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ICMP Router Discovery Protocol
The Cisco IOS software provides a third method, called router discovery, by which the router dynamically learns about routes to other networks using the ICMP Router Discovery Protocol (IRDP). IRDP allows hosts to locate routers. When the device operates as a client, router discovery packets are generated. When the device operates as a host, router discovery packets are received. Our IRDP implementation fully conforms to the router discovery protocol outlined in RFC 1256.
The software is also capable of wire-tapping Routing Information Protocol and Interior Gateway Routing Protocol (IGRP) routing updates and inferring the location of routers from those updates. The server/client implementation of router discovery does not actually examine or store the full routing tables sent by routing devices, it merely keeps track of which systems are sending such data.
You can configure the four protocols in any combination. We recommend that you use IRDP when possible because it allows each router to specify both a priority and the time after which a device should be assumed down if no further packets are received. Devices discovered using IGRP are assigned an arbitrary priority of 60. Devices discovered through RIP are assigned a priority of 50. For IGRP and RIP, the software attempts to measure the time between updates, and assumes that the device is down if no updates are received for 2.5 times that interval.
Each device discovered becomes a candidate for the default router. The list of candidates is scanned and a new highest-priority router is selected when any of the following events occur:
When a higher-priority router is discovered (the list of routers is polled at 5-minute intervals).
When the current default router is declared down.
When a TCP connection is about to time out because of excessive retransmissions. In this case, the server flushes the ARP cache and the ICMP redirect cache, and picks a new default router in an attempt to find a successful route to the destination.
Enabling IRDP Processing
The only required task for configuring IRDP routing on a specified interface is to enable IRDP processing on an interface. Use the following command in interface configuration mode:
Command Purpose
ip irdp
Enable IRDP processing on an interface.
Change IRDP Parameters
When you enable IRDP processing, the default parameters will apply. To optionally change any of these IRDP parameters, use the following commands in interface configuration mode:
Command Purpose
ip irdp multicast
Send IRDP advertisements to the all-systems multicast address (224.0.0.1) on a specified interface.
ip irdp holdtime seconds
Set the IRDP period for which advertisements are valid.
ip irdp maxadvertinterval seconds
Set the IRDP maximum interval between advertisements.
ip irdp minadvertinterval seconds
Set the IRDP minimum interval between advertisements.
ip irdp preference number
Set the IRDP preference level of the device.
ip irdp address address [number]
Specify an IRDP address and preference to proxy-advertise.
The Cisco IOS software can proxy-advertise other machines that use IRDP; however, this is not recommended because it is possible to advertise nonexistent machines or machines that are down.
http://www.cisco.com/warp/public/619/hsrpguide1.html |
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楼主: zero530
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发表于 2002-1-24 14:59
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