12, wide area network

Wan

Note: wherein packet-switched packet switch, not a switch, corresponding to the route.

In the WAN, router only needs to find the corresponding IP address according to the network segment, but when required by a local area network, the need to find the next-hop MAC address, as a local area network, all of the segment number are equal. This is the case for routing and routing are not directly connected, but we consider the network layer when you can ignore this problem, consider routing and routing are directly connected, directly through the IP address you can find the ultimate LAN, in short, MAC address also necessary, WAN MAC level addressing the issue, equivalent to IP addressing problems.

1 Introduction

• Local area network (LAN), metropolitan area network (MAN), wide area network (WAN), the key distinguishing feature is the size: the geographical distance and the number of connected computers

• WAN: Select to accommodate the expected traffic and provide redundancy to avoid data redundancy filled storage space, so that the route does not work

Store and forward

• Each switch receives the packet, it is queued in its memory, and when possible (e.g., when the target is available) and send it out
• Many computers may send packets at the same time

Physical addressing a WAN

• WAN defined frame format and assign physical addresses for the computer
• Hierarchical addressing, e.g.
  • identifying a first portion of a packet switched
  • identifying a second portion of the switch on the computer

The next hop

• Each packet switch determines the target packet according to the destination address
  • local computer or
  • another packet switch
• Only contains information about how to get to the next switch - the next hop

• It is stored in the routing table

• Depending on the target next-hop address, without depending on the source address

Hierarchical addressing and routing

• Routing is the process of the data packet is forwarded to the next hop
• Hierarchical address simplifies routing
  • smaller routing tables
  • Finding more efficient

WAN routing

• Large WAN using internal switches and the external switches

• They must ensure that the routing table:
  • Generic Routing - every possible destination must have the next hop
  • the best route - next hop "shortest path" to reach the destination must point

• A routing table by a default route (up) to simplify

2, the routing table is calculated

• Manual calculation does not apply to large networks
• Static Routing - install program calculates the route and when the switch is activated; these lines will not change.
• Dynamic routing - the switch to start the program will build the initial table; which varies as conditions change.

WAN and graphics

• WAN can be modeled as graphics
• Node is switch: 1,2,3,4
• An edge is a connection between the switch: (1,3) (2,3) (2,4) (3,4)
• Weight is along the "distance" connection

Distributed routing calculation

• All have to do this packet switch
• Two general forms:
  • Link State Routing (LSR): using Dijkstra's algorithm
  • distance vector routing (DVR): using another method
• We will discuss only the method based on Dijkstra's algorithm

Calculates the shortest path

• Dijkstra's algorithm - to find the distance from the source node to every other node in FIG.
• And create the next hop routing table for each switch run this command in the process
• An edge weight represents the "distance"
• Aka - Shortest Path First (SPF) routing

Link state routing

• Each switch on a regular basis on a particular link to other switches of state broadcasting
• Switch collected message, and then Dijkstra algorithm is applied to its network state release
• If the link fails, additional packet switch will detect the link status broadcast and
• Similarly, when the failed link becomes available again, the other packet switch receives this message and recalculate