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1、as-path選路解析
| 屬性 | 傳播范圍 | 默認(rèn)值 | 優(yōu)先值 | 方向 | 備注 |
| weight | 本路由器 | 本路由器32768，其他路由器0 | 大值優(yōu)先 | IN | CISCO私有 |
| Local preference | AS內(nèi)部 | 100：邊界路由器，通告給IBGP鄰居，告知如何離開本AS | 大值優(yōu)先 | OUT | |
| As path | AS之間 | 無 | 短優(yōu)先 | IN、OUT |
修改As path值的方法:
方法一：在OUT方向上修改As path值，例如
ip prefix-list 1 seq 5 permit 33.33.33.0/24
route-map AS permit 10
match ip address prefix-list 1
set as-path prepend 2 2 2 2 2
route-map AS permit 20
router bgp 3
neighbor 23.1.1.2 route-map AS out
方法二：在IN方向上修改As path值，例如
2、實驗拓?fù)?/p>
3、基礎(chǔ)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
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、默認(rèn)選路
根據(jù)BGP選路原則第八條可以知道，R4到達(dá)3.3.3.0/24、33.33.33.0/24都是經(jīng)過R2。
5、在OUT方向上修改As path值
R4到達(dá)3.3.3.0/24、33.33.33.0/24都是經(jīng)過R2，我們通過修改As path值將R4到達(dá)3.3.3.0/24經(jīng)過R2、達(dá)到33.33.33.0/24都是經(jīng)過R1。
R3配置
ip prefix-list 1 seq 5 permit 33.33.33.0/24
route-map AS permit 10
match ip address prefix-list 1
set as-path prepend 2 2 2 2 2
route-map AS permit 20
router bgp 3
neighbor 23.1.1.2 route-map AS out
6、在IN方向上修改As path值
R4到達(dá)3.3.3.0/24、33.33.33.0/24都是經(jīng)過R2，我們通過修改As path值將R4到達(dá)3.3.3.0/24經(jīng)過R2、達(dá)到33.33.33.0/24都是經(jīng)過R1。
R2配置
ip prefix-list 1 seq 5 permit 33.33.33.0/24
route-map AS permit 10
match ip address prefix-list 1
set as-path prepend 2 2 2 2 2
route-map AS permit 20
router bgp 4
neighbor 23.1.1.3 route-map AS in
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