Network Working Group Naiming Shen
Internet Draft Acee Lindem
Expiration Date: February 2002 Jenny Yuan
File name: draft-ietf-isis-igp-p2p-over-lan-00.txt Redback Networks
Alex Zinin
Nexsi Systems
Russ White
Stefano Previdi
Cisco Systems
August 2001
Point-to-point operation over LAN
in link-state routing protocols
draft-ietf-isis-igp-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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
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.
Shen, Zinin, et al Expires February 2002 [Page 1]
INTERNET DRAFT P2P OVER LAN August 2001
1. Introduction
Point-to-point and broadcast are the two predominant 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, flooding of 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
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 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.
Shen, Zinin, et al Expires February 2002 [Page 2]
INTERNET DRAFT P2P OVER LAN August 2001
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.
As a side benefit, unnumbered interface can also be applied over
p2p-over-lan circuits. The advantages of unnumbered point-to-point
links are obvious in the current IP addressing environment where
addresses are a scarce resource. 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.
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 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 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
Shen, Zinin, et al Expires February 2002 [Page 3]
INTERNET DRAFT P2P OVER LAN August 2001
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
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 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
in pure IP routing and forwarding operation.
Since the physically circuit is a broadcast one, the IS-IS protocol
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 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 IP address(es) can be numbered or unnumbered.
4.2 Operation of OSPF
OSPF 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
(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, Zinin, et al Expires February 2002 [Page 4]
INTERNET DRAFT P2P OVER LAN August 2001
4.3 IP forwarding and ARP
Unlike normal point-to-point IGP circuit, the IP nexthop for the
routes using this p2p-over-lan circuit as 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.
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 IP address of ARP request and the target
interface IP address are usually on different subnets. The ARP
should reply only if this is a p2p-over-lan circuit and the source
IP address 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.
4.4 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 Proxy ARP is not enabled
on the circuit. 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.
3. ARP for reference IP address. When a point-to-point link is
configured as unnumbered, the router usually associates with
it a "reference IP address", that is used as the source IP
address in the packets originated for the unnumbered
interface. When such an address is known to a router, the
router may announce its MAC address by sending a gratuitous
ARP message. This solution will also help in the situations
where routers calculate the next-hop addresses for the routes
through point-to-point interfaces. Since the source IP address
in the received routing protocol packet is used as the next-
hop address in the route, forwarding an IP packet along such
a route will lead to an ARP request submission on the LAN
link that will be answered by the remote device.
4. Broadcast/multicast/proprietary.
Shen, Zinin, et al Expires February 2002 [Page 5]
INTERNET DRAFT P2P OVER LAN August 2001
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
mis-configuration.
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.
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 MAY also support
one of the above listed methods for mapping ip addresses on the
link to MAC address, and MUST support proxy ARP on the link. 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
There is obvious 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
overhead. The scalability impact is less of a concern if all the
vLANs are within a single OSPF area or ISIS level.
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.
Shen, Zinin, et al Expires February 2002 [Page 6]
INTERNET DRAFT P2P OVER LAN August 2001
7. 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 interface address 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.
8. Acknowledgments
The authors would like to acknowledge the following individuals:
(in last name alphabetical order) Pedro Marques, Christian Martin,
Danny McPherson, Ajay Patel, Tony Przygienda and Alvaro Retana.
9. 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.
10. 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
Shen, Zinin, et al Expires February 2002 [Page 7]
INTERNET DRAFT P2P OVER LAN August 2001
Jenny Yuan
Redback Networks
350 Holger Way
San Jose, CA, 95134 USA
jenny@redback.com
Alex Zinin
Nexsi Systems
1959 Concourse Drive
San Jose, CA 95131
azinin@nexsi.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
e-mail: riw@cisco.com
Stefano Previdi
Cisco Systems, Inc.
De Kleetlaan 6A
1831 Diegem - Belgium
email: sprevidi@cisco.com
Shen, Zinin, et al Expires February 2002 [Page 8]
