another next hop router) while the datagram is in a queue,
resulting in the queue being flushed and the datagram
discarded.
o In cases where the method of transport is not guaranteed, for
example with PPP where there is no acknowledgement and
retransmission of HDLC frames, a corrupted frame will result in
the loss of the datagram.
2.5 Guaranteeing delivery of Routing Updates
To guarantee delivery of routing updates on the WAN an
acknowledgement and retransmission scheme MUST be used:
o Send a routing update to a next hop router on the WAN.
o The other router responds with an acknowledgement packet.
The original router receives the acknowledgement.
o Otherwise the original router retransmits the update until an
acknowledgement is received.
Retransmission timer values are covered in section 7.
In cases where the routing database is modified before an
acknowledgement is received a new routing update with an
updated sequence number is sent out. If an acknowledgement for
the old routing update is received it is ignored.
o A router only updates its routing database when it receives a
complete update, which may consist of several fragments. Each
fragment is individually acknowledged.
The above mechanism caters for cases where the datagram is lost
because of a frame error or is discarded because of an over-full
queue. The routing update and acknowledgement will eventually both
get through.
In cases where the circuit manager cannot establish a connection, a
mechanism is provided to allow the circuit manager to inform the
routing task of the failure to make a connection so that it can
suppress retransmissions until a circuit becomes available.
2.6 The Routing Database
A requirement of using triggered updates for propagating routing
information is that NO routing information ever gets LOST or
DISCARDED.
The routing database MUST adopt one of the following strategies:
o It must keep ALL alternative routing information it learns from
any routing updates from the LAN and the WAN, so that if the
best route disappears an alternative route (if available) can
replace it as the new best route.
o If the amount of memory this consumes is problematic the routing
application must keep SOME alternative routing information - say
a best route and two alternatives.
If the router ever has to discard routing information about a
route it should note the fact. If the routes that have been
kept disappear because they have become unreachable, the router
MUST issue a request on all interfaces to try and obtain
discarded alternatives.
It is recommended that the request is issued BEFORE all routes
to a destination have been lost.
Entries in the routing database can either be permanent or temporary.
Entries learned from broadcasts on LANs are temporary. They will
expire if not periodically refreshed by further broadcasts.
Entries learned from a triggered response on the WAN are "permanent".
They MUST not time out in the normal course of events. The entries
state MUST be changed to "temporary" by the following events:
o The arrival of a routing update containing the entry set to
unreachable.
The normal hold down timer MUST be started, after which the
entry disappears from the routing database.
o The arrival of a routing update with the entry absent.
If the hold down timer is not already running, the entry MUST be
set to unreachable and the hold down timer started.
o A message sent from the circuit manager, to indicate that it
failed to make a connection in normal running.
The routing table MUST be scanned for all routes via that next
hop router. Aging of these routing entries MUST commence. If
推荐阅读
- 1 路由信息协议的概述
- RIP协议－中兴
- 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
- 2 使用RIP协议处理不连续的子网和VLSM
- 1 RIP协议的基本配置
- 如何将RIP、HELLO和EGP组合起来
- RIP 详细解析选路信息协议