BGP 最佳路径选择之 -- MED

【实验要求】
 

在 AS1 中配置 BGP MED 值， 确保R3上由155.1.37.0/24 网络到155.1.5.0 网络的流量走R1

【实验拓扑】

 

【实验基本配置】

R1:

interface FastEthernet0/0

ip address 155.1.146.1 255.255.255.0

!

interface Serial0/0

ip address 155.1.13.1 255.255.255.0

!

router bgp 2

neighbor 155.1.13.3 remote-as 2

neighbor 155.1.13.3 next-hop-self

neighbor 155.1.146.4 remote-as 1

R3:

interface Serial0/1

ip address 155.1.13.3 255.255.255.0

clockrate 64000

!

interface Serial0/0

ip address 155.1.0.3 255.255.255.0

encapsulation frame-relay

clock rate 64000

frame-relay map ip 155.1.0.5 305 broadcast

no frame-relay inverse-arp

!

interface Loopback0

ip add 155.1.37.3 255.255.255.0

!

router bgp 2

network 155.1.37.0 mask 255.255.255.0

neighbor 155.1.0.5 remote-as 1

neighbor 155.1.13.1 remote-as 2

neighbor 155.1.13.1 next-hop-self

 

R4:

interface FastEthernet0/0

ip address 155.1.146.4 255.255.255.0

!

interface Serial0/0

ip address 155.1.45.4 255.255.255.0

!

router bgp 1

neighbor 155.1.45.5 remote-as 1

neighbor 155.1.45.5 next-hop-self

neighbor 155.1.146.1 remote-as 2

 

R5:

interface Serial0/0

ip address 155.1.0.5 255.255.255.0

encapsulation frame-relay

frame-relay map ip 155.1.0.3 503 broadcast

no frame-relay inverse-arp

!

interface Loopback0

ip address 155.1.5.5 255.255.255.0

!

interface Serial0/1

ip address 155.1.45.5 255.255.255.0

clockrate 64000

!

router bgp 1

network 155.1.5.0 mask 255.255.255.0

neighbor 155.1.0.3 remote-as 2

neighbor 155.1.45.4 remote-as 1

neighbor 155.1.45.4 next-hop-self

【实验验证】

R3#show ip bgp

BGP table version is 3, local router ID is 155.1.37.3

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

 

Network Next Hop Metric LocPrf Weight Path

*> 155.1.5.0/24 155.1.0.5 0 0 1 i

* i 155.1.13.1 0 100 0 1 i

*> 155.1.37.0/24 0.0.0.0 0 32768 i

在未配置weight时R3的bgp路由表，我们发现到达155.1.5.0 的网络下一跳为155.1.0.5，为什么呢？我们回顾一下BGP选路原则, 是不是比较到7步就比较出来了

1.权重：Prefer highest weight (local to router).

2.优先级：Prefer highest local preference (global within AS).

3.起源于本地：Prefer route originated by the local router (next hop = 0.0.0.0).

4.最短AS路径：Prefer shortest AS path.

5.最低的起源属性：Prefer lowest origin code (IGP < EGP < incomplete).

6.最低的MED：Prefer lowest MED.

7.EBGP优于IBGP：Prefer EBGP path over IBGP path.

8.最近的IGP邻居：Prefer the path through the closest IGP neighbor.

9.生存最长的路由：Prefer oldest route for EBGP paths.

10.最低的router-id：Prefer the path with the lowest neighbor BGP router ID.

11.最低的IP地址：Prefer the path with the lowest neighbor IP address.

【MED 配置方法】

下一步配置MED值让流量走R1

配置方法：

R4:

route-map MED permit 10

set metric 100

!

router bgp 1

neighbor 155.1.146.1 route-map MED out

R5:

route-map MED permit 10

set metric 200

!

router bgp 1

neighbor 155.1.0.3 route-map MED out

 

【配置后验证】

 

R3#show ip bgp 下面验证了配置是正确的，BGP选择metric（MED）更小的路由。

BGP table version is 5, local router ID is 155.1.37.3

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

 

Network Next Hop Metric LocPrf Weight Path

*>i155.1.5.0/24 155.1.13.1 100 100 0 1 i

* 155.1.0.5 200 0 1 i

*> 155.1.37.0/24 0.0.0.0 0 32768 i