rfc1195.txt

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Callon                                                          [Page 5]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   Instead, manual configuration is used. The link is statically
   configured with the set of address prefixes reachable via that link,
   and with the method by which they can be reached (such as the DTE
   address to be dialed to reach that address, or the fact that the DTE
   address should be extracted from the IDP portion of the ISO address).

   OSI IS-IS routing makes use of two-level hierarchical routing. A
   routing domain is partitioned into areas. Level 1 routers know the
   topology in their area, including all routers and end systems in
   their area. However, level 1 routers do not know the identity of
   routers or destinations outside of their area. Level 1 routers
   forward all traffic for destinations outside of their area to a level
   2 router in their area. Similarly, level 2 routers know the level 2
   topology, and know which addresses are reachable via each level 2
   router. However, level 2 routers do not need to know the topology
   within any level 1 area, except to the extent that a level 2 router
   may also be a level 1 router within a single area. Only level 2
   routers can exchange data packets or routing information directly
   with external routers located outside of the routing domains.

    +----------------------+-------------------------------+
    |        IDP           |              DSP              |
    +----------------------+-------------------------------+
    .                      .                               .
    .                      .                               .
    .                      .                               .
    +-----+----------------+----------+--------------+-----+
    | AFI |      IDI       |  HO-DSP  |      ID      | SEL |
    +-----+----------------+----------+--------------+-----+

         Figure 1 - ISO Hierarchical Address Structure


   As illustrated in figure 1, ISO addresses are subdivided into the
   Initial Domain Part (IDP), and the Domain Specific Part (DSP). The
   IDP is the part which is standardized by ISO, and specifies the
   format and authority responsible for assigning the rest of the
   address. The DSP is assigned by whatever addressing authority is
   specified by the IDP. The DSP is further subdivided into a "High
   Order Part of DSP" (HO-DSP), a system identifier (ID), and an NSAP
   selector (SEL). The HO-DSP may use any format desired by the
   authority which is identified by the IDP. Together, the combination
   of [IDP, HO-DSP] identify both the routing domain and the area within
   the routing domain. The combination of [IDP,HO-DSP] may therefore be
   referred to as the "Area Address".

   Usually, all nodes in an area have the same area address. However,
   sometimes an area might have multiple addresses. Motivations for



Callon                                                          [Page 6]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   allowing this are:

   - It might be desirable to change the address of an area. The most
     graceful way of changing an area from having address A to having
     address B is to first allow it to have both addresses A and B, and
     then after all nodes in the area have been modified to recognize
     both addresses, then one by one the nodes can be modified to
     "forget" address A.

   - It might be desirable to merge areas A and B into one area. The
     method for accomplishing this is to, one by one, add knowledge of
     address B into the A partition, and similarly add knowledge of
     address A into the B  partition.

   - It might be desirable to partition an area C into two areas, A
     and B (where "A" might equal "C", in which case this example
     becomes one of removing a portion of an area). This would be
     accomplished by first introducing knowledge of address A into
     the appropriate nodes (those destined to become area A), and
     knowledge of address B into the appropriate nodes, and then one
     by one removing knowledge of address C.

   Since OSI addressing explicitly identifies the area, it is very easy
   for level 1 routers to identify packets going to destinations outside
   of their area, which need to be forwarded to level 2 routers.

   In IS-IS, there are two types of routers:

   - Level 1 intermediate systems -- these nodes route based on the ID
     portion of the ISO address. They route within an area. They
     recognize, based on the destination address in a packet, whether
     the destination is within the area. If so, they route towards
     the destination. If not, they route to the nearest level 2 router.

   - Level 2 intermediate systems -- these nodes route based on the area
     address (i.e., on the combination of [IDP, HO-DSP]). They route
     towards areas, without regard to the internal structure of an area.
     A level 2 IS may also be a level 1 IS in one area.

   A level 1 router will have the area portion of its address manually
   configured. It will refuse to become a neighbor with a node whose
   area addresses do not overlap its area addresses. However, if level 1
   router has area addresses A,  B, and C, and a neighbor has area
   addresses B and D, then the level 1 router will accept the other node
   as a neighbor.

   A level 2 router will accept another level 2 router as a neighbor,
   regardless of area address. However, if the area addresses do not



Callon                                                          [Page 7]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   overlap, the link would be considered by both routers to be "level 2
   only", and only level 2 LSPs would flow on the link. External links
   (to other routing domains) must be from level 2 routers.

   IS-IS provides an optional partition repair function. In the unlikely
   case that a level 1 area become partitioned, this function, if
   implemented, allows the partition to be repaired via use of level 2
   routes.

   IS-IS requires that the set of level 2 routers be connected. Should
   the level 2 backbone become partitioned, there is no provision for
   use of level 1 links to repair a level 2 partition.

   In unusual cases, a single level 2 router may lose connectivity to
   the level 2 backbone. In this case the level 2 router will indicate
   in its level 1 LSPs that it is not "attached", thereby allowing level
   1 routers in the area to route traffic for outside of the domain to a
   different level 2 router. Level 1 routers therefore route traffic to
   destinations outside of their area only to level 2 routers which
   indicate in their level 1 LSPs that they are "attached".

   An end system may autoconfigure the area portion of its address by
   extracting the area portion of a neighboring router's address. If
   this is the case, then an endnode will always accept a router as a
   neighbor. Since the standard does not specify that the end system
   MUST autoconfigure its area address, an end system may be configured
   with an area address. In this case the end system would ignore router
   neighbors with non-matching area addresses.

   Special treatment is necessary for broadcast subnetworks, such as
   LANs. This solves two sets of issues: (i) In the absence of special
   treatment, each router on the subnetwork would announce a link to
   every other router on the subnetwork, resulting in n-squared links
   reported; (ii) Again, in the absence of special treatment, each
   router on the LAN would report the same identical list of end systems
   on the LAN, resulting in substantial duplication.

   These problems are avoided by use of a "pseudonode", which represents
   the LAN. Each router on the LAN reports that it has a link to the
   pseudonode (rather than reporting a link to every other router on the
   LAN). One of the routers on the LAN is elected "designated router".
   The designated router then sends out an LSP on behalf of the
   pseudonode, reporting links to all of the routers on the LAN. This
   reduces the potential n-squared links to n links. In addition, only
   the pseudonode LSP includes the list of end systems on the LAN,
   thereby eliminating the potential duplication (for further
   information on designated routers and pseudonodes, see [1]).




Callon                                                          [Page 8]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   The IS-IS provides for optional Quality of Service (QOS) routing,
   based on throughput (the default metric), delay, expense, or residual
   error probability. This is described in greater detail in section
   3.5, and in [1].

1.3 Overview of the Integrated IS-IS

   The integrated IS-IS allows a single routing protocol to be used to
   route both IP and OSI packets. This implies that the same two-level
   hierarchy will be used for both IP and OSI routing. Each area will be
   specified to be either IP-only (only IP traffic can be routed in that
   particular area), OSI-only (only OSI traffic can be routed in that
   area), or dual (both IP and OSI traffic can be routed in the area).

   This proposal does not allow for partial overlap of OSI and IP areas.
   For example, if one area is OSI-only, and an other area is IP-only,
   then it is not permissible to have some routers be in both areas.
   Similarly, a single backbone is used for the routing domain. There is
   no provision for independent OSI and IP backbones.

   Similarly, within an IP-only or dual area, the amount of knowledge
   maintained by routers about specific IP destinations will be as
   similar as possible as for OSI. For example, IP-capable level 1
   routers will maintain the topology within the area, and will be able
   to route directly to IP destinations within the area. However, IP-
   capable level 1 routers will not maintain information about
   destinations outside of the area. Just as in normal OSI routing,
   traffic to destinations outside of the area will be forwarded to the
   nearest level 2 router. Since IP routes to subnets, rather than to
   specific end systems, IP routers will not need to keep nor distribute
   lists of IP host identifiers (note that routes to hosts can be
   announced by using a subnet mask of all ones).

   The IP address structure allows networks to be partitioned into
   subnets, and allows subnets to be recursively subdivided into smaller
   subnets. However, it is undesireable to require any specific
   relationship between IP subnet addresses and IS-IS areas. For
   example, in many cases, the dual routers may be installed into
   existing environments, which already have assigned IP and/or OSI
   addresses. In addition, even if IP addresses are not already pre-
   assigned, the address limitations of IP constrain what addresses may
   be assigned. We therefore will not require any specific relationship
   between IP addresses and the area structure. The IP addresses can be
   assigned completely independently of the OSI addresses and IS-IS area
   structure. As will be described in section 3.2 ("Hierarchical
   Abbreviation of IP Reachability Information"), greater efficiency and
   scaling of the routing algorithm can be achieved if there is some
   correspondence between the IP address assignment structure and the



Callon                                                          [Page 9]

RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   area structure.

   Within an area, level 1 routers exchange link state packets which
   identify the IP addresses reachable by each router. Specifically,
   zero or more [IP address, subnet mask, metric] combinations may be
   included in each Link State Packet. Each level 1 router is manually
   configured with the [IP address, subnet mask, metric] combinations
   which are reachable on each interface. A level 1 router routes as
   follows:

   - If a specified destination address matches an [IP address, subnet
     mask, metric] reachable within the area, the packet is routed via
     level 1 routing.

   - If a specified destination address does not match any [IP address,
     subnet mask, metric] combination listed as reachable within the
     area, the packet is routed towards the nearest level 2 router.

   Flexible use of the limited IP address space is important in order to
   cope with the anticipated growth of IP environments. Thus an area
   (and by implication a routing domain) may simultaneously make use of
   a variety of different address masks for different subnets in the
   area (or domain). Generally, if a specified destination address
   matches more than one [IP address, subnet mask] pair, the more
   specific address is the one routed towards (the one with more "1"
   bits in the mask -- this is known as "best match" routing).

   Level 2 routers include in their level 2 LSPs a complete list of [IP
   address, subnet mask, metric] specifying all IP addresses reachable
   in their area. As described in section 3, this information may be
   obtained from a combination of the level 1 LSPs (obtained from level
   1 routers in the same area), and/or by manual configuration. In
   addition, Level 2 routers may report external reachability
   information, corresponding to addresses which can be reached via
   routers in other routing domains (autonomous systems)

   Default routes may be announced by use of a subnet mask containing
   all zeroes. Default routes should be used with great care, since they
   can result in "black holes". Default routes are permitted only at
   level 2 as external routes (i.e., included in the "IP External
   Reachability Information" field, as explained in sections 3 and 5).
   Default routes are not permitted at level 1.

   The integrated IS-IS provides optional Type of Service (TOS) routing,
   through use of the QOS feature from IS-IS.