此实验 是为了验证ORF（出站路由过滤）
R1配置：
Loopback0 1.1.1.1
Loopback1 192.168.1.1/24
Loopback2 192.168.2.1/24
Loopback3 192.168.3.1/24
Loopback4 192.168.4.1/24
E1/0 10.1.12.1
R2配置：
Loopback 2.2.2.2
E1/0 10.1.12.2
配置IGP（RIP）保持连通性：
R1配置：
router rip
version 2
network 1.0.0.0
network 10.0.0.0
network 192.168.1.0
network 192.168.2.0
network 192.168.3.0
network 192.168.4.0
no auto-summary
R2配置：
router rip
version 2
network 2.0.0.0
network 10.0.0.0
no auto-summary
开始配置BGP：
R1配置：
router bgp 1
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.255
network 10.1.12.0 mask 255.255.255.0
network 192.168.1.0
network 192.168.2.0
network 192.168.3.0
network 192.168.4.0
neighbor 2.2.2.2 remote-as 2
neighbor 2.2.2.2 ebgp-multihop 2
neighbor 2.2.2.2 update-source Loopback0
R2配置：
router bgp 2
bgp router-id 2.2.2.2
bgp log-neighbor-changes
network 2.2.2.2 mask 255.255.255.255 backdoor
network 10.1.12.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 1
neighbor 1.1.1.1 ebgp-multihop 2
neighbor 1.1.1.1 update-source Loopback0
BGP 配置完成，这时可以在R2上看到R1通告的192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24 四条路由。
R2#sh ip bgp
BGP table version is 6, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
r> 10.1.12.0/24 1.1.1.1 0 0 1 i
*> 192.168.1.0 1.1.1.1 0 0 1 i
*> 192.168.2.0 1.1.1.1 0 0 1 i
*> 192.168.3.0 1.1.1.1 0 0 1 i
*> 192.168.4.0 1.1.1.1 0 0 1 i
现在开始在R2上配置ORF过滤掉192.168.1.0/24 192.168.3.0/24，使R2只接收192.168.2.0/24 192.168.4.0/24两条路由
首先配置prefix-list列表：
ip prefix-list 1 seq 5 permit 192.168.2.0/24
ip prefix-list 1 seq 10 permit 192.168.4.0/24
之后在R1/R2上追加配置：
router bgp 1
neighbor 2.2.2.2 capability orf prefix-list receive
*Jul 12 14:44:55.843: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Down Capability changed
*Jul 12 14:44:55.843: %BGP_SESSION-5-ADJCHANGE: neighbor 2.2.2.2 IPv4 Unicast topology base removed from session Capability changed
*Jul 12 14:44:56.271: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Up
配置完成后，会提示 capability changed。
router bgp 2
nei 1.1.1.1 capability orf prefix-list both
nei 1.1.1.1 prefix-list 1 in
*Jul 12 14:35:27.863: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Down Capability changed
*Jul 12 14:35:27.863: %BGP_SESSION-5-ADJCHANGE: neighbor 1.1.1.1 IPv4 Unicast topology base removed from session Capability changed
*Jul 12 14:35:28.823: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up
配置完成后，会提示 capability changed。
R2上的配置：
命令neighbor 1.1.1.1 capability orf prefix-list both 的作用是：激活R2与R1 BGP邻居之间的ORF功能，表明R1与R2 “交换”prefix-list。
命令nei 1.1.1.1 prefix-list 1 in的作用是：R2只接收路由前缀192.168.2.0/24 192.168.4.0/24这两条路由，拒绝其他所有路由。
命令clear ip bgp 1.1.1.1 in prefix-filter 的作用是：迫使R2把本路由器设置的入站方向的prefix-list推送给R1。
R1上的配置：
命令neighbor 2.2.2.2 capability orf prefix-list receive的作用是：接受R2推送过来关于ORF的相关路由过滤设置。
至此，配置完成，可在R1上查看效果：
R1#sh ip bgp neighbors 2.2.2.2 advertised-routes
BGP table version is 6, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 192.168.2.0 0.0.0.0 0 32768 i
*> 192.168.4.0 0.0.0.0 0 32768 i
Total number of prefixes 2
在R1上能看到R1只向R2发送路由192.168.2.0/24 192.168.4.0/24这两条路由，其他的路由都被拒绝，证明R2的配置正确，并且R1收到了R2推送过来的ORF过滤路由配置。
R2#sh ip bgp
BGP table version is 9, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.1.12.0/24 0.0.0.0 0 32768 i
*> 192.168.2.0 1.1.1.1 0 0 1 i
*> 192.168.4.0 1.1.1.1 0 0 1 i
同样的，在R2上也能看到效果，从R1只收到permit的两条路由前缀。
所以实际上ORF是借助prefix-list去实现路由过滤的一个特性，在本地将prefix-list推送给对端，让对端来执行路由前缀的过滤。
