cấu hình ip classless trong router ????

[duylinh]

em có đọc một tài liệu nó nói về câu lệnh sau:

R1(config)# ip classless
các anh cho em hỏi là câu lệnh trên đây được dùng khi nào, và nó có tác dụng gì ?

[manhht]

* By using the ip classless command, you are overriding this behavior; you’re allowing your classful router to use a default route to reach discontiguous subnets.

Mark
* Classful protocols, such as IP RIPv1, understand only classful subnets—you can apply only one subnet mask to a class address.
* Classless protocols, such as RIPv2, EIGRP, OSPF, and IS-IS, do not have this restriction. The ip classless command allows a classful protocol to use a default route; omitting this command will cause the router to drop packets that don’t match a specific destination network entry in the routing table.

[duylinh]

nhưng mà cái em đọc trong mô hình lab ở đây nó lại cấu hình static route chứ không không có cái gì liên quan đên dynamic routing protocol. bác nào có thể lấy một ví dụ cho em hiểu rõ vấn đề này không ạ?

[TheGioiToThatTo]

Classless routing and classful routing relate to the logic used to match the routing table, specifically for when the default route is used. Regardless of the use of any optimized forwarding methods (for instance, CEF), the following statements are true about classless and classful routing:

• Classless routing—When a default route exists, and no specific match is made when comparing the destination of the packet and the routing table, the default route is used.
• Classful routing—When a default route exists, and the class A, B, or C network for the destination IP address does not exist at all in the routing table, the default route is used. If any part of that classful network exists in the routing table, but the packet does not match any of the existing subnets of that classful network, the router does not use the default route and thus discards the packet.

Typically, classful routing works well in enterprise networks only when all the enterprise routes are known by all routers, and the default is used only to reach the Internet-facing routers. Conversely, for enterprise routers that normally do not know all the routes—for instance, if a remote router has only a few connected routes to network 10.0.0.0 and a default route pointing back to a core site— classless routing is required. For instance, in an OSPF design using stubby areas, default routes are injected into the non-backbone areas, instead of advertising all routes to specific subnets. As a result, classless routing is required in routers in the stubby area, because otherwise non-backbone area routers would not be able to forward packets to all parts of the network.

[phamminhtuan]

Chào bạn,

Xem bài lab sau dây.

Mô hình

R1----R2

R1
interface FastEthernet0/0
ip address 172.16.1.1 255.255.255.0
ip route 0.0.0.0 0.0.0.0 172.16.1.2
no ip classless


R2
interface FastEthernet0/0
ip address 172.16.1.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 172.16.3.1 255.255.255.0
duplex auto
speed auto


Ping từ R1 -> R2 (172.16.2.2)
Router#ping 172.16.3.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/50/88 ms


Giờ trên router R1 thêm mạng 172.16.2.1 255.255.255.0
interface FastEthernet0/1
ip address 172.16.2.1 255.255.255.0


R1#ping 172.16.3.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.3.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)


Lí do vì sao vậy?



Trên R1 hiện tại đang dùng classful.
Trên R1 sẽ chỉ hiểu mạng 172.16.0.0/16 => bạn ping 172.16.3.1 (tức cũng là mạng con của 172.16.0.0/16), vậy nó đi ra connected của router (f0/1). Mạng nối với f0/1 của R1 không có IP nào là 172.16.3.1 nên R1 sẽ bỏ gói này.

Giờ trên R1 ta dùng lại câu lệnh ip classless
R1(config)#ip classless

R1#ping 172.16.3.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/35/68 ms




Chú ý: các dòng router hiện tại mặc định đã bật sẵn câu lệnh ip classless nên vấn đề này đã được giải quyết.