當(dāng)鏈路發(fā)生故障后燕锥，可以依靠路由協(xié)議重新收斂，計算出新的路徑轉(zhuǎn)發(fā)數(shù)據(jù)報文托慨，這種方式通常會花費很長的時間暇榴，在幾百毫秒以上，已經(jīng)滿足不了網(wǎng)絡(luò)延時要求特別高的應(yīng)用婆硬〖槔基于RFC5286的LFA FRR（Loop-Free Alternate Fast Reroute）技術(shù)，借助鏈路狀態(tài)協(xié)議的特性查吊，在鏈路故障前就計算出備份路徑，在鏈路故障時不用等待重新路由協(xié)議收斂就可以快速通過備份路徑轉(zhuǎn)發(fā)流量卢佣，能夠保證倒換延時在50毫秒以內(nèi)箭阶。
類型一：Link Protection
當(dāng)滿足不等式D (N，D) < D (N嘹叫，S) + D (S，D)時婆芦，可以形成鏈路級保護喂饥，以前綴10.4.4.4/32為例，主路徑是R1->R4或粮，四臺路由器基本配置：
R1配置
!
interface Loopback1
ip address 10.1.1.1 255.255.255.255
!
router ospf 1
fast-reroute per-prefix enable area 0 prefix-priority high
fast-reroute keep-all-paths
network 10.1.1.1 0.0.0.0 area 0
network 10.10.12.1 0.0.0.0 area 0
network 10.10.13.1 0.0.0.0 area 0
network 10.10.14.1 0.0.0.0 area 0
!
R2配置
interface Loopback1
ip address 10.2.2.2 255.255.255.255
end
!
router ospf 1
network 10.2.2.2 0.0.0.0 area 0
network 10.10.12.2 0.0.0.0 area 0
network 10.10.23.2 0.0.0.0 area 0
network 10.10.24.2 0.0.0.0 area 0
!
R3配置
interface Loopback1
ip address 10.3.3.3 255.255.255.255
!
router ospf 1
network 10.3.3.3 0.0.0.0 area 0
network 10.10.13.3 0.0.0.0 area 0
network 10.10.23.3 0.0.0.0 area 0
network 10.10.34.3 0.0.0.0 area 0
!
R4配置
interface Loopback1
ip address 10.4.4.4 255.255.255.255
!
router ospf 1
network 10.4.4.4 0.0.0.0 area 0
network 10.10.14.4 0.0.0.0 area 0
network 10.10.24.4 0.0.0.0 area 0
network 10.10.34.4 0.0.0.0 area 0
!
** R1在計算鏈路R1–R4的備份路徑的時候捞高，會根據(jù)下面的不等式去計算
D (N，D) < D (N氢哮，S) + D (S型檀，D)，不等式標識的含義如下表**
對于上面這個例子：
- R2 ：10 < 10 + 10 ——> 滿足不等式
- R3 ：20 < 10 + 10 ——> 不滿足不等式
在路由器R1上面查看備份路徑生闲，由于R3不滿足不等式關(guān)系月幌，所以下一跳是R2悬蔽，備份路徑是R1–R2
Flags: RIB, Repair, IntfDj, BcastDj, NodeProt, Downstr
- IntfDj : This shows that repair path used a different interface (Eth0/0) as compared to primary path (Eth1/0).
- BcastDj : This shows that repair path used a different broadcast interface (Eth0/0) as compared to primary path (Eth1/0).
- LC Dj: This flag shows that repair path used a different linecard (Eth0/0, module 0) as compared to primary path (Eth1/0, module 1).
本實驗使用的是CSR1000V模擬器录语，所以沒有LC Dj標記
R1#show version
Cisco IOS XE Software, Version 03.13.00.S – Extended Support Release
Cisco IOS Software, CSR1000V Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 15.4(3)S, RELEASE SOFTWARE (fc11)
Technical Support:
[http://www.cisco.com/techsupport](http://www.cisco.com/techsupport)
Copyright (c) 1986-2014 by Cisco Systems, Inc.
Compiled Mon 28-Jul-14 00:46 by mcpre
類型二：Node Protection
當(dāng)滿足不等式D(N禾乘，D) < D(N，E) + D(E蒲稳，D)時，可以形成節(jié)點級保護
以前綴10.3.3.3/32為例江耀，主路徑是R1->R4->R3祥国，四臺路由器基本配置：
R1配置
!
interface Loopback1
ip address 10.1.1.1 255.255.255.255
!
router ospf 1
fast-reroute per-prefix enable area 0 prefix-priority high
fast-reroute keep-all-paths
network 10.1.1.1 0.0.0.0 area 0
network 10.10.12.1 0.0.0.0 area 0
network 10.10.14.1 0.0.0.0 area 0
!
R2配置
interface Loopback1
ip address 10.2.2.2 255.255.255.255
end
!
router ospf 1
network 10.2.2.2 0.0.0.0 area 0
network 10.10.12.2 0.0.0.0 area 0
network 10.10.23.2 0.0.0.0 area 0
network 10.10.24.2 0.0.0.0 area 0
!
R3配置
interface Loopback1
ip address 10.3.3.3 255.255.255.255
!
router ospf 1
network 10.3.3.3 0.0.0.0 area 0
network 10.10.23.3 0.0.0.0 area 0
network 10.10.34.3 0.0.0.0 area 0
!
R4配置
interface Loopback1
ip address 10.4.4.4 255.255.255.255
!
router ospf 1
network 10.4.4.4 0.0.0.0 area 0
network 10.10.14.4 0.0.0.0 area 0
network 10.10.24.4 0.0.0.0 area 0
network 10.10.34.4 0.0.0.0 area 0
!
R1在計算備份節(jié)點的時候，會根據(jù)不等式去計算D(N啊犬，D) < D(N壁查，E) + D(E，D)
R2 ：10 < 10 + 15 ——> 滿足不等式
在路由器R1上面查看備份路徑康谆，R2為備份下一跳嫉到，備份路徑是R1–R2，并且形成節(jié)點級保護
Flags: RIB, Repair, IntfDj, BcastDj, NodeProt, Downstr
NodeProt : This flag shows that R2 provides node protection against failure of primary next hop R4.
Downstr : This flag shows that R2 is closer to destination than local router R1.
