实验目的:
l 掌握BGP 13条选路
l 掌握选路应用场景
实验说明：
l 通过此实验练习，可以更好的掌握BGP 13条选路
实验环境：
l 三台支持SPSERVICES的IOS的路由器
l 直通线
实验拓扑：
实验步骤：
R1(config)#interface f0/0
R1(config-if)#ip address 12.1.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#int f1/0
R1(config-if)#ip address 13.1.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#int lo 0
R1(config-if)#ip address 1.1.1.1 255.255.255.255
R1(config)#router ospf 110
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 12.1.1.1 0.0.0.0 a 0
R1(config-router)#network 1.1.1.1 0.0.0.0 a 0
R1(config)#router bgp 100
R1(config-router)#bgp router-id 1.1.1.1
R1(config-router)#neighbor 2.2.2.2 remote-as 100
R1(config-router)#neighbor 2.2.2.2 update-source lo 0
R1(config-router)#neighbor 2.2.2.2 next-hop-self
R1(config-router)#neighbor 13.1.1.3 remote-as 200
R2(config)#interface f0/0
R2(config-if)#ip address 12.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#int f1/1
R2(config-if)#ip address 23.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config)#int lo 0
R2(config-if)#ip address 2.2.2.2 255.255.255.255
R2(config)#router ospf 110
R2(config-router)#router-id 2.2.2.2
R2(config-router)#network 12.1.1.2 0.0.0.0 a 0
R2(config-router)#net
R2(config-router)#network 2.2.2.2 0.0.0.0 a 0
R2(config-router)#
*Sep 28 09:03:58.399: %OSPF-5-ADJCHG: Process 110, Nbr 1.1.1.1 on FastEthernet0/0 from LOADING to FULL, Loading Done
R2(config-router)#
R2(config-router)#
R2(config-router)#
R2(config-router)#
R2(config-router)#exit
R2(config)#router bgp 100
R2(config-router)#bgp ro
R2(config-router)#bgp router-id 2.2.2.2
R2(config-router)#nei
R2(config-router)#neighbor 1.1.1.1 remot
R2(config-router)#neighbor 1.1.1.1 remote-as 100
R2(config-router)#nei
R2(config-router)#neighbor 1.1.1.1 up
R2(config-router)#neighbor 1.1.1.1 update-source lo 0
R2(config-router)#nei
R2(config-router)#neighbor 1.1.1.1 ne
R2(config-router)#neighbor 1.1.1.1 next-hop-s
*Sep 28 09:05:01.299: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up
R2(config-router)#neighbor 1.1.1.1 next-hop-s
R2(config-router)#neighbor 1.1.1.1 next-hop-self
R2(config-router)#nei
R2(config-router)#neighbor 23.1.1.3 remot
R2(config-router)#neighbor 23.1.1.3 remote-as 200
R3(config-router)#network 3.3.3.3 mask 255.255.255.255
选路原则1：weight值
没设置weight值前：
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* i3.3.3.3/32 2.2.2.2 0 100 0 200 i
*> 13.1.1.3 0 0 200 i
设置weight值后：
R1(config)#router bgp 100
R1(config-router)#neighbor 2.2.2.2 weight 1
OR
R1(config)#access-list 1 permit 3.3.3.3 0.0.0.0
R1(config)#route-map wei permit 10
R1(config-route-map)#match ip address 1
R1(config-route-map)#set weight 1
R1(config)#route-map wei permit 20
R1(config)#router bgp 100
R1(config-router)#neighbor 2.2.2.2 route-map wei in
验证：
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* >i3.3.3.3/32 2.2.2.2 0 100 1 200 i
* 13.1.1.3 0 0 200 i
选路原则2：第一条选路一致，比较local-preference
使用原则：
1， 只能在IBGP传递，不能在EBGP传递
2， 告诉本路由器或IBGP邻居如何离开本AS
3， 默认local-preference为100，越大越好
没有设置local-preference前：
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* i3.3.3.3/32 2.2.2.2 0 100 0 200 i
*> 13.1.1.3 0 0 200 i
设置local-preference后：
R1(config)#router bgp 100
R1(config-router)#bgp default local-preference 101
OR
R2(config)#access-list 1 permit 3.3.3.3 0.0.0.0
R2(config)#route-map loc permit 10
R2(config-route-map)#match ip address 1
R2(config-route-map)#set local-preference 101
R2(config)#route-map loc permit 20
R2(config)#router bgp 100
R2(config-router)#neighbor 1.1.1.1 route-map loc out
选路原则3：前两条选路一致，比较起源本地
本地起源意味着该路由通过本地路由器起源，
起源的方式：network，redistribution ，agg
当路由器本地起源的路由同时通过其他邻居收到此路由
优先选择本地起源的路由
R3#show ip bgp 3.3.3.3
BGP routing table entry for 3.3.3.3/32, version 2
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to update-groups:
1
Local
0.0.0.0 from 0.0.0.0 (3.3.3.3)
Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best
选路原则4：前三条选路一致，比较AS-path
AS-path原则：
优选最短的AS-path
对EBGP邻居起作用
环境设定：R1和R2同时有条路由100.100.100.100/32发送给R3
R1/R2(config)#int lo 100
R1/R2(config-if)#ip address 100.100.100.100 255.255.255.255
R1/R2(config-router)#network 100.100.100.100 mask 255.255.255.255
此时：
R3#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 100.100.100.100/32
23.1.1.2 0 0 100 i
*> 13.1.1.1 0 0 100 i
设置AS-path选路原则：当As-path经历的AS数约小越优先
R2(config)#access-list 2 permit 100.100.100.100 0.0.0.0
R2(config)#route-map as-path permit 10
R2(config-route-map)#match ip address 2
R2(config-route-map)#set as-path prepend 300 400
R2(config)#route-map as-path permit 20
R2(config)#router bgp 100
R2(config-router)#neighbor 23.1.1.3 route-map as-path out
R3(config)#access-list 1 permit 100.100.100.100 0.0.0.0
R3(config)#route-map as-path permit 10
R3(config-route-map)#match ip address 1
R3(config-route-map)#set as-path prepend last-as 3
R3(config)#route-map as-path permit 20
R3(config)#router bgp 200
R3(config-router)#neighbor 13.1.1.1 route-map as-path in
验证：
R3#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 8
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
1
100 300 400
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
100 100 100 100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external
注：
通过bgp bestpath as-path ignore忽略该规则的选举
选路原则5：前四条选路一致，比较Origin
比较原则：i>e>?
例如：
R3#show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 3.3.3.3/32 0.0.0.0 0 32768 i
100 100 100 100 i
*> 200.200.200.200/32
33.33.33.34 0 32768 ?
选路原则6：前五条选路一致，比较MED
MED规则：
告诉EBGP邻居怎么进入本AS或告诉自己以及IBGP邻居如何离开本AS
从EBGP学习到BGP条目携带的MED只可以传给其他IBGP
从IBGP或下一个EBGP学习到BGP路由条目的MED为0或者其他值，缺省不会传给EBGP邻居，显示为空。
汇总方式生成的BGP条目，MED值为空
实验拓扑：
实验步骤（基于之前的配置）
R3(config)#router bgp 200
R3(config-router)#neighbor 34.1.1.4 remote-as 300
R4(config)#router bgp 300
R4(config-router)#bgp router-id 4.4.4.4
R4(config-router)#neighbor 34.1.1.3 remote-as 200
修改MED值：
R2(config)#route-map med permit 10
R2(config-route-map)#match ip address 2
R2(config-route-map)#set metric 200
R2(config)#route-map med permit 20
R2(config)#router bgp 100
R2(config-router)#neighbor 23.1.1.3 route-map med out
R3(config)#route-map med permit 10
R3(config-route-map)#match ip address 1
R3(config-route-map)#set metric 100
R3(config)#route-map med permit 20
R3(config-router)#neighbor 13.1.1.1 route-map med in
验证：
R3#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 12
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x10840
Advertised to update-groups:
1
100
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, metric 200, localpref 100, valid, external
100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, metric 100, localpref 100, valid, external, best
R4#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
200 100
34.1.1.3 from 34.1.1.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external, best
通过Bgp bestpath med missing-med-worst来显示隐藏的med值
R4(config)#router bgp 300
R4(config-router)#bgp bestpath med missing-as-worst
R4#show ip bgp 100.100.100.100
*Sep 28 10:57:59.611: %SYS-5-CONFIG_I: Configured from console by console
R4#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
200 100
34.1.1.3 from 34.1.1.3 (3.3.3.3)
Origin IGP, metric 4294967295, localpref 100, valid, external, best
注：med比较条件：第一个AS号相同才比较med这个选路原则
通过bgp always-compare-med打破AS相同的限制
R4(config)#router bgp 300
R4(config-router)#bgp always-compare-med
选路原则7：前六条选路一致，比较EBGP邻居优于IBGP邻居学来的路由
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* i3.3.3.3/32 2.2.2.2 0 100 0 200 i
*> 13.1.1.3 0 0 200 i
选路原则8：前七条选路一致，比较最近的IGP邻居
实验拓扑：
实验步骤（基本配置同上，略）：
R3(config-if)#router bgp 200
R3(config-router)#network 3.3.3.3 mask 255.255.255.255
验证：
R4#show ip bgp 3.3.3.3
BGP routing table entry for 3.3.3.3/32, version 3
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
200
1.1.1.1 (metric 2) from 1.1.1.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
200
2.2.2.2 (metric 2) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal
修改从1.1.1.1学到的IGP开销变大
R1(config-router)#int lo 0
R1(config-if)#ip ospf cost 100
R4#show ip bgp 3.3.3.3
BGP routing table entry for 3.3.3.3/32, version 4
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x800
Not advertised to any peer
200
1.1.1.1 (metric 101) from 1.1.1.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, internal
200
2.2.2.2 (metric 2) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal, best
选路原则9：前八条选路一致，EBGP等价负载均衡
注：默认情况下第八条比不出来的时候，直接跳过第九条，默认第九条关闭，当开启时候，通过第九条能实现ebgp的等价负载均衡
R4(config)#router bgp 100
R4(config-router)#network 4.4.4.4 mask 255.255.255.255
此时：
R3#show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 3
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
2
100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, localpref 100, valid, external
100
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, best
R3#show ip route bgp
4.0.0.0/32 is subnetted, 1 subnets
B 4.4.4.4 [20/0] via 23.1.1.2, 00:01:20
开启负载功能
R3(config)#router bgp 200
R3(config-router)#maximum-paths 2
此时：
R3#show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 4
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Multipath: eBGP
Advertised to update-groups:
2
100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, localpref 100, valid, external, multipath
100
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, multipath, best
R3#show ip route bgp
4.0.0.0/32 is subnetted, 1 subnets
B 4.4.4.4 [20/0] via 23.1.1.2, 00:03:49
[20/0] via 13.1.1.1, 00:02:03
选路原则10：前九条选路一致，选择最老的EBGP邻居
R3#show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 5
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x800
Advertised to update-groups:
2
100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, localpref 100, valid, external
100
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, best
验证：
是23.1.1.2从新建立ebgp邻居关系
R2#clear ip bgp *
R3#show ip bgp summary
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
13.1.1.1 4 100 22 26 6 0 0 00:16:53 1
23.1.1.2 4 100 23 28 0 0 0 00:00:23 0
R3#show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 6
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x840
Advertised to update-groups:
2
100
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external
100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, localpref 100, valid, external, best
注：打上bgp bestpath compare-routerid忽略第十条
选路原则11：前十条选路一致，最低的EBGP/IBGP router-id
验证：
打上bgp bestpath compare-routerid忽略第十条验证EBGP router-id
R3#show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 6
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Advertised to update-groups:
2
100
23.1.1.2 from 23.1.1.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external
100
13.1.1.1 from 13.1.1.1 (1.1.1.1)
Origin IGP, localpref 100, valid, external, best
验证IBGP router-id
R4#show ip bgp 3.3.3.3
BGP routing table entry for 3.3.3.3/32, version 8
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x800
Not advertised to any peer
200
2.2.2.2 (metric 2) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal
200
1.1.1.1 (metric 2) from 1.1.1.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
选路原则12：前十一条选路一致，针对于IBGP Cluster_list短的优先
实验拓扑：
实验步骤：
R1(config)#interface f0/0
R1(config-if)#ip address 12.1.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#int lo 0
R1(config-if)#ip address 1.1.1.1 255.255.255.255
R1(config-if)#int lo 100
R1(config-if)#ip address 100.100.100.100 255.255.255.255
R1(config)#router ospf 110
R1(config-router)#network 1.1.1.1 0.0.0.0 a 0
R1(config-router)#network 12.1.1.1 0.0.0.0 a 0
R1(config)#router bgp 100
R1(config-router)#neighbor 2.2.2.2 remote-as 100
R1(config-router)#neighbor 2.2.2.2 update-source lo 0
R1(config-router)#neighbor 2.2.2.2 next-hop-self
R1(config-router)#network 100.100.100.100 mask 255.255.255.255
R2(config)#interface f0/0
R2(config-if)#ip address 12.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#int f1/1
R2(config-if)#ip address 23.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#int f1/0
R2(config-if)#ip address 24.1.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#int lo 0
R2(config-if)#ip address 2.2.2.2 255.255.255.255
R2(config)#router ospf 110
R2(config-router)#router-id 2.2.2.2
R2(config-router)#network 12.1.1.2 0.0.0.0 a 0
R2(config-router)#network 23.1.1.2 0.0.0.0 a 0
R2(config-router)#network 24.1.1.2 0.0.0.0 a 0
R2(config-router)#network 2.2.2.2 0.0.0.0 a 0
R2(config)#router bgp 100
R2(config-router)#bgp router-id 2.2.2.2
R2(config-router)#neighbor 1.1.1.1 remote-as 100
R2(config-router)#neighbor 1.1.1.1 update-source lo 0
R2(config-router)#neighbor 1.1.1.1 route-reflector-client
R2(config-router)#neighbor 4.4.4.4 remote-as 100
R2(config-router)#neighbor 4.4.4.4 update-source lo 0
R2(config-router)#neighbor 4.4.4.4 route-reflector-client
R2(config-router)#neighbor 3.3.3.3 remote-as 100
R2(config-router)#neighbor 3.3.3.3 update-source lo 0
R2(config-router)#neighbor 3.3.3.3 route-reflector-client
R3(config)#interface f1/1
R3(config-if)#ip address 23.1.1.3 255.255.255.0
R3(config-if)#no shutdown
R3(config-if)#int f0/0
R3(config-if)#ip address 34.1.1.3 255.255.255.0
R3(config-if)#no shutdown
R3(config-if)#int lo 0
R3(config-if)#ip address 3.3.3.3 255.255.255.255
R3(config)#router ospf 110
R3(config-router)#router-id 3.3.3.3
R3(config-router)#network 23.1.1.3 0.0.0.0 a 0
R3(config-router)#network 34.1.1.3 0.0.0.0 a 0
R3(config-router)#network 3.3.3.3 0.0.0.0 a 0
R3(config)#router bgp 100
R3(config-router)#bgp router-id 3.3.3.3
R3(config-router)#neighbor 2.2.2.2 remote-as 100
R3(config-router)#neighbor 2.2.2.2 update-source lo 0
R3(config-router)#neighbor 4.4.4.4 remote-as 100
R3(config-router)#neighbor 4.4.4.4 update-source lo 0
R3(config-router)#neighbor 4.4.4.4 route-reflector-client
R4(config)#interface f0/0
R4(config-if)#ip address 34.1.1.4 255.255.255.0
R4(config-if)#no shutdown
R4(config-if)#int f1/0
R4(config-if)#ip address 24.1.1.4 255.255.255.0
R4(config-if)#no shutdown
R4(config-if)#int lo 0
R4(config-if)#ip address 4.4.4.4 255.255.255.255
R4(config)#router ospf 110
R4(config-router)#router-id 4.4.4.4
R4(config-router)#network 34.1.1.4 0.0.0.0 a 0
R4(config-router)#network 24.1.1.4 0.0.0.0 a 0
R4(config-router)#network 4.4.4.4 0.0.0.0 a 0
R4(config)#router bgp 100
R4(config-router)#bgp router-id 4.4.4.4
R4(config-router)#neighbor 3.3.3.3 remot
R4(config-router)#neighbor 3.3.3.3 remote-as 100
R4(config-router)#neighbor 3.3.3.3 update-source lo 0
R4(config-router)#neighbor 2.2.2.2 remote-as 100
R4(config-router)#neighbor 2.2.2.2 update-source lo 0
验证：
R4#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
Local
1.1.1.1 (metric 3) from 3.3.3.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal
Originator: 100.100.100.100, Cluster list: 3.3.3.3, 2.2.2.2
Local
1.1.1.1 (metric 3) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 100.100.100.100, Cluster list: 2.2.2.2
选路原则13：前十二条选路一致，最低的邻居地址的路径
实验拓扑：
实验步骤（略）
验证：
R4#show ip bgp 100.100.100.100
BGP routing table entry for 100.100.100.100/32, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Not advertised to any peer
Local
1.1.1.1 (metric 3) from 3.3.3.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal
Originator: 100.100.100.100, Cluster list: 3.3.3.3
Local
1.1.1.1 (metric 3) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 100.100.100.100, Cluster list: 2.2.2.2
