利用BGP路由的Community属性实现流量分担
1、首先来介绍下网络拓扑图：路由器r1、r2、r3属于 AS 100并且内部运行ospf协议。r4属于AS 200。r5、6、7属于AS 300并且内部运行ospf协议。本例中a路径：1.1.1.1 访问172.16.10.1 走 r1-r2-r4-r5-r7。b路径:1.1.1.1 访问172.16.20.1 走路径r1-r3-r4-r6-r7。介绍完拓扑图后接下来将为大家依次介绍每个路由器的配置！
3、路由器r2的核心配置：#interface GigabitEthernet0/0/0ip address 10.0.12.2 255.255.255.0#interface GigabitEthernet0/0/1ip address 10.0.24.2 255.255.255.0#interface GigabitEthernet0/0/2#interface NULL0#interface LoopBack0ip address 2.2.2.2 255.255.255.255#bgp 100router-id 2.2.2.2peer 10.0.12.1 as-number 100peer 10.0.24.4 as-number 200#ipv4-family unicast undo synchronization network 2.2.2.2 255.255.255.255 peer 10.0.12.1 enable peer 10.0.24.4 enable#ospf 1 router-id 2.2.2.2area 0.0.0.0 network 2.2.2.2 0.0.0.0 network 10.0.12.0 0.0.0.255 network 10.0.24.0 0.0.0.255#路由器r3的核心配置：#interface GigabitEthernet0/0/0ip address 10.0.34.3 255.255.255.0#interface GigabitEthernet0/0/1ip address 10.0.13.3 255.255.255.0#interface GigabitEthernet0/0/2#interface NULL0#interface LoopBack0ip address 3.3.3.3 255.255.255.255#bgp 100router-id 3.3.3.3peer 10.0.13.1 as-number 100peer 10.0.34.4 as-number 200#ipv4-family unicast undo synchronization network 3.3.3.3 255.255.255.255 peer 10.0.13.1 enable peer 10.0.34.4 enable#ospf 1 router-id 3.3.3.3area 0.0.0.0 network 3.3.3.3 0.0.0.0 network 10.0.13.0 0.0.0.255 network 10.0.34.0 0.0.0.255#
4、路由器r4的核心配置：#interface GigabitEthernet0/0/0ip address 10.0.34.4 255.255.255.0#interface GigabitEthernet0/0/1ip address 10.0.24.4 255.255.255.0#interface GigabitEthernet4/0/0ip address 10.0.46.4 255.255.255.0#interface GigabitEthernet4/0/1ip address 10.0.45.4 255.255.255.0#interface LoopBack0ip address 4.4.4.4 255.255.255.255#bgp 200router-id 4.4.4.4peer 10.0.24.2 as-number 100peer 10.0.34.3 as-number 100peer 10.0.45.5 as-number 300peer 10.0.46.6 as-number 300#ipv4-family unicast undo synchronization network 4.4.4.4 255.255.255.255 peer 10.0.24.2 enable peer 10.0.24.2 route-policy med10 export peer 10.0.24.2 advertise-community peer 10.0.34.3 enable peer 10.0.34.3 route-policy med20 export peer 10.0.34.3 advertise-community peer 10.0.45.5 enable peer 10.0.45.5 route-policy 10 import peer 10.0.46.6 enable peer 10.0.46.6 route-policy 10 import#route-policy 10 permit node 10if-match community-filter 10apply ip-address next-hop 10.0.45.5#route-policy 10 permit node 20if-match community-filter 20apply ip-address next-hop 10.0.46.6#route-policy med10 permit node 10if-match community-filter 10apply cost 100#route-policy med10 permit node 20if-match community-filter 20apply cost 200#route-policy med20 permit node 10if-match community-filter 20apply cost 100#route-policy med20 permit node 20if-match community-filter 10apply cost 200#ip community-filter 10 permit 300:100ip community-filter 20 permit 300:200#
7、配置好所有设备后我们在r4上查看172.16.10.0和172.16.20.0网段的community值：<ar4>dis bgp routing-table 172.16.10.1BGP local router ID : 4.4.4.4Local AS number : 200Paths: 2 available, 1 best, 1 selectBGP routing table entry information of 172.16.10.0/24:From: 10.0.45.5 (5.5.5.5)Route Duration: 01h13m49sDirect Out-interface: GigabitEthernet4/0/1Original nexthop: 10.0.45.5Qos information : 0x0Community:<300:100>AS-path 300, origin igp, pref-val 0, valid, external, best, select, active, pre255Advertised to such 4 peers: 10.0.45.5 10.0.46.6 10.0.24.2 10.0.34.3BGP routing table entry information of 172.16.10.0/24:From: 10.0.46.6 (6.6.6.6)Route Duration: 01h13m39sRelay IP Nexthop: 0.0.0.0Relay IP Out-Interface: GigabitEthernet4/0/1Original nexthop: 10.0.45.5Qos information : 0x0Community:<300:100>通过上面输出的信息可以看到172.16.10.1的Community属性值是<300:100>！
