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Versions: 00                                                            
Network Working Group                                       Naiming Shen
Internet Draft                                               Acee Lindem
Expiration Date: December 2001                                Jenny Yuan
File name: draft-shen-isis-ospf-p2p-over-lan-00.txt     Redback Networks
                                                               June 2001





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

               draft-shen-isis-ospf-p2p-over-lan-00.txt



Status of this Memo


    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
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Abstract

    Two different circuit types are commonly used by link state routing
    protocols: point-to-point and broadcast. It is important to identify
    the correct circuit type in forming adjacency with neighbors,
    in flooding link state database packets, in representation of the
    circuit subnet. This document describes a simple mechanism to treat
    the broadcast media as point-to-point connection from IP routing
    protocol point of view if there are only two devices on the LAN
    media.




Shen, Lindem, Yuan       Expires December 2001                  [Page 1]


INTERNET DRAFT             P2P OVER LAN                        June 2001


1. Introduction

    Point-to-point and broadcast media are two most common circuit
    types used by link state routing protocols such as IS-IS [ref1]
    [ref2] and OSPF [ref3]. They are treated differently with respect
    to establishing neighbor adjacencies, link state database flooding,
    representation of the media subnet, SPF calculation and protocol
    packets.  The most important difference is there is a designated
    router concept associated with the broadcast media and pseudo node
    is used to represent the information on the LAN media.

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

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


2. Motivation

    Even though the broadcast media is meant to handle more than two
    devices, there exist cases where only two routers are interconnected
    over the physical or logical broadcast media:

      o   simply only two routers on the physical LAN.
      o   two routers are connected directly back to back using
          broadcast media, mainly for long-haul operation.
      o   only two routers exist on the virtual LAN.

    In any of the above cases, the link state routing protocols will
    normally still treat the circuit as a broadcast type. Thus it will
    have the overhead involved with protocol LAN operation but without
    the benefit of reducing routing information designed for the LAN
    environment.

    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 broadcast media can be
    viewed by the IGP as a mesh of point-to-point connections. As a
    side benefit, unnumbered interface can also be applied over LAN.

Shen, Lindem, Yuan       Expires December 2001                  [Page 2]


INTERNET DRAFT             P2P OVER LAN                        June 2001


3. 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.


3.1 Operation of IS-IS

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

    Since physically the circuit is a broadcast one, the IS-IS packets
    need to have MAC addresses for this p2p-over-lan circuit.  From
    link layer point of view, those packets are IS-IS LAN packets. The
    Multi-destination address, either AllL1ISs or AllL2ISs defined
    in [ref1], is used for the point-to-point IS-IS PDUs.

    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 IP address(es) can be numbered or unnumbered. Note
    that the term "unnumbered" here means this interface sets the
    IP address to any one of the other IP addresses belong to this
    router.

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


3.2 Operation of OSPF

    OSPF routers supporting the capabilities described herein must
    support an additional interface configuration parameter specifying
    the interface topology type. For LAN (i.e., broadcast capable)
    media, the interface may be viewed as a point-to-point interface.
    Both routers on the LAN will simply join the AllSPFRouters
    (224.0.0.5) multicast group and send all OSPF packets to 224.0.0.5.
    This is identical to operation over a physical point-to-point link
    as described in sections 8.1 and 8.2 of [ref3].


Shen, Lindem, Yuan       Expires December 2001                  [Page 3]


INTERNET DRAFT             P2P OVER LAN                        June 2001


3.3 IP forwarding and ARP

    Unlike normal point-to-point IGP circuit, the IP nexthop for the
    routes using this p2p-over-lan circuit as 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.
    Proxy ARP has to be enabled if the address is not the adjacent
    interface IP address.

    In the case where unnumbered IP addresses are used for p2p-over-lan
    circuit, the source IA of ARP request and the target interface IA
    are usually on different subnets. The ARP should reply only if this
    circuit is a p2p-over-lan type and the source IA of the ARP request
    is the same as the neighbor's interface IP address at the other end.
    The neighbor's address is learned from IGP hello exchanges over this
    circuit.


4. Compatibility

    Routers on both sides of the broadcast media connection have to
    support this p2p-over-lan extension in order to establish adjacency
    to each other. Otherwise, the traditional LAN model for the IGP
    has to be used on this media.


5. Scalability Issues

    There is obvious advantage to use this extension if the broadcast
    media between two routers are connected back-to-back. To model
    the LAN as a number of vLANs with this extension does sacrifice
    the scalability property of the LAN representation for link-state
    routing protocols. It will in general increase the link-state
    database size, the amount of packets to be flooded and the
    route calculation time thus the network overall convergence time.

    The network design engineers should carefully balance between the
    need of more precise routing control and the scalability of the
    network. The scalability impact is less of a concern if the LAN
    and routers involved are within a single link-state subdomain
    in hierarchical IGP routing.


6. Security Issues

    This document does not introduce any new security issues to ISIS or
    OSPF. For ARP to support unnumbered IP interface addresses, it needs
    to verify the p2p-over-lan circuit type described in this document
    and to verify the ARP packet source IA to match the IGP adjacency
    interface IP address. This is due to normal ARP sanity check for
    common subnet can not be applied in this case.

Shen, Lindem, Yuan       Expires December 2001                  [Page 4]


INTERNET DRAFT             P2P OVER LAN                        June 2001


7. Acknowledgments

    TBA.




8. 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.



9. Authors' Addresses

    Naiming Shen
    Redback Networks
    350 Holger Way
    San Jose, CA, 95134 USA
    naiming@redback.com

    Acee Lindem
    Redback Networks
    102 Carric Bend Court
    Apex, NC 27502 USA
    acee@redback.com

    Jenny Yuan
    Redback Networks
    350 Holger Way
    San Jose, CA, 95134 USA
    jenny@redback.com








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