BGP路径选择
拓扑图R1配置R2配置
R3配置R4配置测试R1 BGP路由表R4 BGP路由表
拓扑图
网段 网段可以自己设置，为了方便记忆，我是根据路由器的ID定义网段的，接口IP地址的主机位就是路由的ID 例如R1和R2之间的网段是12.1.1.0/24，R1上的g0/0/0的IP地址就是12.1.1.1 24 BGP选路原则 1.若去往目的网络的路由下一跳不可达，则可以忽略此路由 2.Preferred-Valve 优先级以数值高的路由优先 3.Local-Preferece 优先级以数值高的路由优先 4.聚合路由优先级高于数值高的路由优先 5.本地手动聚合路由的优先级高于本地自动聚合的路由 6.本地通过Wetwork 命令引入的路由的优先级高于本地通过Import-route 命令引入的路由 7.AS路径长度最短（最少格数）的路径优先级高 8.比较Origin属性，IGP优先级高于EGP，EGP优先级高于Incomplete 9.选择MED优先级较小的路由 10.EBGP路由优先级高于IBGP路由 11.BGP优先选择到BGP下跳的IGP度量低的路径 当以上全部相同，则为等价路由，可以负载分担（注：AS-PATH必须一致），当负载分担时，以下3条则无效 12.比较Cluster-List 长度，短者优先 13.比较Originator ID(如果没有Originator ID,则用Router ID比较）选择数值较小的路径 14.比较对等体的IP地址，选择IP地址数值最小的路径 BGP可以做策略的地方 AS路径（AS-PATH)属性 越短越优 IGBP EGBP 下一跳（Next-Hop）属性 不改 Local-perf 属性 本地优先级越大越优 分流 IBGP MED属性 值越小链路越优秀 EGBP
R1配置
** IP地址设置**
#
interface GigabitEthernet0/0/0
ip address 12.1.1.1 255.255.255.0
#
interface GigabitEthernet0/0/1
ip address 13.1.1.1 255.255.255.0
#
interface LoopBack0
ip address 1.1.1.1 255.255.255.255
设置BGP
#
bgp 100
router-id 1.1.1.1 //宣告id
peer 2.2.2.2 as-number 200 //和2.2.2.2 bgp200建立邻接
peer 2.2.2.2 ebgp-max-hop 255 //最大跳数设置为255
peer 2.2.2.2 connect-interface LoopBack0
peer 3.3.3.3 as-number 200
peer 3.3.3.3 ebgp-max-hop 255
peer 3.3.3.3 connect-interface LoopBack0
#
ipv4-family unicast
undo synchronization
network 1.1.1.1 255.255.255.255
network 12.1.1.0 255.255.255.0
network 13.1.1.0 255.255.255.0
peer 2.2.2.2 enable
peer 3.3.3.3 enable
由于要跨AS通信，所以设置静态路由
#
ip route-static 2.2.2.2 255.255.255.255 12.1.1.2
ip route-static 3.3.3.3 255.255.255.255 13.1.1.3
R2配置
#
interface GigabitEthernet0/0/0
ip address 12.1.1.2 255.255.255.0
#
interface GigabitEthernet0/0/1
ip address 24.1.1.2 255.255.255.0
#
interface LoopBack0
ip address 2.2.2.2 255.255.255.255
#
bgp 200
router-id 2.2.2.2
peer 1.1.1.1 as-number 100
peer 1.1.1.1 ebgp-max-hop 255
peer 1.1.1.1 connect-interface LoopBack0
peer 3.3.3.3 as-number 200
peer 3.3.3.3 connect-interface LoopBack0
peer 4.4.4.4 as-number 200
peer 4.4.4.4 connect-interface LoopBack0
#
ipv4-family unicast
undo synchronization
network 1.1.1.1 255.255.255.255
network 2.2.2.2 255.255.255.255
network 3.3.3.3 255.255.255.255
network 4.4.4.4 255.255.255.255
network 12.1.1.0 255.255.255.0
network 24.1.1.0 255.255.255.0
peer 1.1.1.1 enable
peer 1.1.1.1 route-policy med export //告诉其它邻居
peer 3.3.3.3 enable
peer 3.3.3.3 next-hop-local
peer 4.4.4.4 enable
peer 4.4.4.4 next-hop-local
#
ospf 1 router-id 2.2.2.2
area 0.0.0.0
network 2.2.2.2 0.0.0.0
network 24.1.1.0 0.0.0.255
路径选择
#
acl number 2000
rule 5 permit source 4.4.4.4 0 //抓4.4.4.4包
# //做策略
route-policy med permit node 10
if-match acl 2000
apply cost + 50 //cost值加50（选择MED优先级较小的路由）
#
route-policy med permit node 20 //放行其它流量
做完策略后，R2上4.4.4.4的med值会变大，可以看理论的第9条，所以4.4.4.4的路由就会选择R3
ip route-static 1.1.1.1 255.255.255.255 12.1.1.1 //回指一条静态路由
R3配置
#
interface GigabitEthernet0/0/0
ip address 13.1.1.3 255.255.255.0
#
interface GigabitEthernet0/0/1
ip address 34.1.1.3 255.255.255.0
#
interface LoopBack0
ip address 3.3.3.3 255.255.255.255
#
bgp 200
router-id 3.3.3.3
peer 1.1.1.1 as-number 100
peer 1.1.1.1 ebgp-max-hop 255
peer 1.1.1.1 connect-interface LoopBack0
peer 2.2.2.2 as-number 200
peer 2.2.2.2 connect-interface LoopBack0
peer 4.4.4.4 as-number 200
peer 4.4.4.4 connect-interface LoopBack0
#
ipv4-family unicast
undo synchronization
network 1.1.1.1 255.255.255.255
network 2.2.2.2 255.255.255.255
network 3.3.3.3 255.255.255.255
network 4.4.4.4 255.255.255.255
network 13.1.1.0 255.255.255.0
network 34.1.1.0 255.255.255.0
peer 1.1.1.1 enable
peer 2.2.2.2 enable
peer 2.2.2.2 next-hop-local
peer 4.4.4.4 enable
peer 4.4.4.4 route-policy local export
peer 4.4.4.4 next-hop-local
#
ospf 1 router-id 3.3.3.3
area 0.0.0.0
network 3.3.3.3 0.0.0.0
network 34.1.1.0 0.0.0.255
#
acl number 2000
rule 5 permit source 1.1.1.1 0
#
route-policy local permit node 10
if-match acl 2000
apply local-preference 200 //Local-Preferece 优先级以数值高的路由优先
#
route-policy local permit node 20
做完策略后1.1.1.1的Local值会变大，可以看理论第三条：Local-Preferece 优先级以数值高的路由优先，所以1.1.1.1的路由会走R3
#
ip route-static 1.1.1.1 255.255.255.255 13.1.1.1
R4配置
#
interface GigabitEthernet0/0/0
ip address 24.1.1.4 255.255.255.0
#
interface GigabitEthernet0/0/1
ip address 34.1.1.4 255.255.255.0
#
interface LoopBack0
ip address 4.4.4.4 255.255.255.255
#
bgp 200
router-id 4.4.4.4
peer 2.2.2.2 as-number 200
peer 2.2.2.2 connect-interface LoopBack0
peer 3.3.3.3 as-number 200
peer 3.3.3.3 connect-interface LoopBack0
#
ipv4-family unicast
undo synchronization
network 4.4.4.4 255.255.255.255
network 24.1.1.0 255.255.255.0
network 34.1.1.0 255.255.255.0
peer 2.2.2.2 enable
peer 3.3.3.3 enable
#
ospf 1 router-id 4.4.4.4
area 0.0.0.0
network 24.1.1.0 0.0.0.255
network 4.4.4.4 0.0.0.0
network 34.1.1.0 0.0.0.255
测试
R1 BGP路由表
R4 BGP路由表
