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ESIS(4)			 BSD Kernel Interfaces Manual		       ESIS(4)

     es-is — End System to Intermediate System Routing Protocol


     The ES-IS routing protocol is used to dynamically map between ISO NSAP
     addresses and ISO SNPA addresses; to permit End and Intermediate Systems
     to learn of each other's existence; and to allow Intermediate Systems to
     inform End Systems of (potentially) better routes to use when forwarding
     NPDUs to a particular destination.

     The mapping between NSAP addresses and SNPA addresses is accomplished by
     transmitting hello PDUs between the cooperating Systems. These PDUs are
     transmitted whenever the configuration timer expires.  When a hello PDU
     is received, the SNPA address that it conveys is stored in the routing
     table for as long as the holding time in the PDU suggests. The default
     holding time (120 seconds) placed in the hello PDU, the configuration
     timer value, and the system type (End System or Intermediate System) may
     be changed by issuing an SIOCSSTYPE ioctl(2), which is defined in

     The protocol behaves differently depending on whether the System is con‐
     figured as an End System or an Intermediate System.

     When an interface requests a mapping for an address not in the cache, the
     SNPA of any known Intermediate System is returned. If an Intermediate
     System is not known, then the all end systems multicast address is
     returned. It is assumed that the intended recipient of the NPDU will
     immediately transmit a hello PDU back to the originator of the NPDU.

     If an NPDU is forwarded by the End System, a redirect PDU will not be
     generated.	 However, redirect PDUs received will be processed. This pro‐
     cessing consists of adding an entry in the routing table. If the redirect
     is towards an Intermediate System, then an entry is made in the routing
     table as well.  The entry in the routing table will mark the NSAP address
     contained in the redirect PDU as the gateway for the destination system
     (if an NET is supplied), or will create a route with the NSAP address as
     the destination and the SNPA address (embodied as a link-level sockaddr)
     as the gateway.

     If the System is configured as an End System, it will report all the
     NSAPs that have been configured using the ifconfig command, and no oth‐
     ers.  It is possible to have more than one NSAP assigned to a given
     interface, and it is also possible to have the same NSAP assigned to mul‐
     tiple interfaces.	However, any NSAP containing an NSEL that is consis‐
     tent with the nsellength option (default one) of any interface will be
     accepted as an NSAP for this System.

     When an interface requests a mapping for an address not in the routing
     table, an error is returned.

     When an NPDU is forwarded out on the same interface that the NPDU arrived
     upon, a redirect PDU is generated.

     To facilitate communications with systems which do not use ES-IS, one may
     add a route whose destination is a sockaddr_iso containing the NSAP in
     question, and the gateway being a link-level sockaddr, either by writing
     a special purpose program, or using the route(8) command e.g.:

     route add -iface -osi     -link

     If the System is configured as an End System and has a single network
     interface which does not support multicast reception, it is necessary to
     manually configure the location of an IS, using the route command in a
     similar way.  There, the destination address should be “default” (spelled
     out literally as 7 ASCII characters), and the gateway should be once
     again be a link-level sockaddr specifying the SNPA of the IS.

     un(4), iso(4), route(8), ifconfig(8)

     End system to Intermediate system routing exchange protocol for use in
     conjunction with the Protocol for providing the connectionless-mode
     network service, ISO, 9542.

     Redirect PDUs do not contain options from the forwarded NPDU which gener‐
     ated the redirect. The multicast address used on the 802.3 network is
     taken from the NBS December 1987 agreements. This multicast address is
     not compatible with the 802.5 (Token Ring) multicast addresses format.
     Therefore, broadcast addresses are used on the 802.5 subnetwork.
     Researchers at the University of Wisconsin are constructing an implemen‐
     tation of the IS-IS routing protocol.

BSD			       November 30, 1993			   BSD

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