BGP 管理距离修改及分析
所谓管理距离（AD）就是指一种路由协议的可信度，对于多种不同的路由协议到同一个目的地的路由信息，路由器首先根据管理距离的大小决定相信哪一个路由协议。管理距离只用于多种协议的情况下会使用，同一路由协议中，选路按照该路由协议的选路机制，而不会用管理距离来选路。管理距离越小，其优先级越高，可信度越高。
如上图，在R6上查看BGP路由表
R6#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B -BGP
D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,E2 - OSPF external type 2
i - IS-IS, su - IS-ISsummary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * -candidate default, U - per-user static route
o - ODR, P - periodicdownloaded static route, H - NHRP, l - LISP
+ - replicated route, % -next hop override
Gateway of last resort is not set
4.0.0.0/32 is subnetted, 1subnets
B 4.4.4.4 [200/0] via 2.2.2.2, 00:00:06
5.0.0.0/32 is subnetted, 1subnets
B 5.5.5.5 [20/0] via 56.1.1.1, 00:03:51
B 192.168.10.0/24 [200/0] via 2.2.2.2, 00:00:06
B 192.168.11.0/24 [200/0] via 2.2.2.2, 00:00:06
B 192.168.12.0/24 [200/0] via 2.2.2.2, 00:00:06
从上述可知，从IBGP邻居R2收到的路由，其管理距离为200，从EBGP邻居R5收到的路由，其管理距离为20。
小结：iBGP的管理距离为200，EBGP的管理距离为20，BGP的本地（Local）AD默认等同于IBGP，本地路由指通过network命令得到。
现通过distance命令修改从IBGP过来的路由管理距离为150；
R6(config)#router bgp 65414
R6(config-router)#distance150 2.2.2.2 0.0.0.0 //修改来至邻居2.2.2.2的管理距离
配置完成后，清邻居关系（需要硬清，软清不起作用，无法观查效果）后，可查看BGP路由情况
R6#clear ip bgp *
R6#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B -BGP
D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,E2 - OSPF external type 2
i - IS-IS, su - IS-ISsummary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * -candidate default, U - per-user static route
o - ODR, P - periodicdownloaded static route, H - NHRP, l - LISP
+ - replicated route, % -next hop override
Gateway of last resort is not set
4.0.0.0/32 is subnetted, 1subnets
B 4.4.4.4 [150/0] via 2.2.2.2, 00:00:46
5.0.0.0/32 is subnetted, 1subnets
B 5.5.5.5 [20/0] via 56.1.1.1,00:00:46
B 192.168.10.0/24 [150/0] via 2.2.2.2, 00:00:46
B 192.168.11.0/24 [150/0] via 2.2.2.2, 00:00:46
B 192.168.12.0/24 [150/0] via 2.2.2.2, 00:00:46
除了指定邻居修改所有的路由，还可以指定相应邻居，后面跟ACL，匹配需要修改的路由的管理距离；
现配置一个ACL，匹配4.4.4.4/32的路由，修改其管理距离为120，修改来至R5的5.5.5.5/32的管理距离为50，其它的管理距离为默认；
R6(config)#access-list 10 permit 4.4.4.4 0.0.0.0 R6(config)#router bgp 65414 R6(config-router)#distance 150 2.2.2.2 0.0.0.0 10 R6(config-router)#distance 50 56.1.1.1 0.0.0.0
R6#clear ip bgp * //硬清，软清看不见效果
R6#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B -BGP
D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,E2 - OSPF external type 2
i - IS-IS, su - IS-ISsummary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * -candidate default, U - per-user static route
o - ODR, P - periodicdownloaded static route, H - NHRP, l - LISP
+ - replicated route, % -next hop override
Gateway of last resort is not set
4.0.0.0/32 is subnetted, 1subnets
B 4.4.4.4 [150/0] via 2.2.2.2, 00:00:16
5.0.0.0/32 is subnetted, 1subnets
B 5.5.5.5 [50/0] via 56.1.1.1, 00:00:16
B 192.168.10.0/24 [200/0] via 2.2.2.2,00:00:16
B 192.168.11.0/24 [200/0] via 2.2.2.2,00:00:16
B 192.168.12.0/24 [200/0] via 2.2.2.2,00:00:16
从上看出，来至R2的4.4.4.4/32路由的管理距离为150，来至R5的5.5.5.5/32路由的管理距离为50，其它的为默认的200；
现在在R5和R2上分别查看BGP的路由情况；
R5#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B -BGP
D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,E2 - OSPF external type 2
i - IS-IS, su - IS-ISsummary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * -candidate default, U - per-user static route
o - ODR, P - periodicdownloaded static route, H - NHRP, l - LISP
+ - replicated route, % -next hop override
Gateway of last resort is not set
1.0.0.0/32 is subnetted, 1subnets
B 1.1.1.1 [20/0] via56.1.1.2, 00:00:25
2.0.0.0/32 is subnetted, 1subnets
B 2.2.2.2 [20/0] via56.1.1.2, 00:00:25
3.0.0.0/32 is subnetted, 1subnets
B 3.3.3.3 [20/0] via56.1.1.2, 00:00:25
B 192.168.10.0/24 [20/0] via56.1.1.2, 00:00:25
B 192.168.11.0/24 [20/0] via56.1.1.2, 00:00:25
B 192.168.12.0/24 [20/0] via56.1.1.2, 00:00:25
R2#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B -BGP
D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,E2 - OSPF external type 2
i - IS-IS, su - IS-ISsummary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * -candidate default, U - per-user static route
o - ODR, P - periodic downloadedstatic route, H - NHRP, l - LISP
+ - replicated route, % -next hop override
Gateway of last resort is not set
4.0.0.0/32 is subnetted, 1subnets
B 4.4.4.4 [200/0] via3.3.3.3, 00:27:30
5.0.0.0/32 is subnetted, 1subnets
B 5.5.5.5 [200/0] via 6.6.6.6, 00:00:34
B 192.168.10.0/24 [200/0] via3.3.3.3, 00:27:30
B 192.168.11.0/24 [200/0] via3.3.3.3, 00:27:30
B 192.168.12.0/24 [200/0] via3.3.3.3, 00:27:30
从上可看出，R2和R5上的路由条目的管理距离为默认值，并没有因R6修改了管理距离而改变；
小结：管理距离只具有本地有意义，不可传递给对等体邻居（包括EBGP和IBGP）
现使用命令distance X Y Z
X为外部路由Distance for routes external to the AS
Y为内部路由Distance for routes internal to the AS
Z为本地路由Distance for local routes（包括network和聚合路由）
现配置以下命令后，将外部的管理距离设置为120，内部的设置为150，本地的为111；
R6(config)#router bgp 65414
R6(config-router)#distancebgp 120 150 111
R6(config-router)#aggregate-address192.168.8.0 255.255.248.0 summary-only
在R6上查看BGP路由表情况：
R6#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B -BGP
D - EIGRP, EX - EIGRPexternal, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA externaltype 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,E2 - OSPF external type 2
i - IS-IS, su - IS-ISsummary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * -candidate default, U - per-user static route
o - ODR, P - periodicdownloaded static route, H - NHRP, l - LISP
+ - replicated route, % -next hop override
Gateway of last resort is not set
4.0.0.0/32 is subnetted, 1subnets
B 4.4.4.4 [150/0] via 2.2.2.2, 00:08:24
5.0.0.0/32 is subnetted, 1subnets
B 5.5.5.5 [120/0] via 56.1.1.1, 00:08:24
B 192.168.8.0/21 [111/0] via 0.0.0.0, 00:00:21, Null0
B 192.168.10.0/24 [150/0] via 2.2.2.2,00:00:21
B 192.168.11.0/24 [150/0] via 2.2.2.2,00:00:21
B 192.168.12.0/24 [150/0] via 2.2.2.2,00:00:21
从上可知，管理距离修改成功。