BGP 最佳路径选择之 -- Local Preference

【实验说明】

在AS2中配置BGP Local Preference 参数， 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】

 

【实验步骤】

• 按照上图配置网络
• 在 R1, R3, R4, and R5 上配置bgp邻居
• R1 应该 peer with R3 and R4
• R5 应该 peer with R3 and R5
• 通告 R3’s 155.1.37.0/24 网络到 BGP
• 通告 R5’s 155.1.5.0？24 网络到 BGP
• 在AS2中配置Local preference 参数，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.

【local preference 配置方法】

下一步配置本地优先级让流量走R1

配置方法：

R3：

route-map LOCAL_PREFERENCE permit 10

set local-preference 200

!

router bgp 2

neighbor 155.1.13.1 route-map LOCAL_PREFERENCE in

 

【验证配置】

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 0 200 0 1 i 注意这里的local Preference被修改为200

*> 155.1.37.0/24 0.0.0.0 0 32768 i

 

R3#show ip route

155.1.0.0/24 is subnetted, 4 subnets

C 155.1.13.0 is directly connected, Serial0/1

C 155.1.0.0 is directly connected, Serial0/0

B 155.1.5.0 [200/0] via 155.1.13.1, 00:06:50

C 155.1.37.0 is directly connected, Loopback0