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