目录
第三章 BGP协议特性与配置
实验 3-3 BGP属性与路径选择1
学习目的
·掌握通过配置AS-Path属性来影响路径选择的方法
·掌握通过修改Origin属性来影响路径选择的配置方法
·掌握通过修改Local-Pref属性来影响路径选择的配置方法
·掌握通过修改MED属性来影响路径选择的配置方法
拓扑图
图3-3 BGP属性与路径选择
场景
你是公司的网络管理员。公司的网络采用了BGP协议接入了两个服务运营商。公司自己采用了私有的AS号64512，ISP1的AS号为100，公司共有2条链路接入ISP1。ISP2的AS号为200，公司租用了一条线路接入ISP2。现在Internet上的部分用户反应访问公司网站的速度较慢，你通过改变BGP的各种属性达到了调整路由走向的目的。
学习任务
步骤一.基础配置与IP编址
给所有路由器配置物理接口及Loopback接口的IP地址和掩码。注意各Loopback 0接口均使用32位掩码。
[R1]interface Serial 1/0/0
[R1-Serial1/0/0]ip address 10.0.12.1 24
[R1-Serial1/0/0]quit
[R1]interface GigabitEthernet 0/0/2
[R1-GigabitEthernet0/0/2]ip address 10.0.15.1 24
[R1-GigabitEthernet0/0/2]quit
[R1]interface GigabitEthernet 0/0/1
[R1-GigabitEthernet0/0/1]ip address 10.0.111.1 24
[R1-GigabitEthernet0/0/1]quit
[R1]interface LoopBack 0
[R1-LoopBack0]ip address 10.0.1.1 32
[R1-LoopBack0]quit
[R2]interface Serial 1/0/0
[R2-Serial1/0/0]ip address 10.0.12.2 24
[R2-Serial1/0/0]quit
[R2]interface Serial 2/0/0
[R2-Serial2/0/0]ip address 10.0.23.2 24
[R2-Serial2/0/0]quit
[R2]interface LoopBack 0
[R2-LoopBack0]ip address 10.0.2.2 24
[R2-LoopBack0]quit
[R3]interface GigabitEthernet 0/0/2
[R3-GigabitEthernet0/0/2]ip address 10.0.15.3 24
[R3-GigabitEthernet0/0/2]quit
[R3]interface Serial 2/0/0
[R3-Serial2/0/0]ip address 10.0.23.3 24
[R3-Serial2/0/0]quit
[R3]interface Serial 3/0/0
[R3-Serial3/0/0]ip address 10.0.35.3 24
[R3-Serial3/0/0]quit
[R3]interface loopback 0
[R3-LoopBack0]ip address 10.0.3.3 32
[R3-LoopBack0]quit
[R4]interface GigabitEthernet 0/0/1
[R4-GigabitEthernet0/0/1]ip address 10.0.114.4 24
[R4-GigabitEthernet0/0/1]quit
[R4]interface GigabitEthernet 0/0/0
[R4-GigabitEthernet0/0/0]ip address 10.0.45.4 24
[R4-GigabitEthernet0/0/0]quit
[R4]interface loopback 0
[R4-LoopBack0]ip address 10.0.4.4 32
[R4-LoopBack0]quit
[R5]interface Serial 1/0/0
[R5-Serial1/0/0]ip address 10.0.35.5 24
[R5-Serial1/0/0]quit
[R5]interface GigabitEthernet 0/0/0
[R5-GigabitEthernet0/0/0]ip address 10.0.45.5 24
[R5-GigabitEthernet0/0/0]quit
[R5]interface loopback 0
[R5-LoopBack0]ip address 10.0.5.5 32
[R5-LoopBack0]quit
配置完成后，测试直连链路的连通性。
<R1>ping -c 1 10.0.12.2
PING 10.0.12.2: 56 data bytes, press CTRL_C to break
Reply from 10.0.12.2: bytes=56 Sequence=1 ttl=255 time=29 ms
--- 10.0.12.2 ping statistics ---
1 packet(s) transmitted
1 packet(s) received
0.00% packet loss
round-trip min/avg/max = 29/29/29 ms
[R1]ping -c 1 10.0.15.3
PING 10.0.15.3: 56 data bytes, press CTRL_C to break
Reply from 10.0.15.3: bytes=56 Sequence=1 ttl=255 time=59 ms
--- 10.0.15.3 ping statistics ---
1 packet(s) transmitted
1 packet(s) received
0.00% packet loss
round-trip min/avg/max = 59/59/59 ms
<R2>ping -c 1 10.0.23.3
PING 10.0.23.3: 56 data bytes, press CTRL_C to break
Reply from 10.0.23.3: bytes=56 Sequence=1 ttl=255 time=32 ms
--- 10.0.23.3 ping statistics ---
1 packet(s) transmitted
1 packet(s) received
0.00% packet loss
round-trip min/avg/max = 32/32/32 ms
[R3]ping -c 1 10.0.35.5
PING 10.0.35.5: 56 data bytes, press CTRL_C to break
Reply from 10.0.35.5: bytes=56 Sequence=1 ttl=255 time=36 ms
--- 10.0.35.5 ping statistics ---
1 packet(s) transmitted
1 packet(s) received
0.00% packet loss
round-trip min/avg/max = 36/36/36 ms
<R4>ping -c 1 10.0.45.5
PING 10.0.45.5: 56 data bytes, press CTRL_C to break
Reply from 10.0.45.5: bytes=56 Sequence=1 ttl=255 time=11 ms
--- 10.0.45.5 ping statistics ---
1 packet(s) transmitted
1 packet(s) received
0.00% packet loss
round-trip min/avg/max = 11/11/11 ms
步骤二.配置IGP及BGP
在AS 64512内部使用OSPF作为IGP，所有设备属于区域0。
R1的G0/0/1和Loopback 0连接的网段运行OSPF。
[R1]router id 10.0.1.1
[R1]ospf 1
[R1-ospf-1]area 0
[R1-ospf-1-area-0.0.0.0]network 10.0.111.1 0.0.0.0
[R1-ospf-1-area-0.0.0.0]network 10.0.1.1 0.0.0.0
[R1-ospf-1-area-0.0.0.0]quit
[R1-ospf-1]quit
在S1上创建Vlan 111，配置Vlanif地址与R1进行互联。
创建Vlan114，配置Vlanif地址与R4进行互联。
互联接口使用Access模式，S1的Vlanif 111、Vlanif 114 和Loopback 0连接的网段运行OSPF。
[S1]router id 10.0.11.11
[S1]vlan batch 111 114
[S1]interface vlan 111
[S1-Vlanif111]ip address 10.0.111.11 24
[S1-Vlanif111]quit
[S1]interface vlan 114
[S1-Vlanif114]ip address 10.0.114.11 24
[S1-Vlanif114]quit
[S1]interface loopback 0
[S1-LoopBack0]ip address 10.0.11.11 32
[S1-LoopBack0]quit
[S1]interface GigabitEthernet 0/0/1
[S1-GigabitEthernet0/0/1]port link-type access
[S1-GigabitEthernet0/0/1]port default vlan 111
[S1-GigabitEthernet0/0/1]quit
[S1]interface GigabitEthernet 0/0/4
[S1-GigabitEthernet0/0/4]port link-type access
[S1-GigabitEthernet0/0/4]port default vlan 114
[S1-GigabitEthernet0/0/4]quit
[S1]ospf 1
[S1-ospf-1]area 0
[S1-ospf-1-area-0.0.0.0]network 10.0.111.11 0.0.0.0
[S1-ospf-1-area-0.0.0.0]network 10.0.114.11 0.0.0.0
[S1-ospf-1-area-0.0.0.0]network 10.0.11.11 0.0.0.0
[S1-ospf-1-area-0.0.0.0]quit
[S1-ospf-1]quit
R4的G0/0/1和Loopback 0连接的网段运行OSPF，
[R4]router id 10.0.4.4
[R4]ospf 1
[R4-ospf-1]area 0
[R4-ospf-1-area-0.0.0.0]network 10.0.114.4 0.0.0.0
[R4-ospf-1-area-0.0.0.0]network 10.0.4.4 0.0.0.0
[R4-ospf-1-area-0.0.0.0]quit
[R4-ospf-1]quit
检查是否学习到其他设备的Loopback 0地址所在的网段。
[R1]display ip routing-table
Route Flags: R - relay, D - download to fib
----------------------------------------------------------------------------
Routing Tables: Public
Destinations : 18 Routes : 18
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.0.1.1/32 Direct 0 0 D 127.0.0.1 LoopBack0
10.0.4.4/32 OSPF 10 2 D 10.0.111.11 GigabitEthernet0/0/1
10.0.11.11/32 OSPF 10 1 D 10.0.111.11 GigabitEthernet0/0/1
10.0.12.0/24 Direct 0 0 D 10.0.12.1 Serial1/0/0
10.0.12.1/32 Direct 0 0 D 127.0.0.1 Serial1/0/0
10.0.12.2/32 Direct 0 0 D 10.0.12.2 Serial1/0/0
10.0.12.255/32 Direct 0 0 D 127.0.0.1 Serial1/0/0
10.0.15.0/24 Direct 0 0 D 10.0.15.1 GigabitEthernet0/0/2
10.0.15.1/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/2
10.0.15.255/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/2
10.0.111.0/24 Direct 0 0 D 10.0.111.1 GigabitEthernet0/0/1
10.0.111.1/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/1
10.0.111.255/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/1
10.0.114.0/24 OSPF 10 2 D 10.0.111.11 GigabitEthernet0/0/1
127.0.0.0/8 Direct 0 0 D 127.0.0.1 InLoopBack0
127.0.0.1/32 Direct 0 0 D 127.0.0.1 InLoopBack0
127.255.255.255/32Direct 0 0 D 127.0.0.1 InLoopBack0
255.255.255.255/32Direct 0 0 D 127.0.0.1 InLoopBack0
[S1]display ip routing-table
Route Flags: R - relay, D - download to fib
----------------------------------------------------------------------------
Routing Tables: Public
Destinations : 9 Routes : 9
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.0.1.1/32 OSPF 10 1 D 10.0.111.1 Vlanif111
10.0.4.4/32 OSPF 10 1 D 10.0.114.4 Vlanif114
10.0.11.11/32 Direct 0 0 D 127.0.0.1 LoopBack0
10.0.111.0/24 Direct 0 0 D 10.0.111.11 Vlanif111
10.0.111.11/32 Direct 0 0 D 127.0.0.1 Vlanif111
10.0.114.0/24 Direct 0 0 D 10.0.114.11 Vlanif114
10.0.114.11/32 Direct 0 0 D 127.0.0.1 Vlanif114
127.0.0.0/8 Direct 0 0 D 127.0.0.1 InLoopBack0
127.0.0.1/32 Direct 0 0 D 127.0.0.1 InLoopBack0
<R4>display ip routing-table
Route Flags: R - relay, D - download to fib
----------------------------------------------------------------------------
Routing Tables: Public
Destinations : 14 Routes : 14
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.0.1.1/32 OSPF 10 2 D 10.0.114.11 GigabitEthernet0/0/1
10.0.4.4/32 Direct 0 0 D 127.0.0.1 LoopBack0
10.0.11.11/32 OSPF 10 1 D 10.0.114.11 GigabitEthernet0/0/1
10.0.45.0/24 Direct 0 0 D 10.0.45.4 GigabitEthernet0/0/0
10.0.45.4/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/0
10.0.45.255/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/0
10.0.111.0/24 OSPF 10 2 D 10.0.114.11 GigabitEthernet0/0/1
10.0.114.0/24 Direct 0 0 D 10.0.114.4 GigabitEthernet0/0/1
10.0.114.4/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/1
10.0.114.255/32 Direct 0 0 D 127.0.0.1 GigabitEthernet0/0/1
127.0.0.0/8 Direct 0 0 D 127.0.0.1 InLoopBack0
127.0.0.1/32 Direct 0 0 D 127.0.0.1 InLoopBack0
127.255.255.255/32Direct 0 0 D 127.0.0.1 InLoopBack0
255.255.255.255/32Direct 0 0 D 127.0.0.1 InLoopBack0
在R1、R4、S1上配置BGP，要求使用各自的Loopback 0接口建立对等体连接，配置的时候使用对等体组，组名为as64512。
默认情况下，BGP的负载分担是关闭的。在所有的路由器上打开负载分担，设置最大同时使用4条等价路径。
[R1]bgp 64512
[R1-bgp]group as64512 internal
[R1-bgp]peer 10.0.11.11 group as64512
[R1-bgp]peer 10.0.11.11 connect-interface LoopBack 0
[R1-bgp]maximum load-balancing 4
[R1-bgp]quit
[S1]bgp 64512
[S1-bgp]group as64512 internal
[S1-bgp]peer 10.0.4.4 group as64512
[S1-bgp]peer 10.0.4.4 connect-interface LoopBack 0
[S1-bgp]maximum load-balancing 4
[S1-bgp]peer 10.0.1.1 group as64512
[S1-bgp]peer 10.0.1.1 connect-interface LoopBack 0
[S1-bgp]quit
[R4]bgp 64512
[R4-bgp]group as64512 internal
[R4-bgp]peer 10.0.11.11 group as64512
[R4-bgp]peer 10.0.11.11 connect-interface LoopBack 0
[R4-bgp]maximum load-balancing 4
[R4-bgp]quit
在R1、R2、R3、R4、R5上配置EBGP，AS的规划如图所示，EBGP全部使用物理接口地址建立对等体关系。
[R1]bgp 64512
[R1-bgp]peer 10.0.12.2 as-number 200
[R1-bgp]peer 10.0.15.3 as-number 100
[R1-bgp]quit
[R2]router id 10.0.2.2
[R2]bgp 200
[R2-bgp]peer 10.0.12.1 as-number 64512
[R2-bgp]peer 10.0.23.3 as-number 100
[R2-bgp]maximum load-balancing 4
[R2-bgp]quit
[R3]router id 10.0.3.3
[R3]bgp 100
[R3-bgp]peer 10.0.23.2 as-number 200
[R3-bgp]peer 10.0.35.5 as-number 100
[R3-bgp]peer 10.0.15.1 as-number 64512
[R3-bgp]maximum load-balancing 4
[R3-bgp]quit
[R4]bgp 64512
[R4-bgp]peer 10.0.45.5 as-number 100
[R4-bgp]quit
[R5]router id 10.0.5.5
[R5]bgp 100
[R5-bgp]peer 10.0.35.3 as-number 100
[R5-bgp]peer 10.0.45.4 as-number 64512
[R5-bgp]maximum load-balancing 4
[R5-bgp]quit
步骤三.配置AS-Path属性
在S1上创建Loopback 1，地址为10.1.11.11/24，使用network命令发布到BGP中。
[S1]interface loopback 1
[S1-LoopBack1]ip address 10.1.11.11 24
[S1-LoopBack1]quit
[S1]bgp 64512
[S1-bgp]network 10.1.11.11 255.255.255.0
[S1]quit
在R2上观察BGP路由表，可看出这时10.1.11.0/24路由是依据AS-Path属性来选择下一跳的。
[R2]display bgp routing-table
BGP Local router ID is 10.0.2.2
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.11.0/24 10.0.12.1 0 64512i
* 10.0.23.3 0 100 64512i
由于R1到R4之间的带宽有限，现在希望R2能经由AS100来访问10.1.11.0/24。
这里通过AS-Path来影响选路。
在R1上创建路由策略as_path，针对10.1.11.0/24这条路由增加2个重复的AS号。
[R1]acl number 2001
[R1-acl-basic-2001]rule 5 permit source 10.1.11.0 0.0.0.255
[R1-acl-basic-2001]quit
[R1]route-policy as_path permit node 10
[R1-route-policy]if-match acl 2001
[R1-route-policy]apply as-path 64512 64512 additive
[R1-route-policy]quit
然后把该策略应用在R1上，让R2从R1上学习到的这条路由的AS-Path有3个值。
[R1]bgp 64512
[R1-bgp]peer 10.0.12.2 route-policy as_path export
[R1-bgp]quit
在R2上观察BGP路由表。
<R2>display bgp routing-table
BGP Local router ID is 10.0.2.2
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.11.0/24 10.0.23.3 0 100 64512i
* 10.0.12.1 0 64512 64512 64512i
此时R2经由 AS100访问10.1.11.0/24网段。
步骤四.配置Origin属性
观察R3的路由表。
<R3>display bgp routing-table
BGP Local router ID is 10.0.3.3
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.11.0/24 10.0.15.1 0 64512i
* i 10.0.35.5 100 0 64512i
到达10.1.11.0/24的下一跳是R1，分析原因。
我们希望R3通过R5访问AS 64512，查看10.1.11.0/24原来的Origin属性是IGP。
这时，我们将R1对R3通告的该路由修改为incomplete。
[R1]route-policy 22 permit node 10
[R1-route-policy]if-match acl 2001
[R1-route-policy]apply origin incomplete
[R1-route-policy]quit
[R1]bgp 64512
[R1-bgp]peer 10.0.15.3 route-policy 22 export
[R1-bgp]quit
策略生效后观察R3的BGP路由表。
<R3>display bgp routing-table
BGP Local router ID is 10.0.3.3
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.1.11.0/24 10.0.35.5 100 0 64512i
* 10.0.15.1 0 64512?
这时R3到达网络10.1.11.0/24的下一跳是R5。
步骤五.配置Local-Pref属性
本地优先属性在选路中有很高的优先级。
通过改变本地优先属性可以影响选路。
在R3上创建Loopback 1，地址为10.1.3.3/24，发布进BGP。
[R3]interface loopback 1
[R3-LoopBack1]ip address 10.1.3.3 255.255.255.0
[R3-LoopBack1]quit
[R3]bgp 100
[R3-bgp]network 10.1.3.3 255.255.255.0
[R3-bgp]quit
在R5上创建Loopback 1，地址为10.1.5.5/24，发布进BGP。
[R5]interface loopback 1
[R5-LoopBack1]ip address 10.1.5.5 255.255.255.0
[R5-LoopBack1]quit
[R5]bgp 100
[R5-bgp]network 10.1.5.5 24
[R5-bgp]quit
在S1上观察路由表。
[S1]display bgp routing-table
BGP Local router ID is 10.0.11.11
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 5
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.1.3.0/24 10.0.1.1 0 100 0 100i
* i 10.0.4.4 100 0 100i
*>i 10.1.5.0/24 10.0.1.1 100 0 100i
* i 10.0.4.4 0 100 0 100i
*> 10.1.11.0/24 0.0.0.0 0 0 i
现在希望到达网络10.1.5.0/24的流量从R4发送到目标，到达网络10.1.3.0/24的流量从R1发送到目标。
在R4上创建路由策略Pref4，匹配路由10.1.5.0/24，将其本地优先属性修改为110。
R1上创建路由策略Pref1，匹配路由10.1.3.0/24，将其本地优先属性修改为110，然后将策略应用到IBGP的对等体组上。
[R4]acl number 2001
[R4-acl-basic-2001]rule 5 permit source 10.1.5.0 0.0.0.255
[R4-acl-basic-2001]quit
[R4]route-policy Pref4 permit node 10
[R4-route-policy]if-match acl 2001
[R4-route-policy]apply local-preference 110
[R4-route-policy]quit
[R4]route-policy Pref4 permit node 20
[R4-route-policy]quit
[R4]bgp 64512
[R4-bgp]peer as64512 route-policy Pref4 export
[R4-bgp]quit
[R1]acl number 2002
[R1-acl-basic-2002]rule 5 permit source 10.1.3.0 0.0.0.255
[R1-acl-basic-2002]quit
[R1]route-policy Pref1 permit node 10
[R1-route-policy]if-match acl 2002
[R1-route-policy]apply local-preference 110
[R1-route-policy]quit
[R1]route-policy Pref1 permit node 20
[R1-route-policy]quit
[R1]bgp 64512
[R1-bgp]peer as64512 route-policy Pref1 export
[R1-bgp]quit
在S1上查看BGP路由表。
[S1]display bgp routing-table
BGP Local router ID is 10.0.11.11
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? – incomplete
Total Number of Routes: 5
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.1.3.0/24 10.0.1.1 0 110 0 100i
* i 10.0.4.4 100 0 100i
*>i 10.1.5.0/24 10.0.4.4 0 110 0 100i
* i 10.0.1.1 0 100 0 100i
*> 10.1.11.0/24 0.0.0.0 0 0 i
可以观察到，此时根据Local-Pref属性进行选路，越高越优先。
步骤六.配置MED属性
删除步骤四中通过修改Origin来影响AS100中对10.1.11.0/24选路的路由策略，本实验中通过修改MED值来影响选路。
[R1]bgp 64512
[R1-bgp]undo peer 10.0.15.3 route-policy 22 export
[R1-bgp]quit
[R1]undo route-policy 22
在R1上创建路由策略med，针对10.1.11.0/24，将MED值修改为100，将该策略应用到对等体R3上。
[R1]route-policy med permit node 10
[R1-route-policy]if-match acl 2001
[R1-route-policy]apply cost 100
[R1-route-policy]quit
[R1]bgp 64512
[R1-bgp]peer 10.0.15.3 route-policy med export
[R1-bgp]quit
在R3上查看BGP路由表。
<R3>display bgp routing-table
BGP Local router ID is 10.0.15.3
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 4
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.1.3.0/24 0.0.0.0 0 0 i
*>i 10.1.5.0/24 10.0.35.5 0 100 0 i
*>i 10.1.11.0/24 10.0.35.5 100 0 64512i
* 10.0.15.1 100 0 64512i
[R3]display bgp routing-table 10.1.11.0
BGP local router ID : 10.0.3.3
Local AS number : 100
Paths: 2 available, 1 best, 1 select
BGP routing table entry information of 10.1.11.0/24:
From: 10.0.35.5 (10.0.5.5)
Route Duration: 00h00m33s
Relay IP Nexthop: 0.0.0.0
Relay IP Out-Interface: Serial3/0/0
Original nexthop: 10.0.35.5
Qos information : 0x0
AS-path 64512, origin igp, localpref 100, pref-val 0, valid, internal, best, select, active, pre 255
Advertised to such 2 peers:
10.0.23.2
10.0.15.1
BGP routing table entry information of 10.1.11.0/24:
From: 10.0.15.1 (10.0.1.1)
Route Duration: 18h52m36s
Direct Out-interface: GigabitEthernet0/0/2
Original nexthop: 10.0.15.1
Qos information : 0x0
AS-path 64512, origin igp, MED 100, pref-val 0, valid, external, pre 255, not preferred for MED
Not advertised to any peer yet
MED值越小越优先。
最后观察现象，可以和步骤四中达到同样的选路效果。
附加实验: 思考并验证
思考在完成了步骤六以后，关闭R1的S1/0/0接口，那么在R2上学习到的关于10.1.11.0/24的MED值是多少？
思考可否使用路由策略将AS-Path属性里的某个AS删除？
最终设备配置
<R1>display current-configuration
[V200R007C00SPC600]
#
sysname R1
#
router id 10.0.2.2
#
interface Serial1/0/0
link-protocol ppp
ip address 10.0.12.1 255.255.255.0
#
interface Serial3/0/0
link-protocol ppp
ip address 10.0.14.1 255.255.255.0
#
interface GigabitEthernet0/0/1
ip address 10.0.111.1 255.255.255.0
#
interface GigabitEthernet0/0/2
ip address 10.0.15.1 255.255.255.0
#
interface LoopBack0
ip address 10.0.1.1 255.255.255.255
#
bgp 64512
peer 10.0.12.2 as-number 200
peer 10.0.15.3 as-number 100
group as64512 internal
peer 10.0.11.11 as-number 64512
peer 10.0.11.11 group as64512
peer 10.0.11.11 connect-interface LoopBack0
#
ipv4-family unicast
undo synchronization
maximum load-balancing 4
peer 10.0.12.2 enable
peer 10.0.12.2 route-policy as_path export
peer 10.0.15.3 enable
peer 10.0.15.3 route-policy med export
peer as64512 enable
peer as64512 route-policy Pref1 export
peer 10.0.11.11 enable
peer 10.0.11.11 group as64512
#
ospf 1
area 0.0.0.0
network 10.0.1.1 0.0.0.0
network 10.0.111.1 0.0.0.0
#
route-policy as_path permit node 10
if-match acl 2001
apply as-path 64512 64512 additive
#
route-policy Pref1 permit node 10
if-match acl 2002
apply local-preference 110
#
route-policy Pref1 permit node 20
#
route-policy med permit node 10
if-match acl 2001
apply cost 100
#
return
<R2>display current-configuration
[V200R007C00SPC600]
#
sysname R2
#
router id 10.0.2.2
#
interface Serial1/0/0
link-protocol ppp
ip address 10.0.12.2 255.255.255.0
#
interface Serial2/0/0
link-protocol ppp
ip address 10.0.23.2 255.255.255.0
#
interface LoopBack0
ip address 10.0.2.2 255.255.255.0
#
bgp 200
peer 10.0.12.1 as-number 64512
peer 10.0.23.3 as-number 100
#
ipv4-family unicast
undo synchronization
maximum load-balancing 4
peer 10.0.12.1 enable
peer 10.0.23.3 enable
#
return
<R3>display current-configuration
[V200R007C00SPC600]
#
sysname R3
#
router id 10.0.3.3
#
interface Serial2/0/0
link-protocol ppp
ip address 10.0.23.3 255.255.255.0
#
interface Serial3/0/0
link-protocol ppp
ip address 10.0.35.3 255.255.255.0
#
interface GigabitEthernet0/0/2
ip address 10.0.15.3 255.255.255.0
#
interface LoopBack0
ip address 10.0.3.3 255.255.255.255
#
interface LoopBack1
ip address 10.1.3.3 255.255.255.0
#
bgp 100
peer 10.0.15.1 as-number 64512
peer 10.0.23.2 as-number 200
peer 10.0.35.5 as-number 100
#
ipv4-family unicast
undo synchronization
network 10.1.3.0 255.255.255.0
maximum load-balancing 4
peer 10.0.15.1 enable
peer 10.0.23.2 enable
peer 10.0.35.5 enable
#
return
<R4>display current-configuration
[V200R007C00SPC600]
#
sysname R4
#
router id 10.0.4.4
#
interface Serial1/0/0
link-protocol ppp
ip address 10.0.14.4 255.255.255.0
#
interface GigabitEthernet0/0/0
ip address 10.0.45.4 255.255.255.0
#
interface GigabitEthernet0/0/1
ip address 10.0.114.4 255.255.255.0
#
interface LoopBack0
ip address 10.0.4.4 255.255.255.255
#
bgp 64512
peer 10.0.45.5 as-number 100
group as64512 internal
peer 10.0.11.11 as-number 64512
peer 10.0.11.11 group as64512
peer 10.0.11.11 connect-interface LoopBack0
#
ipv4-family unicast
undo synchronization
maximum load-balancing 4
peer 10.0.45.5 enable
peer as64512 enable
peer as64512 route-policy Pref4 export
peer 10.0.11.11 enable
peer 10.0.11.11 group as64512
#
ospf 1
area 0.0.0.0
network 10.0.114.4 0.0.0.0
network 10.0.4.4 0.0.0.0
#
route-policy Pref4 permit node 10
if-match acl 2001
apply local-preference 110
#
route-policy Pref4 permit node 20
#
return
<R5>display current-configuration
[V200R007C00SPC600]
#
sysname R5
#
router id 10.0.5.5
#
interface Serial1/0/0
link-protocol ppp
ip address 10.0.35.5 255.255.255.0
#
interface GigabitEthernet0/0/0
ip address 10.0.45.5 255.255.255.0
#
interface LoopBack0
ip address 10.0.5.5 255.255.255.255
#
interface LoopBack1
ip address 10.1.5.5 255.255.255.0
#
bgp 100
peer 10.0.35.3 as-number 100
peer 10.0.45.4 as-number 64512
#
ipv4-family unicast
undo synchronization
network 10.1.5.0 255.255.255.0
maximum load-balancing 4
peer 10.0.35.3 enable
peer 10.0.45.4 enable
#
return