一、实验要求：
1、AS1去AS2走R2
2、AS2去AS1走R4
3、AS1去AS3走R6
4、AS2去AS3走R6
5、AS3去AS2走R5
6、AS3去AS1走R5
7、每个选路规则只能使用一次
解决以上步骤的选路方法（事先没有规划）：
1、local-preference
2、MED
3、Weight
4、Origin
5、AS-path
6、next-hop
二、BGP的选路规则：
1、weight，范围0-65535，默认为0或32768，大优；
2、Local-preference，范围0-65535，大优；
3、本地起源，起源与本地最优先；
4、As-path，经过AS的数量越少越忧，并且EBGP邻居可添加AS的数量；
5、起源码小忧，i-igp=0,e-egp=1,?-incomplete=2；
6、MED，小优；
7、EBGP路由优于IBGP路由，联邦EBGP与普通IBGP不能使用该条进行比较，此时联邦EBGP路由与IBGP相同；
8、最近的IGP邻居，iIGP的metric最小的优；
9、如果配置了maximum-path [ibgp]n，如果存在多条等价路径，会同时加载到路由表；
10、最老的路由，前提必须是external路由，联邦内的EBGP路由当做IBGP 处理；
11、最小的router-id优；
12、多条路径的router-id相同，那么选择cluster-list长度最短的；
13、BGP优选来自最低邻居IP地址的路由，BGP neighbor配置的那个IP地址。
三、配置：
R1#sho run | s pre
ip prefix-list AS2 seq 5 permit 3.3.3.0/24
ip prefix-list AS2 seq 10 permit 4.4.4.0/24
route-map AS2 permit 10
match ip address prefix-list AS2
set local-preference 80 //解决第一步是用的是local-preference
R1#sho run | s bgp
router bgp 1
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
network 1.1.1.1 mask 255.255.255.255
neighbor 2.2.2.2 remote-as 1
neighbor 2.2.2.2 update-source Loopback0
neighbor 2.2.2.2 next-hop-self
neighbor 100.1.13.3 remote-as 2
neighbor 100.1.13.3 route-map AS2 in
R2#sho run | s pre
ip prefix-list AS3 seq 5 permit 6.6.6.0/24
ip prefix-list AS3 seq 10 permit 7.7.7.0/24
ip prefix-list AS3 seq 15 permit 5.5.5.0/24
route-map AS3 permit 10
match ip address prefix-list AS3
set weight 10 //解决第三步用的是weight
R2#sho run | s bgp
router bgp 1
bgp log-neighbor-changes
network 2.2.2.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 1
neighbor 1.1.1.1 update-source Loopback0
neighbor 1.1.1.1 next-hop-self
neighbor 100.1.24.4 remote-as 2
neighbor 100.1.25.5 remote-as 3
neighbor 100.1.26.6 remote-as 3
neighbor 100.1.26.6 route-map AS3 in
R3#sho run | s pre
ip prefix-list AS1 seq 5 permit 1.1.1.0/24
ip prefix-list AS1 seq 10 permit 2.2.2.0/24
route-map AS1 permit 10
match ip address prefix-list AS1
set metric 100 //解决第二步用的是MED
R3#sho run | s bgp
router bgp 2
bgp log-neighbor-changes
network 3.3.3.0 mask 255.255.255.0
neighbor 4.4.4.4 remote-as 2
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 next-hop-self
neighbor 100.1.13.1 remote-as 1
neighbor 100.1.13.1 route-map AS1 in
R4#sho run | s pre
ip prefix-list AS3 seq 5 permit 5.5.5.0/24
ip prefix-list AS3 seq 10 permit 6.6.6.0/24
ip prefix-list AS3 seq 15 permit 7.7.7.0/24
route-map AS3 permit 10
match ip address prefix-list AS3
set origin incomplete //解决第四步用的是origin
R4#sho run | s bgp
router bgp 2
bgp log-neighbor-changes
network 4.4.4.0 mask 255.255.255.0
neighbor 3.3.3.3 remote-as 2
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 next-hop-self
neighbor 100.1.24.2 remote-as 1
neighbor 100.1.45.5 remote-as 3
neighbor 100.1.45.5 route-map AS3 in
neighbor 100.1.46.6 remote-as 3
R5#sho run | s bgp
router bgp 3
bgp log-neighbor-changes
network 5.5.5.0 mask 255.255.255.0
neighbor 6.6.6.6 remote-as 3
neighbor 6.6.6.6 update-source Loopback0
neighbor 6.6.6.6 next-hop-self
neighbor 7.7.7.7 remote-as 3
neighbor 7.7.7.7 update-source Loopback0
neighbor 7.7.7.7 next-hop-self
neighbor 100.1.25.2 remote-as 1
neighbor 100.1.45.4 remote-as 2
R6#sho run | s prefix-list AS1
ip prefix-list AS1 seq 5 permit 1.1.1.0/24
ip prefix-list AS1 seq 10 permit 2.2.2.0/24
route-map AS1 permit 10
match ip address prefix-list AS1
set as-path prepend 1 //第五步用的是as-path
R6#sho run | s prefix-list AS2
ip prefix-list AS2 seq 5 permit 3.3.3.0/24
ip prefix-list AS2 seq 10 permit 4.4.4.0/24
route-map AS2 permit 10
match ip address prefix-list AS2
set ip next-hop 5.5.5.5 //第六步用的是next-hop
R6#SHO run | s bgp
router bgp 3
bgp log-neighbor-changes
network 6.6.6.0 mask 255.255.255.0
neighbor 5.5.5.5 remote-as 3
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 next-hop-self
neighbor 7.7.7.7 remote-as 3
neighbor 7.7.7.7 update-source Loopback0
neighbor 7.7.7.7 next-hop-self
neighbor 100.1.26.2 remote-as 1
neighbor 100.1.26.2 route-map AS1 in
neighbor 100.1.46.4 remote-as 2
neighbor 100.1.46.4 route-map AS2 in
//在第六步，R6去AS2，没有使用next-hop时，加表并优的是R4，加表不优的是R5，由于使用了next-hop，
加表并优的路由变成R5，所以才会看到BGP表中R6去AS2的优和不优的加表路由都成了R5。但实验结果能过达到要求。
R6#traceroute 3.3.3.3 source 6.6.6.6 numeric
Type escape sequence to abort.
Tracing the route to 3.3.3.3
VRF info: (vrf in name/id, vrf out name/id)
1 100.1.56.5 0 msec 1 msec 0 msec
2 100.1.45.4 0 msec 1 msec 0 msec
3 100.1.34.3 1 msec * 1 msec
//测验结果正确，能满足第六步要求
R7#sho run | s bgp
router bgp 3
bgp log-neighbor-changes
network 7.7.7.0 mask 255.255.255.0
neighbor 5.5.5.5 remote-as 3
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 next-hop-self
neighbor 6.6.6.6 remote-as 3
neighbor 6.6.6.6 update-source Loopback0
neighbor 6.6.6.6 next-hop-self
注意：
本实验的第六步选路方案虽实现了要求，但选路方案并不佳。所以，实验前应对实验中所要用到的选路规则进行推演，以便选出最好的选路方案。
原文地址：http://blog.51cto.com/13596342/2095313
