Basic knowledge of routers must-see for network administrators

A network administrator is a person who has been trained in the system. You need to know the following knowledge, and it is also necessary to understand, so that you can quickly deal with the problem of router failure.

1. Management distance

Administrative distance refers to the routing credibility of a routing protocol. Each routing protocol assigns a trust level in turn from high to low reliability. This trust level is called administrative distance. For routing information from two different routing protocols to a destination, the router first decides which protocol to trust based on the administrative distance.

2. Routing redistribution

Route redistribution is usually configured on routers that are responsible for learning routes from one autonomous system and then broadcasting to another autonomous system. If you are using IGRP or EIGRP, route redistribution is usually performed automatically.

3. What types of access tables are supported

An access list can be determined by its number. The specific protocol and its corresponding access table number are as follows: ◎IP standard access table number: 1～99◎IP extended access table number: 100～199◎IPX standard access table number: 800～899◎IPX extended access table number: 1000 ～1099◎AppleTa lk access table number: 600～699. In Cisco IOS Release 11.2 or above, you can use the well-known access table to determine the access table numbered from 1 to 199.

4. Access table

The access table can allow or prohibit data packets from entering or outputting to the destination. The access table is a series of rules added by the administrator to control the input and output of data packets in the router. It is not generated by the router itself. The entry of the access table is executed sequentially, that is, when the data packet arrives, it is first to see whether it is constrained by the first entry, if not, then the sequence is executed downward; if it matches the first entry, no matter Whether it is allowed or forbidden, there is no need to perform the following check. There can only be one access table for each protocol of each interface.

5. Multiple routing protocols

When two different routing protocols want to exchange routing information, multiple routing protocols are used. Of course, routing redistribution can also exchange routing information. There is no need to use multiple routing protocols in the following situations: Upgrade from the old version of the Interior Gateway Protocol (IGP) to the new version of IGP. You want to use another routing protocol but you must keep the original protocol.

6. Link state routing protocol

The link state routing protocol is more suitable for large-scale networks, but due to its complexity, routers require more C P U resources. It can find broken links or newly connected routers in a shorter time, making the convergence time of the protocol shorter than that of the distance vector routing protocol. Generally, if it does not receive the HE L LO message from the neighboring station within 10 seconds, it considers that the neighboring station is unreachable. A link-state router sends update messages to its neighbors to notify all links it knows. It determines that the metric value of the optimal path is a numerical cost, and the value of this cost is generally determined by the link bandwidth. The link with the smallest cost is considered optimal. In the shortest path first algorithm, the value of the maximum possible cost can be almost unlimited. If there is no change in the network, the router only needs to periodically refresh the routing table that has not been updated (the period can be from 30 minutes to 2 hours).

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