Network Working Group                Naiming Shen, Ed (Redback Networks)
Internet Draft                                  Alex Zinin, Ed (Alcatel)
Expiration Date: January 2005
                                                               July 2004

                  Point-to-point operation over LAN
                   in link-state routing protocols


Status of this Memo

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   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.

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

    Acee Lindem
    Redback Networks
    102 Carric Bend Court
    Cary, NC 27519 USA

    Jenny Yuan
    Redback Networks
    350 Holger Way
    San Jose, CA, 95134 USA

    Russ White
    Cisco Systems, Inc.
    7025 Kit Creek Rd.
    Research Triangle Park, NC 27709

    Stefano Previdi
    Cisco Systems, Inc.
    De Kleetlaan 6A
    1831 Diegem - Belgium


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

1. Introduction

   Point-to-point and broadcast are the two predominant circuit
   types used by link state routing protocols such as ISIS [ref1]
   [ref2] and OSPF [ref3, ref5]. They are treated differently with
   respect to establishing neighbor adjacencies, flooding link-state
   information, representation of the topology, SPF calculation 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

   Compared with broadcast circuits, point-to-point circuits
   afford more straightforward IGP operation. There is no designated

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   router involved and there is no representation of the pseudo-node
   or network LSA in the link state database. For ISIS, 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.

   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.

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 election can be omitted.
   Flooding can be done as in p2p links without the need of using
   "LSA reflection" by the DR in OSPF or periodic CSNPs in ISIS.

   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.

   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

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   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 IPv6 network, link-local address
   used in ISIS [ref4] and OSPFv3 [ref5] 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

      1. Source IP address - The interface address can be used by
         the router as the source IP address in locally originated
         IP packets destined for that subnet or having 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 address using the ARP/ND 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 (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

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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 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 ISIS

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

   Since physically the circuit is a broadcast one, the ISIS protocol
   packets need to have MAC addresses for this p2p-over-lan circuit.
   From link layer point of view, those packets are ISIS LAN packets.
   The Multi-destination address including AllISs, AllL1ISs and AllL2ISs
   defined in [ref1] can be used for link layer encapsulation, the
   use of AllISs is recommended.

   The circuit needs to have IP address(es) and the p2p IIH over this
   circuit MUST include the IP interface address(es) as defined in
   [ref2]. 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) [ref4].

4.2 Operation of OSPF and OSPFv3

   OSPF and OSPFv3 [ref5] 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 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 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 [ref3].

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4.3 ARP and ND

   Unlike 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
   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 subnet of the source address of the
   ARP request to match the local interface address, this check needs
   to be relaxed for the unnumbered p2p-over-lan circuits. The
   mis-configuration detection is handled by the IGPs and is described
   in section 4.5. In 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 [ref6].

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.

4.5 Detection of mis-configuration

   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

   If the circuit is configured as point-to-point type and receives
   LAN hello packets, the router MUST discard the incoming packets; If
   the circuit is a LAN type and receive point-to-point hello packets,
   it MUST discard the incoming packets. If the system ID or the
   router ID of incoming hello packet does not match the system ID or
   the router ID of already established adjacency over this p2p-over-lan
   circuit, it MUST discard the packet. The implementation should offer
   logging and debugging information of the above events.

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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 use this extension on the LANs
   that are connected back-to-back or only contain two routers,
   however there are tradeoffs 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. Network design
   engineers should carefully balance between the associated

   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
   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 ISIS,
   OSPF, ARP or ND. Implementations may have 'source address subnet
   checks' which 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:

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   (in last name alphabetical order) Pedro Marques, Christian Martin,
   Danny McPherson, Ajay Patel, Jeff Parker, Tony Przygienda,
   Alvaro Retana and Pekka Savola.

9. Normative References

   [ref1] ISO.  Information Technology - Telecommunications and
          Information Exchange between Systems - Intermediate System
          to Intermediate System Routing Exchange Protocol for
          Use in Conjunction with the Protocol for Providing the
          Connectionless-Mode Network Service.  ISO, 1990.

   [ref2] R. Callon.  Use of OSI ISIS for Routing in TCP/IP and Dual
          Environments.  INTERNET-RFC, Internet Engineering Task Force,
          December 1990.

   [ref3] J. Moy. OSPF Version 2. Technical Report RFC2328 Internet
          Engineering Task Force, 1998.

   [ref4] Hopps, C., "Routing IPv6 with IS-IS",
          draft-ietf-isis-ipv6-05.txt, work in progress.

   [ref5] Coltun, R., Ferguson, D. and J. Moy, "OSPF for IPv6",
          RFC 2740, December 1999.

   [ref6] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
          for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [ref7] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.

10. Editors' Addresses

    Naiming Shen
    Redback Networks
    350 Holger Way
    San Jose, CA, 95134 USA

    Alex Zinin
    Sunnyvale, CA, USA

Intellectual Property Considerations

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