link is also actively being used to carry user data.
This memo addresses all the above problems.
The approach taken is to modify the routing protocols so as to send
information on the WAN only when there has been an update to the
routing database OR a change in the reachability of a next hop router
is indicated by the task which manages connections on the WAN.
Because datagrams are not guaranteed to get through on all WAN media,
an acknowledgement and retransmission system is required to provide
reliability.
This memo describes the modifications required for Bellman-Ford (or
distance vector) algorithm information broadcasting protocols, such
as IP RIP [1,2] or Netware RIP and SAP [3] on the WAN. The protocols
run unmodified on Local Area Networks (LANs) or fixed point-to-point
links, and so interoperate transparently with implementations
adhering to the original specifications.
2. Running Routing Protocols on the WAN
2.1 Overview
Multiprotocol routers are used on connection oriented Wide Area
Networks (WANs), such as X.25 packet switched networks and ISDN
networks, to interconnect LANs. By using the multiplexing properties
of the underlying WAN technology, several LANs can be interconnected
simultaneously through a single physical interface on the router.
A circuit manager provides an interface between the connectionless
network layers (IP, IPX, CLNP etc) and the connection oriented WAN
(X.25 or ISDN). Figure 1 shows a schematic representative stack
showing the relationship between routing protocols, the network
layers, the circuit manager and the connection oriented WAN.
-------------- --------- ---------
RIPRIPSAP
-------------- --------- ---------
--------------
UDP
--------------
-------------- ----------------
IPIPX
-------------- ----------------
-------------------------------------------
Circuit Manager
-------------------------------------------
---------------------------
Connection Oriented
WAN stack
---------------------------
A WAN circuit manager will support a variety of network layer
protocols, on its upper interface. On its lower interface, it
may support one or more subnetworks. A subnetwork may support a
number of Virtual Circuits.
Figure 1. Representative Multiprotocol Router stack
The router has a translation table which relates the network layer
address of the next hop router to the physical address used to
establish a Virtual Circuit (VC) to it. Datagrams may be
encapsulated in a header to distinguish the network layer protocol
[5].
The circuit manager takes datagrams from the connectionless network
layer protocols and (if one is not currently available) opens a VC to
the next hop router. A VC can carry all traffic between two end-
point routers for a given network layer protocol (or with appropriate
encapsulation all network layer protocols). An idle timer is used to
close the VC when the datagrams stop arriving at the circuit manager.
Running routing protocols on the WAN has traditionally consisted of
making small modifications to the methods used on LANs. Where
routing information would be broadcast periodically on a LAN
interface, it is converted to a series of periodic updates sent to a
list of addresses on the WAN.
This memo targets two areas:
o Eliminating the overkill inherent in periodic transmission of
routing updates.
o Overcoming the bandwidth limitations on the WAN: the number of
simultaneous VCs to next hop routers and restricted data
throughput which the WAN link can support.
The first of these is overcome by transmitting routing updates
推荐阅读
- 1 路由信息协议的概述
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- TRIP协议描述
- rip协议理解
- 小结 配置RIP和IGRP协议
- 2 RIP协议的基本配置
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