BGP路由选择实验

实验拓扑：

R1与R2之间是EBGP
R1与R3之间是EBGP
R2与R3之间是IBGP





然后做如下配置
在R1上通告1.1.1.0 网络
R1：

interface Loopback0
ip address 1.1.1.1 255.255.255.0
!
interface Serial0/0
ip address 172.16.12.1 255.255.255.0
clock rate 64000
!

interface Serial0/1
ip address 172.16.13.1 255.255.255.0
clock rate 64000
!
router bgp 1
network 1.1.1.0 mask 255.255.255.0
neighbor 172.16.12.2 remote-as 2
neighbor 172.16.13.3 remote-as 2
!

R2：

interface Loopback0
ip address 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
ip address 172.16.23.2 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 172.16.12.2 255.255.255.0
clock rate 2000000
!
router bgp 2
neighbor 172.16.12.1 remote-as 1
neighbor 172.16.23.3 remote-as 2

R3：

interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/0
ip address 172.16.23.3 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 172.16.13.3 255.255.255.0
clock rate 2000000
!
router bgp 2
neighbor 172.16.13.1 remote-as 1
neighbor 172.16.23.2 remote-as 2

实验验证
在R2上查看BGP路由表，注意show ip bgp结果，
【问题1】通过R2到R1的1.1.1.0 网络为什么选择172.16.12.1？

R2#show ip bgp
BGP table version is 2, local router ID is 2.2.2.2
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
* i1.1.1.0/24 172.16.13.1 0 100 0 1 i
*> 172.16.12.1 0 0 1 i

【答案1】原因是172.16.13.1 不可达，如下面命令所示

R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Advertised to update-groups:
2
1
172.16.13.1 (inaccessible) from 172.16.23.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal
1
172.16.12.1 from 172.16.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best

【问题2】如何配置，才能让172.16.13.1 可达?
【答案2】按照下面配置
R2:

R2(config)#router bgp 2
R2(config-router)#neighbor 172.16.23.3 next-hop-self

R3(config)#router b 2
R3(config-router)#neighbor 172.16.23.2 next-hop-self

此时再查看BGP路由

R2#clear ip bgp * soft
R2#show ip bgp
BGP table version is 2, local router ID is 2.2.2.2
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
* i1.1.1.0/24 172.16.23.3 0 100 0 1 i
*> 172.16.12.1 0 0 1 i
R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Advertised to update-groups:
2
1
172.16.23.3 from 172.16.23.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal
1
172.16.12.1 from 172.16.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best

【问题4】此时为什么还是选择172.16.23.3 ？
【答案4】原因是 EBGP优于IBGP

【实验2】在R2上，如何修改AS-Path ，使到达1.1.1.0 网络的选择172.16.12.1

R2(config)#ip prefix-list 1 permit 1.1.1.0/24
R2(config)#route-map as
R2(config-route-map)#match ip address prefix-list 1
R2(config-route-map)#set as-path prepend 65000 65001
R2(config-route-map)#router bgp 2
R2(config-router)#neighbor 172.16.12.1 route-map as in
R2(config-router)#end
R2#clear ip bgp * soft
R2#show ip b
BGP table version is 3, local router ID is 2.2.2.2
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
*>i1.1.1.0/24 172.16.23.3 0 100 0 1 i 变成了上面这条路由
* 172.16.12.1 0 0 65000 65001 1 i

此时在R2上查看到R3的路由已经没有了

R2#show ip bgp neighbors 172.16.23.3 advertised-routes

Total number of prefixes 0

在R3上查看bgp路由，也只剩余1条

R3#show ip bgp
BGP table version is 2, local router ID is 3.3.3.3
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
*> 1.1.1.0/24 172.16.13.1 0 0 1 i
R3#