPs.

   The set of reachable addresses obtained from level 1 LSPs is compared
   with the configured reachable addresses. Redundant information
   obtained from level 1 LSPs is not included in level 2 LSPs. Generally
   it is expected that the level 2 configured information will specify
   more inclusive addresses (corresponding to a subnet mask with fewer
   bits set to 1). This will therefore allow one configured
   address/submask pair (or a small number of such pairs) to
   hierarchically supercede the information corresponding to multiple
   entries in level 1 LSPs.

   The manually configured addresses are included in level 2 LSPs only
   if they correspond to at least one address which is reachable in the
   area. For manually configured level 2 addresses, the associated
   metric values to announce in level 2 LSPs are also manually
   configured. The configured addresses will supercede reachable address
   entries from level 1 LSPs based only on the IP address and subnet
   mask -- metric values are not considered when determining if a given
   configured address supercedes an address obtained from a level 1 LSP.

   Any address obtained from a level 1 LSP which is not superceded by
   the manually configured information is included in the level 2 LSPs.
   In this case, the metric value announced in the level 2 LSPs is
   calculated from the sum of the metric value announced in the
   corresponding level 1 LSP, plus the distance from the level 2 router
   to the appropriate level 1 router. Note: If this sum results in a
   metric value greater than 63 (the maximum value that can be reported
   in level 2 LSPs), then the value 63 must be used. Delay, expense, and
   error metrics (i.e., those TOS metrics other than the default metric)
   will be included only if (i) the level 2 router supports the specific



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   TOS; (ii) the path from the level 2 router to the appropropriate
   level 1 router is made up of links which support the specific TOS;
   and (iii) the level 1 router which can reach the address directly
   also supports the specific TOS for this route, as indicated in its
   level 1 LSP.

   In general, the same [IP address, subnet mask] pair may be announced
   in level 1 LSPs sent by multiple level 1 routers in the same area. In
   this case (assuming the entry is not superceded by a manually
   configured entry), then only one such entry shall be included in the
   level 2 LSP. The metric value(s) announced in level 2 LSPs correspond
   to the minimum of the metric value(s) that would be calculated for
   each of the level 1 LSP entries.

   A level 2 router will have IP addresses which are directly reachable
   via its own interfaces. For purposes of inclusion of IP reachable
   address information in level 2 LSPs, these "directly reachable"
   addresses are treated exactly the same as addresses received in level
   1 LSPs.

   Manually configured addresses may hierarchically supercede multiple
   level 1 reachable address entries. However, there may be some IP
   addresses which match the manually configured addresses, but which
   are not reachable via level 1 routing. If a level 2 router receives
   an IP packet whose IP address matches a manually configured address
   which it is including in its level 2 LSP, but which is not reachable
   via level 1 routing in the area, then the packet must be discarded.
   In this case, an error report may be returned (as specified in RFC
   1009), with the reason for discard specifying destination
   unreachable.






           Figure 2 - An Example Routing Domain (not shown)

   An example is illustrated in figure 2. Suppose that the network
   number for the entire routing domain is 17 (a class A network).
   Suppose each area is assigned a subnet number consisting of the next
   8 bits. The area may be further subdivided by assigning the next
   eight bits to each LAN in the area, giving each a 24 bit subnet mask
   (counting the network and subnet fields). Finally 8 bits are left for
   the host field. Suppose that for a particular area (given subnet
   number 17.133) there are a number of IP capable level 1 routers
   announcing (in the special IP entry in their level 1 LSPs) subnets
   17.133.5, 17.133.43, and 17.133.57.



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   Suppose that in this example, in order to save space in level 2 LSPs,
   the level 2 routers in this area are configured to announce subnet
   17.133. Only this one address needs to be announced in level 2 LSPs.
   Thus if an IP packet comes along for an address in subnet 17.133.5,
   17.133.43 or 17.133.57, then other level 2 routers, in other areas,
   will know to pass the traffic to this area.

   The inclusion of 17.133 in level 2 LSPs means that the three subnet
   addresses starting with 17.133 do not all have to be listed
   separately in level 2 LSPs.

   If any traffic comes along that is for an unreachable address such as
   17.133.124.7, then level 2 routers in other areas in this particular
   domain will think that this area can handle this traffic, will
   forward traffic to level 2 routers in this area, which will have to
   discard this traffic.

   Suppose that subnet number 17.133.125 was actually reachable via some
   other area, such as the lower right hand area. In this case, the
   level 2 router in the left area would be announcing (in its level 2
   LSPs according to manually configured information) reachability to
   subnet 17.133. However, the level 2 router in the lower right area
   would be announcing (in its level 2 LSPs according to information
   taken from its received level 1 LSPs), reachability to subnet
   17.133.125. Due to the use of best match routing, this works
   correctly. All traffic from other areas destined to subnet 17.133.125
   would be sent to the level 2 router in the lower right area, and all
   other traffic to subnet 17.133 (i.e., traffic to any IP address
   starting with 17.133, but not starting with 17.133.125) would be sent
   to the level 2 router in the leftmost area.

3.3 Addressing Routers in IS-IS Packets

   The IS-IS packet formats explicitly require that OSI-style addresses
   of routers appear in the IS-IS packets. For example, these addresses
   are used to determine area membership of routers. It is therefore
   necessary for all routers making use of the IS-IS protocol to have
   OSI style addresses assigned. For IP-only routers, these addresses
   will be used only in the operation of the IS-IS protocol, and are not
   used for any other purpose (such as the operation of EGP, ICMP, or
   other TCP/IP protocols).

   For OSI-only and dual routers, assignment of NSAP addresses is
   straight forward, but is outside of the scope of this specification.
   Address assignment mechanisms are being set up by standards bodies
   which allow globally unique OSI NSAP addresses to be assigned. All
   OSI-only and dual routers may therefore make use of normal OSI
   addresses in the operation of the IS-IS protocol.



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   For IP-only routers, there are two ways in which NSAP addresses may
   be obtained for use with the IS-IS protocol.

   1) For those environments in which OSI is being used, or in which it
      is anticipated that OSI will be used in the future, it is
      permissible to obtain NSAP address assignments in the normal
      manner, assign normal NSAP addresses to IP-only routers, and use
      these addresses in the operation of IS-IS. This approach is
      recommended even for pure IP routing domains, as it will simplify
      future migration from IP-only to dual operation.

   2) In some cases, routers may have only TCP/IP addresses, and it may
      be undesireable to have to go through the normal mechanisms for
      assignment of NSAP addresses. Instead, an alternate mechanim is
      provided below for algorithmically generating a valid OSI style
      address from existing IP address and autonomous system number
      assignments.

   Where desired, for IP-only routers, for use in IS-IS packet formats
   only, OSI-style addresses (compatible with the USA GOSIP version 2.0
   NSAP address format [9]) may be derived as follows:

        AFI       1 octet       value "47" (specifies ICD format)

        ICD       2 octet       value "00 05" (specifies Internet/Gosip)

        DFI       1 octet       value "xx"

        AA        3 octets      value "xx xx xx" (specifies special
                                IP-only use of NSAPs)

        Reserved  2 octets      must be "00 00"

        RD        2 octets      contains autonomous system number

        Area      2 octets      must be assigned as described below

        ID        6 octets      must be assigned as described below

        SEL       1 octet       used as described below

   The AFI value of "47" and the ICD value of "00 05" specifies the
   Gosip Version 2.0 addressing format. The DFI number of "xx" and the
   AA of "xx xx xx" specify that this special NSAP address format is
   being used, solely for IS-IS packet formats in an IP-only
   environment. The reserved field must contain "00 00", as specified in
   GOSIP version 2.0.