這篇是在寫NHRP提供的路由捷徑，NHRP是 Next Hop Resolution Protocol 的縮寫。重要觀念是必須先有DMVPN，才能實現NHRP，也就是說NHRP是依附於DMVPN之上的。
NHRP經常用來提升NBMA網路效率，卻也有致命性的缺點，目前還無法基於NHRP以優化群組廣播。NHRP能講的東西也滿多的，在這篇文章我只寫NHRP的捷徑路由就好。
Figure 1-1
Example 1-1：R1、R2、R3、R4配置如下，這是一個DMVPN標準配置。
Example 1-2：DMVPN配置完後，在R1「show dmvpn」能看到spoke的註冊。
Example 1-3：從R3路由表可以看到它學到1.1.1.1 / 32網路、4.4.4.4 / 32網路。
在初始配置完成後，R3追蹤4.4.4.4，會先走10.0.0.1(R1-Hub)，才走10.0.0.4(R4-Spoke2)，然後「sh ip nh sho」的輸出中會看到儲存了一筆NHRP資訊。
再追蹤4.4.4.4一次，因為已經儲存NHRP資訊，所以會直接走10.0.0.4(R4-Spoke2)，這就是NHRP捷徑的特色。
當然，如果在R1沒有做「ip nhrp redirect」、或在R3、R4沒有做「ip nhrp shortcut」，當流量從R3到R4，必須經過NHRP hub(R1)。
Example 1-4
另外可以用「clear ip nhrp shotcut」來清除NHRP捷徑資訊，不過這是清除全部NHRP捷徑資訊，也可以僅清除特定NHRP捷徑資訊。
It is NHRP provides routing shortcut. NHRP is short for Next Hop Resolution Protocol. Have DMVPN is the first, next implement NHRP. It is important ideas. In other words, NHRP is DMVPN-based.
NHRP is often used to improve NBMA network efficiency. But also have deadly shortcoming. NHRP-based can't to optimize multicast currently. NHRP is very wide range, I will write NHRP routing shortcut only.
Figure 1-1
Example 1-1：R1, R2, R3 and R4 configuration are as follow, this is standard DMVPN configuration.
Example 1-2：After implement DMVPN configuration. The command 「show dmvpn」 in R1, you can see spoke are registered.
Example 1-4：You can see R3 has learned network. There are 1.1.1.1 / 32、4.4.4.4 / 32 from routing table.
After initial configuration. R3 traceroute 4.4.4.4 , first hop is 10.0.0.1(R1-Hub), next is 10.0.0.4(R4-Spoke2). And output of 「sh ip nh sho」 can see that stored a NHRP information.
R3 traceroute 4.4.4.4 again. Because R3 has stored NHRP information, then packets were toward 10.0.0.4(R4-Spoke2). It is NHRP shortcut features.
Of course, if R1 has not input 「ip nhrp redirect」, or R3, R4 has not input 「ip nhrp shortcut」. From R3 to R4 when traffic produced, packets are must pass NHRP hub(R1).
Example 1-4
And command 「clear ip nhrp shortcut」, it can clear NHRP shortcut information. But all NHRP information will cleared. Also you can clear particular NHRP information.
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