Internet Routing

Routing process can summarize for a node finds to every possible destination path. Routing appeared in from the first layer to the seventh levels of each layer. People are familiar with the routing is to appear on the third floor (network layer), so we also only discuss the third floor IP routing.

Exchange routing information agreement connecting many of the world's routers, although these router is not a kind, through the routing table may be able to provide their common network view. Routing tables for router storage reaching a destination network office all the needed the necessary information.

Routing protocol

All kinds of routing protocol is used to fill in network routing tables. Like BGP, OSPF, RIP and ISIS such agreement can transmit to all the routers a correct and consistent network view.

Routing protocol want to achieve the goal

Could you imagine if every router has storage from its nodes that can reach each target required information, probably the router will accumulate a huge routing tables. Since the physical (CPU, memory) limit router hard sometimes it is absolutely impossible to handle a large routing tables. So in does not affect the ability reach every destination in the situation, we will make the routing table minimized. For example, a router through connected to another router a ds1s link connection to the Internet, then the router can be on Internet information is stored all nodes, or it can also will all ds1s serial links outside non-local all the information does not store. That is no router in its routing table store any relevant data "package" will look for non-local network destinations information, but these "bag" send to the other end of the router serial links, by the router to provide the necessary information. We often associate like this example we speak in serial ds1s link at the other end of the router called "Gateway of Mr. Resort". This simple tricks for routing table can save 30 orders of magnitude entries. Routing information is not necessary too frequently in router exchanged between. Usually the blender routing table to any router can provide the poor memory and CPU brought many unnecessary stress. The information copied should not influence the router forwards operation. Although no need every millisecond refresh every routing table, of course also cannot every week to refresh routing tables. Routing is an important goal is for hosts provide can accurately reflect the current state of the network a routing tables.

The router's most important operation is will receive packets sent to the right path. Without routing bag may lead to loss of data. While routing table inconsistencies will lead to routing loops and make a packet on two adjacent interface between sending by circulating.

People were so hope all routers can have rapid convergence. Convergence can be defined as measurement informally all routers get consistent network view of the speed of the unit. Hope to people have minimal convergence times, because so on the network for each router even in network topology (namely network view) was serious change circumstances can accurately reflect current network topology. When the topology of the network are changed, each router must transmit data to help other routers to convergence in the right network view. But in refresh routing table when fast convergence, there also exist its problem. If a link in quickly vibration (while disconnected, while closed), it will produce large amounts of the installation and cancellation request. This link will finally exhausted every router resources on the network, because other router is forced rapid installing or cancel the routing. Therefore, even rapid convergence is the routing protocol target, it also is not all network problem's panacea.

Distance vector routing

Distance vector routing protocol to distribute a neighbor routers all record form for < > a list of goals, overhead. These records for network in each are not our node of the other nodes on the overhead fu this value. It is worth noting the information only distributed to source router's neighbors router. Here the adjacent routers are often in the physical, but in eBGP there is also applicable to logic on the situation. Spending meaning from the source router to goal node of the link cost combined. Source router periodically refresh its distance vector record and the records of adjacent routers distributed to it. Adjacent routers will past receives the recording and now comparative, if the past overhead router will receive smaller along the past the distance vector record refers to the path to send output.

Many distance vector in actual use will hit the infinity of problem. For example, we will assume that all link has a cost unit and each pair of adjacent node the link between corresponding to a unit. If the router X connected to router Y and router Y connected to router Z is shown in figure 1, we will find the infinity of problem. Y know to Z to have 1 unit costs and X know to Z to 2 units overhead. Hypothesis link YZ closed, this article link overhead becomes infinite (figure 2). Now Y know the spending is reached Z infinite, it will be the distance vector routing send X. Hypothesis X sent to Y at a distance vector routing claims it reaches Z to 2 units overhead. Now Y will think it can pass X reached Z, it will be sent to X a distance vector routing refresh, says that it reached Z overhead is three units (such as figure 3). Please note that X did not think of to its this Y by distance vector routing is to send it across the distance vector routing Y the computative. This is distance vector routing serious defect, they haven't been improved structure in not contained routing obstacle of information. As legend will continuously shown router change to Z path information. X and Y the two routers will always exchange the relevant Z router's path information or until spending unit value reached a certain predetermined infinite values (for example, in RIP is 15).

X - - - - Y - - - - Z

Y: 1 X: 1 X: 2

Z: 2:1 Y Z: 1

Figure [1]

X - - - - Y - - * * -- - Z

Y: 1 & NBS

P; < - - - Z: infinite

Z: 2 - - - > X: 1

Figure [2]

X - - - - Y - - * * -- - Z

Z: · (from Y) - > X: 1

Y: 1 < - - - Z: 3

Figure [3]

Use path vector routing can solve the infinity of problem. Each distance vector also includes he passed path (figure 4). Routers if receives a path vector contains own refresh record, the router will refresh this record (figure 5). Border Gateway Protocol (The Border Gateway Protocol) is used to solve The above method of The infinity of problem. Obviously, if you want to make the routing table contains router the transmission AS (Autonomous by telephone under 0120-2714540 on the Internet) route information, you will have to add a routing table for more information. Therefore BGP designer decided to sacrifice a little routers can withstand a storage space and processing ability.

X - - - - Y - - - - Z

Y: 1 (Y) X: 1 (X) X: 2 (YX)

Z: 2 (YZ) Z: 1 (Z) Y: 1 (Y)

Figure  [4]

X - - - - Y - - * * -- - Z

Y: 1 (Y) X: 1 (X)

Z: 2 (Y Z) Z: infinity

Graph [5]

Another · problem solution is separation range. Main idea is, if adjacent routers in to destination on the path to the second node, the router to the adjacent routers broadcast this path. This solution can use just examples. Because to Z path from X through Y again to Z, and because of the neighboring router Y is X, so this path X from the radio, not be broadcast. Y,

Link-state routing

A router in use link-state routing, it will to all other routers on the network distribute it to its neighboring router distance. This makes each router need not know from one source node to destination node expenses, the router can produce a routing tables. Loop problem won't appear, because each router has own the whole network topology. Main thoughts is a router have three parts of the record contains source router (itself), adjacent routers and to adjacent routers overhead. Therefore, if the routers A through A expenses for 3 links to routers B, and routers A through A expenses for five links to router C, so the router will all the routers to Internet radio link-state package (LSPs) and. Each router will from receives the LSPs in calculate the path to a destination node of the shortest path.

Obviously, LSP is an integral part of the convergence process. If to network joined the wrong LSP. Will lead to wrong routing information (that causes longer than the original bag along the path transport) even produce routing black holes. If the router C to other routers broadcast a to his adjacent routers path information, but when this link disconnect when router C withdrew just on the radio. Unfortunately the second LSP first come first come first LSP and, when other router's routing table cannot correctly reflect the network topology, but only until another correct LSP come.

In order to solve this problem, LSP introduced sequence yards. So all the routers on the network will be with some value as the initial value to initialize their sequence yards, then in broadcast their LSP. This is solved just of problem.

When using sequence yards will encounter sequence code space is limited to this problem. LSPs can use the sequence of codes are set to limited value. Therefore, when sequence yards after, have to reach maximum from minimum start again. This gives the router in comparative link-state current records and refresh record when difficult, because sequence yards of the has priority. In order to solve this problem, you can define a maximum for LSP aging time. That is to say, if the router in X time didn't receive refresh record, it will existing record discarded and to wait updated records. Should pay attention to must make to the destination path information is invalid. For example, when router Y link to one inning, when the link disconnect network router Y to router Z broadcast the link of information, this situation the router at the net in that they can still reach Z. If they are among the largest aging time receive less than refresh record, they will assume the link to Y has unreachable. So all the router's routing table will be consistent, router Y and Z can also use limited sequence yards.

Sequence yards of initialization is also the problem in another important aspect. Hypothesis router Y reboot, while network began to recount the path. When the router's link-state protocols began the work, it must know reinitializes the its sequence yards why value to make it and other router is consistent. Therefore, it is broadcast with a special initialization set path information. This article records will tell other routers it need that sequence yards, and other router will tell it.