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Home › Network Protocol Suite Directory Index › ISO Protocols in OSI 7 Layers Reference Model › ISO Network Layer

ES-IS: End System to Intermediate System Routing Exchange Protocol

End System to Intermediate System Routing Exchange Protocol (ES-IS), developed by ISO, permits End Systems and Intermediate Systems to exchange configuration and routing information to facilitate the operation of the routing and relaying functions of the Network Layer in the ISO network environment. In an ISO network, there are End Systems, Intermediate Systems, Areas and Domains. End systems are user devices. Intermediate systems are routers. Routers are organized into local groups called "areas", and several areas are grouped together into a "domain". ES-IS, working in conjunction with CLNP, IS-IS, and IDRP, provides complete routing over the entire network.

ES-IS provides solutions for the following practical problems:

  1. For end systems to discover the existence and reachability of intermediate systems that can route NPDUs to destinations on subnetworks other than the one(s) to which the end system is directly connected.
  2. For end systems to discover the existence and reachability of other end systems on the same subnetwork.
  3. For intermediate systems to discover the existence and reachability of end systems on each of the subnetworks to which they are directly connected.
  4. For end systems decide which intermediate system to use to forward NPDUs to a particular destination when more than one intermediate system is accessible.

ES-IS provides two types of information to Network entities which support its operation: a) Configuration Information, Which permits End Systems to discover the existence and reachability of Intermediate Systems and permits Intermediate Systems to discover the existence and reachability of End Systems; and b) Route Redirection Information which allows Intermediate Systems to inform End Systems of (potentially) better paths to use when forwarding NPDUs to a particular destination. A Network Entity may choose to support either the Configuration Information, the Route Redirection Information, neither, or both.

Protocol Structure

ES-IS Protocol Data Unit contains the following:

ES-IS Header Network address Subnetwork address Option

ES-IS Header:

1 byte 1 byte 1 byte 1 byte 1 byte 2 bytes 2 bytes
NLPID Length Version Reserved 0 0 0 Type H-Time Checksum
  • NLPID C Network Layer Protocol Identification. The value of this field shall be 1000 0010
  • Length C Length Indicator is the length of the entire PDU
  • Version C Protocol ID extension. This identifies a standard version of ISO xxxx, End System to Intermediate System Routing Exchange Protocol for use in conjunction with ISO 8473.
  • Reserved C Must be zero.
  • Type CThe Type code field identifies the type of the protocol data unit.
  • H-Time C Holding time field specifies for how long the receiving Network entity should retain the configuration/routing information contained in this PDU.
  • Checksum C Error checking which is computed on the entire PDU header.

Related protocols: IS-IS, CLNP, IDRP

Sponsor Source: ES-IS is defined in ISO (http://www.iso.org)

Reference: http://www.javvin.com/protocol/rfc995.pdf: End System to Intermediate System Routing Exchange Protocol for use in conjunction with ISO 8473

‹ ISO Network LayerupIS-IS: Intermediate System to Intermediate System Routing Protocol ›