BGP分解实验·21——BGP选路原则之本地优先级

当使用BGP路径属性——本地优先级，进行路由优选时，优选“本地优先级”数值较大的那个。（eBGP之间更新不携带这个属性）

实验拓扑如下：

在未实现本地优先级策略前，先在各个BGP之间配置完成基本连接。

R1的基础配如下：

hostname R1
!
interface Loopback0
 ip address 10.1.0.1 255.255.255.0
!
interface Loopback1
 ip address 10.1.1.1 255.255.255.0
!
interface Ethernet0/0
 ip address 100.100.12.1 255.255.255.0
 no shut
!
interface Ethernet0/1
 ip address 100.105.15.1 255.255.255.0
 no shut
!
router bgp 100
 network 10.1.0.0 mask 255.255.255.0
 network 10.1.1.0 mask 255.255.255.0
 aggregate-address 10.1.0.0 255.255.254.0 summary-only
 neighbor 100.100.12.2 remote-as 234
 neighbor 100.105.15.5 remote-as 500
!
end

R2的基础配如下：

hostname R2
!
interface Ethernet0/0
 ip address 100.100.12.2 255.255.255.0
 duplex auto
!
interface Ethernet0/1
 ip address 10.23.88.2 255.255.255.0
 duplex auto
!
router bgp 234
 neighbor 10.23.88.3 remote-as 234
 neighbor 10.23.88.3 next-hop-self
 neighbor 100.100.12.1 remote-as 100
!
end

R3的基础配如下：

hostname R3
!
interface Loopback0
 ip address 10.3.3.3 255.255.255.255
!
interface Ethernet0/0
 ip address 10.23.88.3 255.255.255.0
 no shut
!
interface Ethernet0/1
 ip address 10.34.88.3 255.255.255.0
 no shut
!
router bgp 234
 network 10.3.3.3 mask 255.255.255.255
 neighbor IBGP peer-group
 neighbor IBGP remote-as 234
 neighbor IBGP route-reflector-client
 neighbor 10.23.88.2 peer-group IBGP
 neighbor 10.34.88.4 peer-group IBGP
!
end

R4的基础配如下：

hostname R4
!
interface Ethernet0/0
 ip address 10.34.88.4 255.255.255.0
 no shut
!
interface Ethernet0/1
 ip address 100.200.45.4 255.255.255.0
 no shut
!
router bgp 234
 neighbor 10.34.88.3 remote-as 234
 neighbor 10.34.88.3 next-hop-self
 neighbor 100.200.45.5 remote-as 500
!
end

R5的基础配如下：

hostname R5
!
interface Loopback2
 ip address 10.1.2.5 255.255.255.0
!
interface Loopback3
 ip address 10.1.3.5 255.255.255.0
!
interface Ethernet0/0
 ip address 100.200.45.5 255.255.255.0
 no shut
!
interface Ethernet0/1
 ip address 100.105.15.5 255.255.255.0
 no shut
!
router bgp 500
 network 10.1.2.0 mask 255.255.255.0
 network 10.1.3.0 mask 255.255.255.0
 aggregate-address 10.1.2.0 255.255.254.0 summary-only
 neighbor 100.105.15.1 remote-as 100
 neighbor 100.200.45.4 remote-as 234
!
end

R1 ~ R5的BGP表如下：

使用本地优先级的作用在R2上应用eBGP入站策略，针对来自eBGP对等体的路由设置本地优先级设置为222；相同情况的R4针对目标网络10.1.3.0/24的路由设置本地优先级为333。

R2新增的配置如下：

conf t
!
route-map LocPrf permit 10
 set local-preference 222
!
router bgp 234
 neighbor 100.100.12.1 route-map LocPrf in
!
end
cle ip b * so

R4新增的配置如下：

conf t
!
ip prefix-list R5L3 seq 5 permit 10.1.2.0/23
!
route-map LOCPRF permit 10
 match ip address prefix-list R5L3
 set local-preference 333
!         
route-map LOCPRF permit 20
!
router bgp 234
 neighbor 100.200.45.5 route-map LOCPRF in
!
end
cle ip b * so

更新策略后，在R3上验证本地优先级和BGP路由优选路径如下：

R3测试数据转发路径如下：

由此可见，BGP路径属性——本地优先级，主要应用在多宿主网络环境中控制本地出站流量。

如果一条BGP路由携带P周知团体属性值0xFFFFFF03，该路由只能传递给联邦内部子AS的对等体；如果一条BGP路由携带P周知团体属性值0xFFFFFF02，该路由不会通告给任何对等体。