Shulou(Shulou.com)06/01 Report--

1. Routing table

1) what is the routing table

A routing table is a table of routing information stored on a router or other Internet network device that contains paths to specific network terminals and, in some cases, metrics related to these paths.

2) routing table classification

(1) static routing table

The fixed routing table set in advance by the system administrator is called the static routing table, which is generally set according to the configuration of the network when the system is installed, and it will not change with the change of the network structure in the future.

(2) dynamic routing table

The dynamic routing table is the routing table that the router adjusts automatically according to the operation of the network system. According to the functions provided by the routing Protocol (Routing Protocol), the router automatically learns and remembers the operation of the network, and automatically calculates the best path for data transmission when needed.

2. Routing table entry

Destination address: used to identify the destination address or network of an IP packet.

Network mask: the address that, together with the destination address, identifies the destination host or the network segment on which the router is located.

Pre: identifies the priority at which a route is added to the IP routing table. There may be multiple routes to a destination, but the existence of priority allows them to choose a high-priority route to take advantage of.

Cost: routing cost. When multiple routes to a destination have the same priority, the one with the lowest routing cost will become the optimal route.

Interface: output interface, indicating the interface from which the IP packet will be forwarded on this router.

Nexthop: next-hop IP address, indicating the next router through which the IP packet passes.

3. Related algorithms of routing table

1) Total routing algorithm.

When using a global routing algorithm, each router has all the information of all other routers in the network and the traffic status of the network. These algorithms are also known as LS (link-state) algorithms.

2) distributed routing algorithm

When using a distributed routing algorithm, each router has only information about the router to which it is directly connected-- but not for each router in the network. These algorithms are also known as DV (distance vector) algorithms.

Distance vector routing algorithm, also known as maximum flow algorithm, is used as an algorithm by distance vector protocol. For example, RIP is a distributed routing protocol based on distance vector. The router using this algorithm must master the distance table, which tells the furthest and distance of each node in the network. This information in the distance table is updated from time to time according to the change of the adjacent contact information. The amount of data in the table is equal to all the contacts in the network (except itself). The columns in this table represent the neighbors directly connected to it, and the rows represent all destinations in the network. Each data includes the path and distance / or time on which the packet is transmitted to each destination on the Internet (we call this "cost"). The metric formula in that algorithm is the number of hops, the waiting time, the number of packets flowing out, and so on.

In the distance vector routing algorithm, neighboring routers exchange their routing table backups periodically. When the network topology changes, routers will inform each other of the change information in a timely manner.

The idea of the link-state algorithm is to require all the routers participating in the link-state routing protocol in the network to grasp all the topology information of the network and record it in the routing database. In the link-state algorithm, the routing database is essentially a topological graph of network structure, which is composed of a set of nodes and a set of edges. In the network topology diagram, the node represents the router in the network and the edge represents the physical link between the routers. In the network topology diagram, different attributes can be attached to each link, such as the status, distance, or cost of the link. If none of the routers keep the same network topology diagram, then the routing table generated by each router is also the best, and there is no wrong route or circular route.

Feature 1: send information to all routers in this autonomous system. The method used here is flooding (Flooding), in which routers send information to all neighboring routers through all output ports. Each router sends this information to all its neighboring routers (but not the one that just sent the message).

Feature 2: the information sent is the link state of all routers adjacent to this router, but this is only part of the information that the router knows. The so-called "link state" means that this router is adjacent to those routers and the "metric" of the link. For OSPF, the "metric" of link state is mainly used to represent cost, distance, delay, bandwidth and so on.

Feature 3: only when the link state changes, the router uses flooding to send this information to all routers.

OSPF uses a distributed link-state protocol.

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