经常划分子网的企业要注意了，路由器负荷加重。
什么是CIDR，简称无类域间路由
基于VLSM子网划分后引发的路由条目剧增、路由“爆炸”、路由器负担加重。
当网络规模比较小的时候，这个问题看似不是很严重。但是在大的网络中，如果所有的下级网络都做子网拆分，这个时候将会使得核心位置路由器上的路由表变得分厂的庞大。这个数据容量可以达到十几MB或者几百MB，甚至更多！如果一台路由器的路由表有数百MB，那么如果要在路由表中查看到一个具体的路由条目，也就要在几千几万条路由条目中找到一个具体的路由条目，路由的效率就会变得非常低。我们可以在互联网中可以找到一些给我们测试或者查看的核心路由器的路由条目，要下载路由条目需要很长很长的时间。
路由器对路由条目的查找、更新就会变得非常的中，路由器处理数据的能力就会大大下降。这种情况在运营商变得更加可见，对于ipv4日益耗竭的今天，ip地址的不断拆分引来的一个很重大的问题
为了使在大规模的网络中，效率变得更加高，就要使用路由汇总。CIDR和VLSM其实是一个互为逆的运算
以下为实验拓扑
配置好后通过sh ip route，可以看到的是R4路由器有各个子网的路由，而R5路由器却只有一条192.168.49.0/25的汇总路由，所有通过R5去其余R1~R4的路由器均可以以这条RIP路由
R 192.168.49.0 [120/1] via 201.0.0.1, 00:00:02, Serial1/2
201.0.0.0/24 is variably subnetted, 2 subnets, 2 masks
往出接口S1/2发送数据。这样已达到了减少路由条目，增快查看效率的速度。
路由汇总的关键点，还是在于R4路由器的接口S1/3下配置的命令ip summary-address rip 192.168.49.0 255.255.255.128的效果，还请细细体会。
R1的相关设置：
R1#sh run
Building configuration...
Current configuration : 1348 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
clock timezone CST 8
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip source-route
!
!
ip cef
no ip domain lookup
no ipv6 traffic interface-statistics
no ipv6 cef
!
multilink bundle-name authenticated
!
redundancy
!
interface Loopback0
ip address 192.168.49.1 255.255.255.224
!
interface Ethernet0/0
no ip address
shutdown
!
interface Ethernet0/1
no ip address
shutdown
!
interface Ethernet0/2
no ip address
shutdown
!
interface Ethernet0/3
no ip address
shutdown
!
interface Serial1/0
no ip address
shutdown
no fair-queue
serial restart-delay 0
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.49.97 255.255.255.252
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router rip
version 2
network 192.168.49.0
no auto-summary
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
control-plane
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
exception data-corruption buffer truncate
end
---------------------------------------------------------------
R1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, 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 downloaded static route, + - replicated route
Gateway of last resort is not set
192.168.49.0/24 is variably subnetted, 8 subnets, 3 masks
C 192.168.49.0/27 is directly connected, Loopback0
L 192.168.49.1/32 is directly connected, Loopback0
R 192.168.49.32/27 [120/2] via 192.168.49.98, 00:00:21, Serial1/2
R 192.168.49.64/27 [120/2] via 192.168.49.98, 00:00:21, Serial1/2
C 192.168.49.96/30 is directly connected, Serial1/2
L 192.168.49.97/32 is directly connected, Serial1/2
R 192.168.49.100/30 [120/1] via 192.168.49.98, 00:00:21, Serial1/2
R 192.168.49.104/30 [120/1] via 192.168.49.98, 00:00:21, Serial1/2
R 201.0.0.0/24 [120/1] via 192.168.49.98, 00:00:21, Serial1/2
R 202.0.0.0/24 [120/2] via 192.168.49.98, 00:00:21, Serial1/2
=================================================================
R2的相关设置：
R2#sh run
Building configuration...
Current configuration : 1350 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R2
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
clock timezone CST 8
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip source-route
!
!
ip cef
no ip domain lookup
no ipv6 traffic interface-statistics
no ipv6 cef
!
multilink bundle-name authenticated
!
!
redundancy
!
!
interface Loopback0
ip address 192.168.49.33 255.255.255.224
!
interface Ethernet0/0
no ip address
shutdown
!
interface Ethernet0/1
no ip address
shutdown
!
interface Ethernet0/2
no ip address
shutdown
!
interface Ethernet0/3
no ip address
shutdown
!
interface Serial1/0
no ip address
shutdown
no fair-queue
serial restart-delay 0
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.49.101 255.255.255.252
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router rip
version 2
network 192.168.49.0
no auto-summary
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
exception data-corruption buffer truncate
end
R2#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, 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 downloaded static route, + - replicated route
Gateway of last resort is not set
192.168.49.0/24 is variably subnetted, 8 subnets, 3 masks
R 192.168.49.0/27 [120/2] via 192.168.49.102, 00:00:14, Serial1/2
C 192.168.49.32/27 is directly connected, Loopback0
L 192.168.49.33/32 is directly connected, Loopback0
R 192.168.49.64/27 [120/2] via 192.168.49.102, 00:00:14, Serial1/2
R 192.168.49.96/30 [120/1] via 192.168.49.102, 00:00:14, Serial1/2
C 192.168.49.100/30 is directly connected, Serial1/2
L 192.168.49.101/32 is directly connected, Serial1/2
R 192.168.49.104/30 [120/1] via 192.168.49.102, 00:00:14, Serial1/2
R 201.0.0.0/24 [120/1] via 192.168.49.102, 00:00:14, Serial1/2
R 202.0.0.0/24 [120/2] via 192.168.49.102, 00:00:14, Serial1/2
=================================================================
R3的相关配置：
R3#sh run
Building configuration...
Current configuration : 1350 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R3
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
clock timezone CST 8
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip source-route
!
ip cef
no ip domain lookup
no ipv6 traffic interface-statistics
no ipv6 cef
!
multilink bundle-name authenticated
!
!
redundancy
!
interface Loopback0
ip address 192.168.49.65 255.255.255.224
!
interface Ethernet0/0
no ip address
shutdown
!
interface Ethernet0/1
no ip address
shutdown
!
interface Ethernet0/2
no ip address
shutdown
!
interface Ethernet0/3
no ip address
shutdown
!
interface Serial1/0
no ip address
shutdown
no fair-queue
serial restart-delay 0
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.49.105 255.255.255.252
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router rip
version 2
network 192.168.49.0
no auto-summary
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
control-plane
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
exception data-corruption buffer truncate
end
R3#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, 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 downloaded static route, + - replicated route
Gateway of last resort is not set
192.168.49.0/24 is variably subnetted, 8 subnets, 3 masks
R 192.168.49.0/27 [120/2] via 192.168.49.106, 00:00:11, Serial1/2
R 192.168.49.32/27 [120/2] via 192.168.49.106, 00:00:11, Serial1/2
C 192.168.49.64/27 is directly connected, Loopback0
L 192.168.49.65/32 is directly connected, Loopback0
R 192.168.49.96/30 [120/1] via 192.168.49.106, 00:00:11, Serial1/2
R 192.168.49.100/30 [120/1] via 192.168.49.106, 00:00:11, Serial1/2
C 192.168.49.104/30 is directly connected, Serial1/2
L 192.168.49.105/32 is directly connected, Serial1/2
R 201.0.0.0/24 [120/1] via 192.168.49.106, 00:00:11, Serial1/2
R 202.0.0.0/24 [120/2] via 192.168.49.106, 00:00:11, Serial1/2
========================================================
R4的相关配置：
R4#sh run
Building configuration...
Current configuration : 1389 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R4
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
clock timezone CST 8
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip source-route
!
ip cef
no ip domain lookup
no ipv6 traffic interface-statistics
no ipv6 cef
!
multilink bundle-name authenticated
!
redundancy
!
!
interface Ethernet0/0
no ip address
shutdown
!
interface Ethernet0/1
no ip address
shutdown
!
interface Ethernet0/2
no ip address
shutdown
!
interface Ethernet0/3
no ip address
shutdown
!
interface Serial1/0
ip address 192.168.49.98 255.255.255.252
serial restart-delay 0
!
interface Serial1/1
ip address 192.168.49.102 255.255.255.252
serial restart-delay 0
!
interface Serial1/2
ip address 192.168.49.106 255.255.255.252
serial restart-delay 0
!
interface Serial1/3
ip address 201.0.0.1 255.255.255.0
ip summary-address rip 192.168.49.0 255.255.255.128
serial restart-delay 0
!
router rip
version 2
network 192.168.49.0
network 201.0.0.0
no auto-summary
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
exception data-corruption buffer truncate
end
R4#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, 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 downloaded static route, + - replicated route
Gateway of last resort is not set
192.168.49.0/24 is variably subnetted, 9 subnets, 3 masks
R 192.168.49.0/27 [120/1] via 192.168.49.97, 00:00:04, Serial1/0
R 192.168.49.32/27 [120/1] via 192.168.49.101, 00:00:05, Serial1/1
R 192.168.49.64/27 [120/1] via 192.168.49.105, 00:00:08, Serial1/2
C 192.168.49.96/30 is directly connected, Serial1/0
L 192.168.49.98/32 is directly connected, Serial1/0
C 192.168.49.100/30 is directly connected, Serial1/1
L 192.168.49.102/32 is directly connected, Serial1/1
C 192.168.49.104/30 is directly connected, Serial1/2
L 192.168.49.106/32 is directly connected, Serial1/2
201.0.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 201.0.0.0/24 is directly connected, Serial1/3
L 201.0.0.1/32 is directly connected, Serial1/3
R 202.0.0.0/24 [120/1] via 201.0.0.2, 00:00:19, Serial1/3
============================================================
R5的相关配置：
R5#sh run
Building configuration...
Current configuration : 1353 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R5
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
clock timezone CST 8
mmi polling-interval 60
no mmi auto-configure
no mmi pvc
mmi snmp-timeout 180
ip source-route
!
ip cef
no ip domain lookup
no ipv6 traffic interface-statistics
no ipv6 cef
!
multilink bundle-name authenticated
!
redundancy
!
interface Loopback0
ip address 202.0.0.1 255.255.255.0
!
interface Ethernet0/0
no ip address
shutdown
!
interface Ethernet0/1
no ip address
shutdown
!
interface Ethernet0/2
no ip address
shutdown
!
interface Ethernet0/3
no ip address
shutdown
!
interface Serial1/0
no ip address
shutdown
no fair-queue
serial restart-delay 0
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/2
ip address 201.0.0.2 255.255.255.0
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
router rip
version 2
network 201.0.0.0
network 202.0.0.0
no auto-summary
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
exception data-corruption buffer truncate
end
R5#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, 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 downloaded static route, + - replicated route
Gateway of last resort is not set
192.168.49.0/25 is subnetted, 1 subnets
R 192.168.49.0 [120/1] via 201.0.0.1, 00:00:11, Serial1/2
201.0.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 201.0.0.0/24 is directly connected, Serial1/2
L 201.0.0.2/32 is directly connected, Serial1/2
202.0.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 202.0.0.0/24 is directly connected, Loopback0
L 202.0.0.1/32 is directly connected, Loopback0
========================================================
R1#ping 202.0.0.1 source 192.168.49.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 202.0.0.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.49.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/20/24 ms
R2#ping 202.0.0.1 source 192.168.49.33
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 202.0.0.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.49.33
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/18/20 ms
R3#ping 202.0.0.1 source 192.168.49.65
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 202.0.0.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.49.65
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/19/24 ms
可以看到的是全网互通！