One of the basic knowledge of Cisco Routing Technology

Basic knowledge of Cisco Routing

Router <1>

The simplest network can be imagined as a single-line bus. Each computer can send packets to the bus to communicate with each other. However, as the number of computers in the network increases, this is not feasible and will cause many problems:

1. bandwidth resources are exhausted.
2. Each computer wastes a lot of time processing unrelated broadcast data.
3. The network becomes unmanageable, and any errors may paralyze the entire network.
4. Each computer can listen to communications from other computers.

Network segments can solve these problems, but you must also provide a mechanism for computers in different network segments to communicate with each other, this usually involves transferring data between network segments in some ISO network protocol layers. Let's take a look at the network protocol layer and the router location.

We can see that the router is at the network layer. This document assumes that the network layer protocol is IPv4, because it is the most popular protocol and involves a similar concept as other network layer protocols.

I. Routing and bridging

The bridge/switch between a route and a two-layer route is a high-level concept, which does not involve the physical details of the network. In a routable network, each host has the same network-layer address format (such as an IP address), regardless of whether it is running on Ethernet, ring, FDDI, or WAN. The network layer address consists of the network address and host address.

A bridge can only connect to networks with the same (or similar) data link layer. Different routers can connect to any two networks, as long as the host uses the same network layer protocol.

Vro <2>

2. Connecting the network layer and data link layer

The data link layer is under the network layer. In order to connect them, the "bonding" Protocol is required. ARP (Address Resolution Protocol) is used to map network layer (Layer 3) addresses to data link layer (Layer 2) addresses. RARP (Reverse Address Resolution Protocol) is the opposite.

Although ARP has nothing to do with the network layer protocol, it is usually used to resolve IP addresses; the most common data link layer is Ethernet. Therefore, the following ARP and RARP examples are based on IP and Ethernet, but note that these concepts are the same for other protocols.

1. Address Resolution Protocol

The network layer address is an abstract ing defined by the network administrator. It does not care about which data link layer protocol is used in the lower layer. However, network interfaces can only communicate with each other based on Layer 2 addresses, and Layer 2 addresses are obtained from Layer 3 addresses through ARP.

Instead of sending each packet, ARP requests are required to respond to ARP requests cached in the local ARP table. This reduces the number of ARP packets in the network. ARP is easy to maintain and is a simple protocol.

2. Introduction

If interface A wants to send data to interface B and A only knows the IP address of B, it must first find the physical address of B, it sends A physical address of ARP Broadcast Request B containing the IP address of B. After receiving the broadcast, interface B responds to the physical address of.

Note: although all interfaces receive information, only B should return the request, which ensures that the response is correct and the expired information is avoided. It should be noted that when A and B are not in the same network segment, A sends ARP requests only to the next hop router rather than directly to B. After receiving ARP packets, note that the sender's local ARP table for the host where the ARP request is received. Generally, when A wants to communicate with B, B may also need to communicate with.

3. IP address conflict

IP address conflict is the most common problem caused by ARP. This is because the IP addresses of two different hosts are the same. In any Interconnected Network, IP addresses must be unique. At this time, two ARP responses are received, indicating different hardware addresses. This is a serious error and there is no simple solution.

To avoid such errors, interface A sends an ARP request containing its IP address when it first tries. If no response is received, it assumes that the IP address is not used. Assume that interface B has used this IP address, then B will send an ARP response, and A will know that this IP address has been used, and it will no longer be able to use this IP address, instead, an error message is returned. In this case, another problem occurs. Assume that host C contains the ing of the IP address, which is mapped to the hardware address of B. After receiving the ARP broadcast from interface, update the ARP table to point it to the hardware address of. To solve this problem, B sends an ARP request broadcast again, so host C updates its ARP table and points it to the hardware address of B again. At this time, the network status is back to the previous status. Maybe C has sent the IP group that should be sent to B to A. Unfortunately, however, IP addresses provide unguaranteed transmission, so there will be no major problems.

4. Manage ARP cache tables

The ARP cache table is a correct list, indexed by IP address. The table can be managed using the command arp. Its Syntax includes:

Add a static table entry to the table -- arp-s

Delete A table entry from a table -- arp-d

Display table item -- arp-

Dynamic table items in ARP tables (no manually added table items) are usually automatically deleted after a period of time. The length of this period is determined by the specific TCP/IP implementation.

5. Use of static ARP addresses

A typical use of static ARP addresses is to set up an independent printing server. These devices are usually configured through telnet, but they need an IP address first. There is no obvious way to tell the device this information, as if it can only be set using its serial port. However, it is inconvenient to find a suitable terminal and serial cable and set the baud rate and parity.

Suppose we want to set an IP address P-IP for A print server, and we know its hardware address P-hard, create A static ARP table entry on workstation A to map the P-IP to P-hard, in this way, even though the print server does not know its own IP address, all data pointing to the P-IP will be sent to P-hard. Now we can telnet to the P-IP and configure its IP address, and then delete the static ARP table entry.

Sometimes the printing server is configured in one subnet and used in another subnet. The method is similar to the above. Assuming its IP address is A P-IP, we assign it A temporary IP address T-IP for this Web site, create A temporary ARP table entry on workstation A to map the T-IP to P-hard, then telnet to the T-IP, configure the print server with the IP address P-IP. Next, you can put it in another subnet. Do not forget to delete the static ARP table entry.

6. Proxy ARP

You can use proxy ARP to avoid configuring route tables on each host. This is especially useful when using subnets. Note that not all hosts can understand subnets. The basic idea is to send ARP requests to hosts not on the subnetwork. The ARP Proxy Server (usually the gateway) responds to the gateway's hardware address.

Proxy ARP simplifies host management, but increases network traffic (not obvious) and may require a large ARP cache, each IP address that is not on this network is created with a table item mapped to the hardware address of the gateway. In the host using proxy ARP, The world is like a large physical network without a router.

Article entry: aaadxmmm responsible editor: aaadxmmm