操縱BGP AS-PATH 屬性案例（配圖+詳細驗證過程）

BGP總是優先選擇具有最短AS路徑的路由。本案例中，我們將配置R1，將1.0.0.0通告給R2、R3之前預先添加兩個額外的AS路徑到1.0.0.0上（AS300、AS400）

 

具體配置如下：

//// r1 ////

int f2/0

  ip ad 192.1.1.1 255.255.255.0

 

int f3/0

  ip ad 193.1.1.1 255.255.255.0

 

int lo0

  ip ad 1.1.1.1 255.255.255.0

 

int lo1

  ip ad 2.2.2.2 255.255.255.0

 

router bgp 100

  no syn

  neighbor 192.1.1.2 remote-as 200

  neighbor 193.1.1.3 remote-as 200

  network 1.0.0.0

  network 1.0.0.0

  neighbor 193.1.1.3 route-map aspath out

 

access-list 1 permit 1.0.0.0 0.255.255.255

 

route-map aspath 10

  match ip ad 1

  set as-path prepend 300 400

route-map aspath 20

 

 

 

//// r2 ////

int f2/0

  ip ad 192.1.1.2 255.255.255.0

 

int f4/0

  ip ad 194.1.1.2 255.255.255.0

 

router os 1

  netw 0.0.0.0 255.255.255.255 a 0

  passive-interface f2/0

 

router bgp 200

  no syn

  neighbor 192.1.1.1 remote-as 100

  neighbor 194.1.1.4 remote-as 200

  neighbor 195.1.1.3 remote-as 200

  neighbor 194.1.1.4 next-hop-self

 

 

//// r3 ////

int f3/0

  ip ad 193.1.1.3 255.255.255.0

 

int f5/0

  ip ad 195.1.1.3 255.255.255.0

 

router os 1

  netw 0.0.0.0 255.255.255.255 a 0

  passive-interface f3/0

 

router bgp 200

  no syn

  neighbor 193.1.1.1 remote-as 100

  neighbor 195.1.1.4 remote-as 200

  neighbor 195.1.1.4 next-hop-self

  neighbor 194.1.1.2 remote-as 200

 

 

//// r4 ////

int f4/0

  ip ad 194.1.1.4 255.255.255.0

 

int f5/0

  ip ad 195.1.1.4 255.255.255.0

 

int lo0

  ip ad 4.4.4.4 255.255.255.0

 

router os 1

  netw 0.0.0.0 255.255.255.255 a 0

 

router bgp 200

  no syn

  neighbor 194.1.1.2 remote-as 200

  neighbor 195.1.1.3 remote-as 200

  netw 4.0.0.0

 

 

 

驗證：

 

r3#sh ip b
   Network          Next Hop            Metric LocPrf Weight Path
*>i1.0.0.0          192.1.1.1                0    100      0 100 i
*                   193.1.1.1                0             0 100 300 400 i
* i2.0.0.0          192.1.1.1                0    100      0 100 i
*>                  193.1.1.1                0             0 100 i
*>i4.0.0.0          195.1.1.4                0    100      0 i

 

 

r3#sh ip ro

B    1.0.0.0/8 [200/0] via 192.1.1.1, 00:05:23
B    2.0.0.0/8 [20/0] via 193.1.1.1, 00:05:23
     4.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O       4.4.4.4/32 [110/65] via 195.1.1.4, 00:10:38, Serial5/0
B       4.0.0.0/8 [200/0] via 195.1.1.4, 00:10:13
C    193.1.1.0/24 is directly connected, Serial3/0
O    192.1.1.0/24 [110/192] via 195.1.1.4, 00:10:38, Serial5/0
C    195.1.1.0/24 is directly connected, Serial5/0
O    194.1.1.0/24 [110/128] via 195.1.1.4, 00:10:38, Serial5/0