Wednesday, May 20, 2009

Introduction of Routing

Introduction of Routing
IP Routing is process move packet to another networks using router. Câu này là mở màn Routing Basic, giáo trình Sybex. Có thể nhận xét, routing bản chất là đưa gói tin tới Interface mà nó cần đến :d. Vâng mục đích cuối cùng của Routing là vậy.
Tuy nhiên ko phải Routing lúc nào cũng dùng router. Điển hình là win2k3Se cũng có routing &Remote. Trong Xp có command route. Cái này cũng như vậy trong *nix. Tuy nhiên sẽ nghiên cứu trên nền tảng Công nghệ Cisco. Ok

Routing and Routed Protocol:
Routing Protocol is used by routers dynamically find all the networks in the internetwork and ensure that all route have the same routing table. Routing Protocol detemines the packet through an internetwork. Ex: Rip, IGRP, EIGRP, OSPF
Routed Protocol is used to send data through the establised enterprise. Routed Protocol assign the interface and determine method of packet delivery. Ex: IP, IPX, ApplleTalk.
Routing Protocol:
How the Routing Protocol Works.
There Step:
1. The Routing Protocol send the information about the router or network within autonomous system, such as RIPv1. IGRP, EIGRP, between AS as BGP.
2. The routing table recevice update from routing protocol andand provide the forwarding process with information on request.
3. The forwarding process determines path.
Mục đích của Routing Protocol là tạo ra được routing table ( thảo luận sau)
Criteria to decission:
1. Metric: used to calculate the best path to the remote network
2. Administrative distance: If more than one routing process on router, router use AD to select protocol which will update in routing table
3. Prefix length: will use route where is the most number subnetmask bits match the destination networks.
Type of routing protocol:
The first difference between protocols that send subnetmask in the update and the older protocols do not. So, there ara label classless and classfull routing.
- Classfull Routing: Classfull là mang tính lớp, nghĩa là nó sẽ tuân theo tính phân lớp của đianj chỉ IP. Do dó sẽ ko gửi subnetmask. Hiểu đơn giản là thế :d
Classfull routing protocol do not carry the subnetmask in the update. The older distance vector protocol tend to be classfull. Incclude RIPv1 and EIGRP.
Characteristics:
+Summarization occours at the network boundary
+Exchanged between foregin network are sumuazarized and IANA classfull network.
+In the same network, subnet routes are exchanged by router. without mask
+All router interfaces must share the subnet mask for interfaces in the same IANA
classful network. ???
There is an entry in the routing table for a specific subnet, the datagram is forwarded to that destination. If the destination subnet is unknown, the datagram is dropped.
If there is a default network configured for the router, you might expect it to be used. However, the default network is only used in classful routing if there is no knowledge of the network at any level. Thus, if the major network is known, the datagram is dropped even if there is a default network.
Forwarding decission:
1. Send the datagram to the subnet if there is entry in the routing table
2. If no entry in routing table, discard datagram
3. If there is an entry for major network but not for the special network, discard datagram
4. If there is an entry for major network but not for the special network, do not consider the dafault network and discard datagram
5. If there is no entry for either the major network on the subnet, but there is a default route, forward to the default route.
Classless Routing:
Benefit:
+ Utilization of space address might be inefficient.
+ VLSM (Variable-length subnet mask) is not possible within the network.
+ Lead to very large routing tables and overloaded network links.
Characteristics:
+Router interfaces within the same network can have different subnet masks (VLSM).
+The classless routing protocols support the use of classless interdomain routing (CIDR).
+Some routes can be summarized within the major IANA classful network number.
Routing Table:
The Routing Table Fields:
+Network: as static routes or were learned via a routing protocol as dynamic routes
+Outgoing Interface: which interface to send the datagram and the routing update came
+Metric: a value that is assigned to each path based on the criteria specified in the routing
protocol.
+Next Hop
How to Keep the Routing Table Current and Correct:
Use update mess.
RIP sends out the entire routing table every 30 seconds to a broadcast address. OSPFupdate contain information only about network changes and are sent only when that change occurs, using multicast addressing. OSPF sends a summarization of the entire routing table 30 minutes after the last update was sent.

.........to be continue

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