1、as-path选路解析
| 属性 | 传播范围 | 默认值 | 优先值 | 方向 | 备注 |
| weight | 本路由器 | 本路由器32768，其他路由器0 | 大值优先 | IN | CISCO私有 |
| Local preference | AS内部 | 100：边界路由器，通告给IBGP邻居，告知如何离开本AS | 大值优先 | OUT | |
| As path | AS之间 | 无 | 短优先 | IN、OUT | |
| MED | AS内部 | 0：通告给EBGP，告知如何进入本AS | 小优先 | OUT |
修改MED值的方法:
2、实验拓扑
3、基础配置
R1配置
interface Loopback0
ip address 1.1.1.1 255.255.255.0
interface Serial0/0
ip address 13.1.1.1 255.255.255.0
interface Serial0/1
ip address 14.1.1.1 255.255.255.0
router ospf 110
router-id 1.1.1.1
log-adjacency-changes
network 1.1.1.0 0.0.0.255 area 0
network 14.1.1.0 0.0.0.255 area 0
router bgp 4
no synchronization
bgp router-id 1.1.1.1
neighbor 4.4.4.4 remote-as 4
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 next-hop-self
neighbor 13.1.1.3 remote-as 3
no auto-summary
R2配置
interface Loopback0
ip address 2.2.2.2 255.255.255.0
interface Serial0/1
ip address 23.1.1.2 255.255.255.0
interface FastEthernet1/0
ip address 24.1.1.2 255.255.255.0
router ospf 110
router-id 2.2.2.2
network 2.2.2.0 0.0.0.255 area 0
network 24.1.1.0 0.0.0.255 area 0
router bgp 4
no synchronization
bgp router-id 2.2.2.2
neighbor 4.4.4.4 remote-as 4
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 next-hop-self
neighbor 23.1.1.3 remote-as 3
no auto-summary
R3配置
interface Loopback0
ip address 3.3.3.3 255.255.255.0
interface Loopback1
ip address 33.33.33.33 255.255.255.0
interface Serial0/0
ip address 13.1.1.3 255.255.255.0
interface Serial0/1
ip address 23.1.1.3 255.255.255.0router bgp 3
no synchronization
bgp router-id 3.3.3.3
network 3.3.3.0 mask 255.255.255.0
network 33.33.33.0 mask 255.255.255.0
neighbor 13.1.1.1 remote-as 4
neighbor 23.1.1.2 remote-as 4
no auto-summary
R4配置
interface Loopback0
ip address 4.4.4.4 255.255.255.0
interface Serial0/1
ip address 14.1.1.4 255.255.255.0
interface FastEthernet1/0
ip address 24.1.1.4 255.255.255.0
router ospf 110
router-id 4.4.4.4
network 4.4.4.0 0.0.0.255 area 0
network 14.1.1.0 0.0.0.255 area 0
network 24.1.1.0 0.0.0.255 area 0
router bgp 4
no synchronization
bgp router-id 4.4.4.4
network 4.4.4.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 4
neighbor 1.1.1.1 update-source Loopback0
neighbor 2.2.2.2 remote-as 4
neighbor 2.2.2.2 update-source Loopback0
no auto-summary
4、默认选路
根据BGP选路原则第十条可以知道，R3到达4.4.4.0/24默认经过R1。
5、修改MED值让R3访问4.4.4.0/24的时候从R2经过
R1配置
ip prefix-list 1 seq 5 permit 4.4.4.0/24
route-map MED permit 10
match ip address prefix-list 1
set metric 10
route-map MED permit 20
router bgp 4
neighbor 13.1.1.3 route-map MED out
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