操纵BGP多出口区分属性(MED)案例.doc

操纵BGP多出口区分属性（MED）案例

MED属性与本地优先级属性（localpref）不同，MED在AS间交换，但进入AS的MED不能离开该AS。AS使用Localpref来影响它自已的出站选择处理。MED是用来影响另一个AS的出站选择。MED越小，路由优先级越高。

R1将1.0.0.0的MED属性在通告给R3之前设置为50，在通告给R2前设置为100。

//// 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 192.1.1.2 route-map setmed100 out
  neighbor 193.1.1.3 route-map setmed50 out

access-list 1 permit 1.0.0.0 0.255.255.255

route-map setmed50 10
  match ip ad 1
  set metric 50
route-map setmed50 20
  set metric

route-map setmed100 10
  match ip ad 1
  set metric 100
route-map setmed100 20
  set metric

//// 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

验证：
r2#sh ip bgp
   Network          Next Hop            Metric LocPrf Weight Path
*>i1.0.0.0          193.1.1.1               50    100      0 100 i
*                   192.1.1.1              100             0 100 i
* i2.0.0.0          193.1.1.1                0    100      0 100 i
*>                  192.1.1.1                0             0 100 i
*>i4.0.0.0          194.1.1.4                0    100      0 i
r2#

注意MED值比较低的被选为最佳路径。

r2#sh ip ro
B    1.0.0.0/8 [200/50] via 193.1.1.1, 00:02:00
B    2.0.0.0/8 [20/0] via 192.1.1.1, 00:02:00
     4.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O       4.4.4.4/32 [110/65] via 194.1.1.4, 00:41:09, Serial4/0
B       4.0.0.0/8 [200/0] via 194.1.1.4, 00:17:42
O    193.1.1.0/24 [110/192] via 194.1.1.4, 00:41:09, Serial4/0
C    192.1.1.0/24 is directly connected, Serial2/0
O    195.1.1.0/24 [110/128] via 194.1.1.4, 00:41:09, Serial4/0
C    194.1.1.0/24 is directly connected, Serial4/0