11.5 Distance Vector Routing(距离矢量路由协议介绍-理论).

ICND v2.0—5-2
2
Objectives
Upon completing this lesson, you will be able to:
• Describe（描述） the features offered（提供的功 能） by distance vector routing protocols and give examples of each • Describe the issues associated（引发的问题） with distance vector routing and identify solutions to those issues （这些问题的解决方案）
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-9
Inconsistent Routing Entries (Cont.)
• Router C concludes（推断） that the best path to network 10.4.0.0 is through router B.
© 2002, Cisco Systems, Inc. All rights reserved.
Distance Vector Routing
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© 2002, Cisco Systems, Inc. All rights reserved.
ICND v2.0—5-3
Distance Vector Routing Protocols
距离：多远 矢量：哪个方向
• Routers pass periodic copies of routing table to neighbor routers and accumulate distance vectors.
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-7
Inconsistent Routing Entries
距离矢量路由协议会出现 路由条目不一致的情况
路由条目不一致
• Each node maintains the distance from itself to each possible destination network. • 每一个节点自行维护它到每一个可能的目标网络的距离
ICND v2.0—5-12
Defining a Maximum 规定一个最大的数
• Define a limit on the number of hops to prevent infinite loops. 定义一个跳数的上限防止无穷大的环路
© 2002, Cisco Systems, Inc. All rights reserved.
ICND v2.0—5-13
Routing Loops 路由环路
去往10.4.0.0的数据包
• Packets for network 10.4.0.0 bounce (loop) between routers B and C.
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-14
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-10
Inconsistent Routing Entries (Cont.)
Baidu Nhomakorabea
• Router A updates its table to reflect the new but erroneous（不正确的） hop count.
累计
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-4
周期性地（RIP：30S）
Sources of Information and Discovering Routes 信息来源和路由获取
• Routers discover the best path to destinations from each neighbor.
Split Horizon 水平分割
• It is never useful to send information about a route back in the direction from which the original information came.
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-5
Selecting the Best Route with Metrics度量值
© 2002, Cisco Systems, Inc. All rights reserved.
ICND v2.0—5-6
Maintaining维护 Routing Information
路由器的更新进程更新路由表
路由表更新
拓扑变化引起路由表更新
路由器A在下一个更新周期 发出路由表更新信息
• Updates proceed step-by-step from router to router.
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-8
Inconsistent Routing Entries (Cont.)
• Slow convergence produces inconsistent routing.
• 慢的收敛会引起路由条目的不一致
© 2002, Cisco Systems, Inc. All rights reserved. ICND v2.0—5-11
Count to Infinity 距离运算到无穷大
• Hop count for network 10.4.0.0 counts to infinity.
© 2002, Cisco Systems, Inc. All rights reserved.