Packet forwardingBasic concept:
Router parameter：
Number of hops: The numeral of routers, the smaller the hop count, the better the path
Bandwidth: The transmission system rate of a link, usually in Mb/s (megabits per second)
Delay: The sentence it takes for a packet to offer from the rootage host to the finish host. The smaller the delay, the major.
Load: The measure of information passing through the router or line per building block clock
Reliability: The metric is the bit wrongdoing range during packet transmission. The bit error rate is an indicator of the truth of data transmission. The bit misplay rate = the error in transmission / the total number of codes transmitted * 100%. Of course, the littler the bit error rate, the more trusty the description.
overhead: the cost of packet transmittal, often measured reciprocally proportional to bandwidth
Valuation router ground: Routing algorithms are right, stable, fair, simple, applicable to topology and dealings changes.
Assortment of routingThe router uses a table-determined routing algorithmic rule. The routing table is generated along with the routing algorithm. When the router wants to beam an Information processing bundle, information technology mustiness query the routing table to determine the next address to be conveyed by the mail boat.
Route aggregation is to reduce routing entries. The engineering science used is CIDR (Democratic Inter-Area Routing), which is to solve the problem of generating multiple routes after subnetting and adding routing entries.
An example:
RG router, there should be 6 subnets on the left and 6 subnets along the right before merging. A total of 12 routing entries need to be written. The routing table is too bloated.
Change the mental picture to a err: the first subnet on the left side of the picture should be 156.26.0.0/24
Autonomous system：
The Internet uses a bedded routing protocol and translates the entire Internet into many another smaller autonomous systems (AS). The routing inside the free system is known as intra-domain routing, autonomy. Routing between systems is called inter-domain routing.
The most great feature of an autonomous system is that it has the right to settle which routing protocol should be victimised in the system.
The sovereign organization divides the internet route into two layers.
Internet routing protocol classification 
Interior Gateway Protocol IGP is divided into RIP and OSPF
External Gateway Protocol EGP is mainly BGP
RIP routing information protocol：
Convergence: Update and learning between routing tables to make information technology stable
uses the distance vector algorithmic rule to stage the distance with hops
Pull work process：
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Shortest path prototypic protocol OSPF:
"Open" indicates that the OSPF protocol is not controlled by a vendor but is publicly published.
The shortest path prototypic is due to the use of the shortest path algorithm SPF proposed by Dijkstra
OSPF features：
- Using a distributed link state protocol; the distance vector algorithmic rule used by RIP
- Connect body politic metrics mainly refer to cost, outdistance, delay, broadband, etc.
- This information is sent to altogether routers by flooding when the link state changes.
- All OSPF routers exchange link state information and eventually establish aLink up state database(LSDB, each router has), this database is actually the topology map of the gross network (if the sub-area is only when the local anaesthetic area) (not a routing table), from each one router calculates its own table according to the database!
- OSPF can subdivide an autonomous organisation into several little areas called Areas. Each orbit has a 32-bit area identifier (equivalent to an IP speech such as 0.0.0.0). The figure of a regional router is non More than 200
Class of autonomous system regions：
Benefits: Flooding is accustomed limit the scope of link state information to each district rather than the entire autonomous system of rules. Therefore, routers within each zone only know the complete topology within the zone and do non know the topology of other zones.
The trunk area is generally called country 0.
OSPF protocol slaying process：
- Initialization outgrowth: OSPF allows each router to exchange the link country summary selective information of the database with the database description packet and the neighbouring router. The compendious information mainly indicates which connectedness state info of the router has been written into the database.
- Network operation process: American Samoa long as there is a alter in the link of a router, the router will usance the link status update packet to update the link position to the full network by flooding. To assure that the inter-group communication state database is consistent across the network, OSPF specifies that The link condition is updated once all other time, for example 30 minutes.
External Gateway Communications protocol EGP: Mainly Border Gateway Protocol BGP
RIP is also a vector-based routing protocol, and OSPF is a link-state routing communications protocol.
Running process of BGP routing protocol：
Note that exclusive the changes are updated, revolve about the increments, because the table is too large
When updating a bundle: only one route sack be added to a packet, but multiple routes can be revoked.
routing inside an autonomous system is referred to as
Source: https://programmersought.com/article/1712768179/
