Although VPN is a distance vector routing protocol, it adopts a non-periodic and partial update method during the update process, rip and IGRP use periodic updates to test the neighbor status. Therefore, it is necessary to use a method to verify the existence of the neighbor. The "hello" packet is used in the network. It is sent every five seconds. Because the time is much less than 30 seconds of RIP and 90 seconds of IGRP, the speed of convergence of the network is much faster. To prevent global synchronization, there is a very small random latency, so this value is not an absolute 5 seconds. The Hello packet is sent in multicast 224.0.0.10 mode. In multi-point (Multipoint) environments, such as X.25, frame-relay, and ATM networks, the link bandwidth is lower or lower than the T1 speed. In this environment, it is Unicast (unicast) sent in 60 seconds. Hello packets do not need to be answered. We can change the default time period by using IP hello-interval, and the kernel port is used on the interface. When we do not receive the hello packet sent by our neighbor within a certain period of time, we think that the neighbor does not exist. This time is called the holddown timer, which is usually set to three times the Hello time. The value on the broadcast link is 15 seconds, and on the nbma link is 180 seconds. Of course, you can also use the IP hold-Time Command to adjust it. We can observe the following results when using the show ip vpn command: R2 # SH ip EI nei
IP-EIGRP neighbors for process 1.
H address interface hold uptime srtt rto q seq type
(Sec) (MS) CNT num
1 10.1.1.1 et0/0 14 00:00:23 100 600 0 13
0 172.16.0.3 ET1/0 14 00:00:43 140 840 0 11 There are several new concepts in the above example: SRTT, RTO, Q, seq. A student of NA once asked me this question, but this is actually mentioned in IE. The average return time of SRTT (smooth round-trip time) refers to the average time spent by the router to send the VPN packet to the neighbor and to receive the confirmation of the message from the neighbor, in milliseconds (MS. RTO (Retransmission timeout) re-transmission times out. When the message is sent out in multicast mode, the ACK response from the neighbor is not received after the RTO time period, the next packet will be re-transmitted in the unicast (unicast) mode, so it is called the Retransmission timeout time. Q (queue count) indicates the number of queues. When an update, query, or response message is sent to a neighbor, copies of the same packet will be placed in the queue, after RTO times out, the copy will be sent if the response is not received by the other party. SEQ (sequence number) number, which records the sequence number of the last update, query, or response message received from the neighbor.
