Network Working Group N. Shen, Ed.

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.

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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
mesh of point-to-point connections.

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The IP unnumbered configuration is widely used in networks. It
enables IP processing on a point-to-point interface without an
explicit IP address. The IP unnumbered interface can "borrow" the IP
address of another interface on the node. The advantages of
unnumbered point-to-point links are obvious in the current IP
addressing environment where addresses are a scarce resource. The
unnumbered interface can also be applied over p2p-over-lan circuits.
Separating the concept of network type from media type will allow
LANs, e.g., ethernet, to be unnumbered and realize the IP address
space savings. Another advantage is in simpler network management
and configuration. In the case of an IPv6 network, a link local
address used in IS-IS [RFC5308] and OSPFv3 [RFC5340] serves the same
purpose.

3. IP Multi-Access Subnets

When an IP network includes multi-access segments, each segment is
usually assigned a separate subnet, and each router connected to it
is assigned a distinct IP address within that subnet. The role of
the IP address assigned to a multi-access interface can be outlined
as follows:

1. Source IP address - The interface address can be used by the
router as the source IP address in locally originated IP
packets that are destined for that subnet or have a best path
next hop on that subnet.

2. Destination IP address - The interface address can be used by
other devices in the network as a destination address for
packets to router applications (examples include telnet, SMTP,
TFTP, OSPF, BGP, etc).

3. Next-hop identifier - If other routers connected to the same
segment need to forward traffic through the router, the
corresponding routes in their routing tables will include the
router's interface IP address. This address will be used to
find the router's MAC (Media Access Control) address using the
ARP/ND (Address Resolution Protocol / Neighbor Discovery)
protocol. Effectively, the interface IP addresses help other
routers find the data-link layer details that are required to
specify the destination of the encapsulating data-link frame
when it is sent on the segment.

The IP addressing scheme includes an option that allows the
administrators to not assign any subnets to point-to-point links
(links connecting only two devices and using protocols like PPP,
SLIP, or HDLC for IP encapsulation). This is possible because the
routers do not need next-hop identifiers on point-to-point links

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(there is only one destination for any transmission), and an
interface-independent IP address can be used as the source and
destination. Using the unnumbered option for a point-to-point link
essentially makes it a purely topological entity used only to reach
other destinations.

4. Point-to-Point Connection over LAN Media

The idea is very simple: provide a configuration mechanism to inform
the IGP that the circuit is type point-to-point, irrespective of the
physical media type. For the IGP, this implies that it will send
protocol packets with the appropriate point-to-point information, and
it expects to receive protocol packets as they would be received on a
point-to-point circuit. Over LAN media, the MAC header must contain
the correct multicast MAC address to be received by the other side of
the connection. For vLAN environments, the MAC header must also
contain the proper vLAN ID.

In order to allow LAN links used to connect only two routers to be
treated as unnumbered point-to-point interfaces, the MAC address
resolution and nexthop IP address issues need to be addressed.

4.1. Operation of IS-IS

This p2p-over-lan circuit extension for IS-IS is only concerned with
pure IP routing and forwarding operation.

Since physically the circuit is a broadcast one, the IS-IS protocol
packets need to have MAC addresses for this p2p-over-lan circuit.
From a link-layer point of view, those packets are IS-IS LAN packets.
The Multi-destination address including AllISs, AllL1ISs, and
AllL2ISs, defined in [ISO10589], can be used for link-layer
encapsulation; the use of AllISs is recommended.

The circuit needs to have IP address(es), and the p2p IS-IS Hello
(IIH) over this circuit MUST include the IP interface address(es) as
defined in [RFC1195]. The IPv4 address(es) included in the IIHs is
either the IP address assigned to the interface in the case of a
numbered interface or the interface-independent IP address in the
case of an unnumbered interface. The IPv6 addresses are link-local
IPv6 address(es) [RFC5308].

4.2. Operation of OSPF and OSPFv3

OSPF and OSPFv3 [RFC5340] routers supporting the capabilities
described herein should support an additional interface configuration
parameter specifying the interface topology type. For a LAN (i.e.,
broadcast-capable) interface, the interface may be viewed as a

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point-to-point interface. Both routers on the LAN will simply join
the AllSPFRouters multicast group and send all OSPF packets with a
destination address of AllSPFRouters. AllSPFRouters is 224.0.0.5 for
OSPF and FF02::5 for OSPFv3. This is identical to operation over a
physical point-to-point link as described in Sections 8.1 and 8.2 of
[RFC2328].

4.3. ARP and ND

Unlike a normal point-to-point IGP circuit, the IP nexthop for the
routes using this p2p-over-lan circuit as an outbound interface is
not optional. The IP nexthop address has to be a valid interface or
internal address on the adjacent router. This address is used by a
local router to obtain the MAC address for IP packet forwarding. The
ARP process has to be able to resolve the internal IPv4 address used
for the unnumbered p2p-over-lan circuits. For the ARP implementation
(which checks that the subnet of the source address of the ARP
request matches the local interface address), this check needs to be
relaxed for the unnumbered p2p-over-lan circuits. The
misconfiguration detection is handled by the IGPs and is described in
Section 4.5. In the IPv6 case, the ND resolves the MAC for the
link-local address on the p2p-over-lan circuit, which is part of the
IPv6 neighbor discovery process [RFC4861].

4.4. Other MAC Address Resolution Mechanisms

In more general cases, while p2p-over-lan circuit is used as an
unnumbered link, other MAC address resolution mechanisms are needed
for IP packet forwarding; for example, if link state IGP is not
configured over this p2p-over-lan link, or if the mechanism described
in Section 4.3 is not possible. The following techniques can be used
to acquire the MAC address and/or the next-hop IP address of the
remote device on an unnumbered point-to-point LAN link.

1. Static configuration. A router can be statically configured
with the MAC address that should be used as the destination MAC
address when sending data out of the interface.

2. MAC address gleaning. If a dynamic routing protocol is running
between the routers connected to the link, the MAC address of
the remote device can be taken from a data-link frame carrying
a packet of the corresponding routing protocol.

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4.5. Detection of Misconfiguration

With this p2p-over-lan extension, the difference between a LAN and a
point-to-point circuit can be made purely by configuration. It is
important to implement the mechanisms for early detection of
misconfiguration.

If the circuit is configured as the point-to-point type and receives
LAN hello packets, the router MUST discard the incoming packets; if
the circuit is a LAN type and receives point-to-point hello packets,
it MUST discard the incoming packets. If the system ID or the router
ID of an incoming hello packet does not match the system ID or the
router ID for an established adjacency over a p2p-over-lan circuit,
the packet MUST be discarded. Furthermore, if OSPF hello suppression
(as described in [RFC1793]) is active for the adjacency, the hello
suppression MUST be terminated for a period of RouterIntervalSeconds.
After this interval, either the neighbor adjacency will time out and
an adjacency may be formed with a neighbor with a different router
ID, or hello suppression may be renegotiated. The implementation
should offer logging and debugging information of the above events.

5. Compatibility Considerations

Both routers on a LAN must support the p2p-over-lan extension and
both must have the LAN segment configured as a p2p-over-lan circuit
for successful operation. Both routers SHOULD support at least one
of the above listed methods for mapping IP addresses on the link to
MAC address. If a proprietary method of IP address to MAC address
resolution is used by one router, both routers must be capable of
using the same method. Otherwise, the link should be configured as a
standard LAN link, with traditional IGP LAN models used.

6. Scalability and Deployment Considerations

While there is advantage to using this extension on the LANs that are
connected back to back or only contain two routers, there are trade
offs when modeling a LAN as multiple vLANs and using this extension
since one does sacrifice the inherent scalability benefits of multi-
access networks. In general, it will increase the link state
database size, the amount of packets flooded, and the route
calculation overhead.

Deployment of the described technique brings noticeable benefits from
the perspective of IP address usage: the network management and the
router configuration. Note, however, that use of the IP unnumbered

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option for point-to-point LAN links inherits the same problems as
those present for serial links, i.e., not being able to ping or
monitor a specific interface between routers.

7. Security Considerations

This document does not introduce any new security issues to IS-IS,
OSPF, ARP, or ND. Implementations may have 'source address subnet
checks' that need to be relaxed as described in Section 4.3. These
are used to manage misconfigurations, not so much to secure ARP -- if
an attacker would be attached to the LAN, (s)he could pick a subnet-
wise correct address as well.

If one router on a link thinks that a LAN should be either broadcast
or p2p-over-lan, and the other router has a different opinion, the
adjacencies will never form, as specified in Section 4.5. There are
no fallbacks at either end to resolve the situation, except by a
manual configuration change.

8. Acknowledgments

The authors would like to acknowledge the following individuals (in
alphabetical order by last name): Pedro Marques, Christian Martin,
Danny McPherson, Ajay Patel, Jeff Parker, Tony Przygienda, Alvaro
Retana, and Pekka Savola.

9. Normative References

[ISO10589] ISO, "Intermediate System to Intermediate System intra-
domain routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)",
International Standard 10589:2002, Second Edition, 2002.

[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.

[RFC1793] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
1793, April 1995.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.

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[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, October
2008.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.

Contributors

The following individuals are the authors that contributed to the
contents of this document.

Acee Lindem
Cisco Systems
7025 Kit Creek Road
Research Triangle Park, NC 27709
USA
EMail: [email protected]

Jenny Yuan
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: [email protected]

Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
EMail: [email protected]

Stefano Previdi
Cisco Systems, Inc.
De Kleetlaan 6A
1831 Diegem - Belgium
EMail: [email protected]

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Editors' Addresses

Naiming Shen
Cisco Systems
225 West Tasman Drive
San Jose, CA 95134
USA
EMail: [email protected]

Alex Zinin
Alcatel-Lucent
750D Chai Chee Rd, #06-06
Technopark@ChaiChee
Singapore 469004

EMail: [email protected]

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