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Neighbor Reachability Protocol

《TCP IP路由技术(第2卷)(第2版)英文版》本书是有关Cisco外部路由协议和高级IP路由主题的权威指南,是Cisco路由与交换领域实属罕见的经典著作。本书在上一版的基础上进行了全面更新,其可读性、广度和深度相较于上一版有了相当大的改进。本节为大家介绍Neighbor Reachability Protocol。

作者:【美】Jeff Doyle(杰夫 多伊尔)来源:人民邮电出版社|2017-11-05 18:17

【新品产上线啦】51CTO播客,随时随地,碎片化学习

Neighbor Reachability Protocol

After a gateway has acquired a neighbor, it maintains the neighbor relationship by sending periodic Hello messages. The neighbor responds to each Hello with an I-H-U message. RFC 904 does not specify a standard period between Hellos; IOS uses a default period of 60 seconds, which can be changed with the command timers egp.

When three Hello/I-H-U message pairs have been exchanged, the neighbor state changes from Down to Up (see Example 1-2). The neighbors can then exchange network reachability information, as described in the next section.

Example 1-2 debug ip egp transactions Command Output Displays a Two-Way Handshake Success and Resulting EGP State Transitions

If an active neighbor sends three sequential messages without receiving a response, the neighbor state transitions to Down. The gateway sends three more Hellos at the normal Hello interval; if there is still no response, the state changes to Cease. The gateway sends three Neighbor Cease messages at 60-second intervals. If the neighbor responds to any of the messages with a Cease Acknowledgment, or does not respond at all, the gateway transitions the neighbor state to Idle and waits 5 minutes before transitioning back to Acquire and attempting to reacquire the neighbor. Example 1-3 shows this sequence of events.

Example 1-3 The Neighbor at 192.168.16.2 Has Stopped Responding. The Interval Between Each of the Unacknowledged EGP Messages Is 60 Seconds

Example 1-4 shows another example of a dead neighbor, except this time a core gateway (192.168.16.2) in the passive mode is discovering the dead neighbor (192.168.16.1).

Example 1-4 Neighbor 192.168.16.1 Has Stopped Responding. The Debug Messages Are Taken from 192.168.16.2, a Gateway in Passive Mode

When the gateway does not receive a Hello within the 60-second Hello interval, it tries to “wake up” its neighbor. Because a gateway in passive mode cannot send Hellos, it sends a Poll message. The gateway then waits for one Poll interval. (The IOS default Poll interval is 180 seconds, or 3 minutes.) If no response is received, it sends another Poll and waits another Poll interval. If there still is no response, the gateway changes the neighbor state to Down and then immediately to Cease. As in Example 1-3, three Cease messages are sent and the neighbor state is changed to Idle.


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