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Neighbor Acquisition 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 Acquisition Protocol。

作者:【美】Jeff Doyle(杰夫 多伊尔)来源:人民邮电出版社|2017-11-05 18:15

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Neighbor Acquisition Protocol

Before EGP neighbors can exchange reachability information, they must establish that they are compatible. This function is performed by a simple two-way handshake in which one neighbor sends a Neighbor Acquisition Request message , and the other neighbor responds with a Neighbor Acquisition Confirm message .

None of the RFCs specify how two EGP neighbors initially discover each other. In practice,an EGP gateway learns of its neighbor by manual configuration of the neighbor’sIP address. The gateway then unicasts an Acquisition Request message to the configured neighbor. The message states a Hello interval, the minimum interval between Hello messages that the gateway is willing to accept from the neighbor, and a Poll interval, the minimum interval that the gateway is willing to be polled by the neighbor for routing updates. The neighbor’s responding Acquisition Confirm message contains its own values for the same two intervals. If the neighbors agree on the values, they are ready to exchange network reachability information.

When a gateway first learns of a neighbor, it considers the neighbor to be in the Idle state. Before sending the first Acquisition Request, the gateway transitions the neighbor to the Acquire state; when the gateway receives an Acquisition Confirm, it transitions the neighbor to the Down state.

A gateway can refuse to accept a neighbor by responding with a Neighbor Acquisition Refuse message rather than an Acquisition Confirm message. The Refuse message can include a reason for the refusal, such as a lack of table space, or it can refuse for an unspecified reason.

A gateway can also break an established neighbor relationship by sending a Neighbor Cease message . As with the Refuse message , the originating gateway has the option of including a reason for the Cease or leaving the reason unspecified. A neighbor receiving a Neighbor Cease message responds with a Neighbor Cease Acknowledgment .

The last case of a Neighbor Acquisition procedure is a case in which a gateway sends an Acquisition Request but the neighbor does not respond. RFC 888 suggests retransmitting the Acquisition message “at a reasonable rate, perhaps every 30 seconds or so.” The Cisco now-defunct EGP implementation does not just repeat unacknowledged messages over a constant period. Rather, it retransmits an unacknowledged Acquisition message 30 seconds after the original transmission. It then waits 60 seconds before the next transmission. If no response is received within 30 seconds of the third transmission, the gateway transitions the neighbor state from Acquire to Idle (see Example 1-1). The gateway remains in the Idle state for 300 seconds (5 minutes) and then transitions to Acquire and starts the process all over.

Note The EGP examples shown throughout these sections are from IOS 12.1.

Example 1-1 debug ip egp transactions Command Output Displays EGP State Transitions

Notice in Example 1-1 that each EGP message has a sequence number. The sequence number allows EGP message pairs (such as Neighbor Acquisition Request/Confirm, Request/Refusal, and Cease/Cease-Ack pairs) to be identified. The next section details how the sequence numbers are used.

When two EGP gateways become neighbors, one is the active neighbor and one is the passive neighbor. Active gateways always initiate the neighbor relationship by sending Neighbor Acquisition Requests. Passive gateways do not send Acquisition Requests; they only respond to them. The same is true for Hello/I-Heard-You message pairs, described in the following section: The active neighbor sends the Hello, and the passive neighbor responds with an I-Heard-You (I-H-U). A passive gateway can initiate a Neighbor Cease message, however, to which the active gateway must reply with a Cease Acknowledgment message.

A core gateway, which can be a neighbor of routers in several other autonomous systems, might be the active gateway of one neighbor adjacency and the passive gateway of another neighbor adjacency. The Cisco EGP implementation uses the AS numbers as the determining factor: The neighbor whose AS number is lower will be the active neighbor.


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