【语法】： ip prefix-list list-name [ seq seq-value ] { deny|permit} network/len [ ge ge-value ] [ le le-value ]
ge，大于等于 le，小于等于 如果只出现ge，范围从 ge-value --> 32 如果只出现le,范围从length–>le-value
【分析】 前缀列表中的ge ，le 的理解 假定有三条路由 172.16.0.0/16 172.16.10.0/24 172.16.11.0/24
其实ACL也有方法定义一个范围，以实现类似prefix-list的功能。 access-list 10 permit 199.172.0.0 0.0.3.0,这里这个0.0.3.0，即起了一个定义范围的作用，0表示match,1表示不关心。这样的话，199.172.0.0 这个被定义物里，前16个bit和最后8个bit都是被定死的，唯独第三段的最后2bit是不关心的，可以变换，所以结果就是: 199.172.0.0 199.172.1.0 199.172.2.0 199.172.3.0 per 199.172.1.0 0.0.254.0 奇数路由 per 199.172.0.0 0.0.254.0 偶数路由
在进行路由过滤和地址限制方面，Prefix-list占用CPU的资源比采用access-list要少很多， 它提供更快的访问列表装入速度，目前IOS版本11.1CC(17)，11.3(3)和12.0都提供该项特性。
Prefix-list保存了access-list的多项重要特性： 1、Permit和Deny; 2、最先匹配法则； 3、基于Prefix长度的过滤（精确匹配或range匹配）
Prefix-list可以采用增量方式从新定义，也就是说，它里面的条目可以单独增加或删除，而无需像access-list一样，一删就得将整个access-list删掉重写。
一个空的/不存在 的acl & prefix预设是permit all ，一个不存在 的 route-map 预设是deny all 一个acl 如果配置一条以上permit或deny语句，最后一笔为隐藏的deny all route-map预设deny all(即使为空)
以下为各种例子，以如下的网络架构为网络环境： R1 (s1/0,192.168.1.1) =============== (s1/0,192.168.1.2) R2
Case Study: ACL R1#conf t R1(config)#line vty 0 4 R1(config-line)#password cisco R1(config-line)#login R1(config-line)#access-class 3 in
R2#telnet 192.168.1.1 Password : R1> 没有阻挡可以直接进去
Summary: 空的(未定义的)acl默认允许所有的主机。
Case Study: Route-map R1(config)#router ospf 100 R1(config-router)#default-information originate route-map sense R1(config-router)#exit R2#sh ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets O 172.16.1.0 [110/65] via 192.168.1.1, 00:32:38, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0
注: 不存在的route-map预设deny all。所以这个case的route-map永远返回不满足要求。
R1(config)#router ospf 100 R1(config-router)#default-information originate route-map sense R1(config-router)#exit R1(config)#route-map sense permit 10 R2#sh ip route Gateway of last resort is 192.168.1.6 to network 0.0.0.0 172.16.0.0/24 is subnetted, 2 subnets O 172.16.1.0 [110/65] via 192.168.1.1, 00:32:38, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0
O*E2 0.0.0.0/0 [110/1] via 192.168.1.6, 00:00:12, Serial1/1
R1(config)#router ospf 100 R1(config-router)#default-information originate route-map sense R1(config-router)#exit R1(config)#route-map sense permit 10 R1(config-route-map)#match ip add 1 R2#sh ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets O 172.16.1.0 [110/65] via 192.168.1.1, 00:32:38, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0
R1(config)#router ospf 100 R1(config-router)#default-information originate route-map sense R1(config-router)#exit R1(config)#route-map sense deny 10 R2#sh ip route Gateway of last resort is 192.168.1.6 to network 0.0.0.0 172.16.0.0/24 is subnetted, 2 subnets O 172.16.1.0 [110/65] via 192.168.1.1, 00:32:38, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0 O*E2 0.0.0.0/0 [110/1] via 192.168.1.6, 00:00:12, Serial1/1
R1(config)#router ospf 100 R1(config-router)#default-information originate route-map sense R1(config-router)#exit R1(config)#route-map sense deny 10 R1(config-route-map)#match ip add 1 R2#sh ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets O 172.16.1.0 [110/65] via 192.168.1.1, 00:32:38, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0
Case Study: Prefix-list R1(config)#int lo0 R1(config-if)#ip add 172.16.33.1 255.255.255.0 R1(config-if)#no shut R1(config-if)#ip ospf network point-to-point R1(config-if)#exit R1(config)#router ospf 100 R1(config-router)#redistribute connected subnets R1(config-router)#distribute-list prefix sense out connected R1(config)# R2#sh ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 4 subnets O 172.16.44.0 [33/65] via 192.168.1.1, 00:07:54, Serial1/0 O E2 172.16.33.0 [55/20] via 192.168.1.1, 00:00:01, Serial1/0 O IA 172.16.1.0 [44/65] via 192.168.1.1, 00:07:54, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0
R1(config)#ip prefix-list sense seq 5 deny 172.16.33.0/24 rtb#sh ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets O 172.16.44.0 [33/65] via 192.168.1.1, 00:09:15, Serial1/0 O IA 172.16.1.0 [44/65] via 192.168.1.1, 00:09:15, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0 Summary: 空的prefix-list预设允许所有的路由。
Case Study: Integrate Route-map and ACL R1(config)#router ospf 100 R1(config-router)#default-information originate route-map sense R1(config-router)#exit R1(config)#route-map sense deny 5 R1(config-route-map)#match ip address 33 R1(config-route-map)#
R2#sh ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets O 172.16.1.0 [110/65] via 192.168.1.1, 00:32:38, Serial1/0 C 172.16.2.0 is directly connected, Loopback0 192.168.1.0/30 is subnetted, 1 subnets C 192.168.1.0 is directly connected, Serial1/0
总结: 至少定义一条permit或deny语句才能使用acl或route-map的隐含deny功能。空的acl预设允所有的路由。
