Point-to-Point Operation over LAN in Link State Routing Protocols
RFC 5309

Network Working Group                                       N. Shen, Ed.
Request for Comments: 5309                                 Cisco Systems
Category: Informational                                    A. Zinin, Ed.
                                                          Alcatel-Lucent
                                                            October 2008

                   Point-to-Point Operation over LAN
                    in Link State Routing Protocols

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   The two predominant circuit types used by link state routing
   protocols are point-to-point and broadcast.  It is important to
   identify the correct circuit type when forming adjacencies, flooding
   link state database packets, and representing the circuit
   topologically.  This document describes a simple mechanism to treat
   the broadcast network as a point-to-point connection from the
   standpoint of IP routing.

1.  Introduction

   Point-to-point and broadcast are the two predominant circuit types
   used by link state routing protocols such as IS-IS [ISO10589]
   [RFC1195] and OSPF [RFC2328] [RFC5340].  They are treated differently
   with respect to establishing neighbor adjacencies, flooding link
   state information, representing the topology, and calculating the
   Shortest Path First (SPF) and protocol packets.  The most important
   differences are that broadcast circuits utilize the concept of a
   designated router and are represented topologically as virtual nodes
   in the network topology graph.

   Compared with broadcast circuits, point-to-point circuits afford more
   straightforward IGP operation.  There is no designated router
   involved, and there is no representation of the pseudonode or network
   Link State Advertisement (LSA) in the link state database.  For IS-
   IS, there also is no periodic database synchronization.  Conversely,
   if there are more than two routers on the LAN media, the traditional
   view of the broadcast circuit will reduce the routing information in
   the network.

Shen & Zinin                 Informational                      [Page 1]
RFC 5309                      P2P over LAN                  October 2008

   When there are only two routers on the LAN, it makes more sense to
   treat the connection between the two routers as a point-to-point
   circuit.  This document describes the mechanism to allow link state
   routing protocols to operate using point-to-point connections over a
   LAN under this condition.  Some implications related to forwarding IP
   packets on this type of circuit are also discussed.  We will refer to
   this as a p2p-over-lan circuit in this document.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Motivation

   Even though a broadcast circuit is meant to handle more than two
   devices, there are cases where only two routers are connected over
   either the physical or logical LAN segment:

      1. The media itself is being used for point-to-point operation
         between two routers.  This is mainly for long-haul operation.
      2. There are only two routers on the physical LAN.
      3. There are only two routers on a virtual LAN (vLAN).

   In any of the above cases, the link state routing protocols will
   normally still treat the media as a broadcast circuit.  Hence, they
   will have the overhead involved with protocol LAN operation without
   the benefits of reducing routing information and optimized flooding.

   Being able to treat a LAN as a point-to-point circuit provides the
   benefit of reduction in the amount of information routing protocols
   must carry and manage.  DR/DIS (Designated Router / Designated
   Intermediate System) election can be omitted.  Flooding can be done
   as in p2p links without the need for using "LSA reflection" by the DR
   in OSPF or periodic Complete Sequence Number Packets (CSNPs) in IS-
   IS.

   Also, if a broadcast segment wired as a point-to-point link can be
   treated as a point-to-point link, only the connection between the two
   routers would need to be advertised as a topological entity.

   Even when there are multiple routers on the LAN, an ISP may want to
   sub-group the routers into multiple vLANs, since this allows them to
   assign different costs to IGP neighbors.  When there are only two
   routers in some of the vLANs, this LAN can be viewed by the IGP as a