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Distance vector routing loop issues

Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.
如果在改變中的網路收歛緩慢，將導致不一致的路由表格未被更新，在此情形下，路由迴圈就可能發生。

An example is as follows:
請看下面的例子：
1. Just before the failure of Network 1, all routers have consistent knowledge and correct routing tables. The network is said to have converged. For Router C, the preferred path to Network 1 is by way of Router B, and the distance from Router C to Network 1 is 3.
1. 就在網路1發生問題之前，所有路由器都有一致的知識和正確的路由表格，此網路被稱為已收歛狀態。對路由器C而言，到網路1的優先路徑是經由路由器B，且從路由器C到網路1的距離是3。

2. When Network 1 fails, Router E sends an update to Router A. Router A stops routing packets to Network 1, but Routers B, C, and D continue to do so because they have not yet been informed of the failure. When Router A sends out its update, Routers B and D stop routing to Network 1. However, Router C has not received an update. For Router C, Network 1 can still be reached through Router B.
2. 當網路1發生問題，路由器E發送一個更新給路由器A。路由器A停止繞送封包給網路1，但是路由器B、C和D仍然持續繞送，因為他們尚未被通知此問題。當路由器A發送出它的更新，路由器B和D停止繞送到網路1。然而路由器C沒有收到更新，對路由器C而言，網路1仍然可經由路由器B到達。

3. Now Router C sends a periodic update to Router D, which indicates a path to Network 1 by way of Router B. Router D changes its routing table to reflect this incorrect information, and sends the information to Router A. Router A sends the information to Routers B and E, and the process continues. Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.
3. 現在路由器C發送出定期更新給路由器D，它指出到網路1可經由路由器B。路由器D改變它的路由表格以反應此錯誤的資訊，並且將資訊發送到路由器A。路由器A發送資訊給路由器B和E，這個程序持續地進行。任何通往網路1的封包現在將從路由器C到B到A到D，然後又回到C，於是形成一個迴圈。

Packet tracer 5.1

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The information from CISCO
http://www.cisco.com/web/learning/netacad/course_catalog/PacketTracer.html

Basic static routing

CONFIG HOST





Host A
C:>ipconfig /ip 192.168.1.2 255.255.255.0 //Ip Address
C:>ipconfig /ip 192.168.1.1 //defoult gateway



Host B
C:>ipconfig /ip 192.168.3.2 255.255.255.0 //Ip Address
C:>ipconfig /dg 192.168.3.1 //default gateway


CONFIG ROUTER



Router 1
Router>enable
Router #config t
Router(config)#hostname R1
R1(config)#int e0 //entry ethernet 0 port
R1(config-if)#ip address 192.168.1.1 255.255.255.0 //ethernet 0 ip
R1(config-if)#no shutdown //ethernet 0 port
R1(config-if)#exit
R1(config)#int s0/0 //entry serial0/0
R1(config-if)#ip address 192.168.2.1 255.255.255.0 //serial0/0 ip
R1(config-if)#clock rate 64000 //DCE
R1(config-if)#no shutdown //serial0/0 port
R1(config-if)#exit



Router 2
Router>enable
Router#config t
Router(config)# hostname R2
R2(config)#int e0
R2(config-if)#ip address 192.168.3.1 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#int s0/0
R2(config-if)# ip address 192.168.2.2 255.255.255.0
R2(config-if)#no shutdown
R2(config)#exit





STATIC ROUTING


R1(config)#ip route 192.168.3.0 255.255.255.0 192.168.2.2
R1(config)#exit


R2(config)#ip route 192.168.1.0 255.255.255.0 192.168.2.1
R2(config)#exit



You can "show ip route"

Look up routing table