Three switches - one routing, forwarding times
The single-arm routing
First, the sequence
First of all, we want to understand the three-tier exchange knowledge, we need to understand (Layer device) switch (data link layer) and (three-tier equipment) routers (network layer) of the relevant knowledge (data frame) forwarding on the second floor foundation, three Layer (packet) to forward the principle of understanding, comprehension MAC address table, knowledge ARP cache and routing tables.
When it comes to the understanding of the three switches can not be separated and divided VLAN Trunk link, simply, the purpose of VLAN division is mainly produced in order to solve the network broadcast storms, which leads to the result that belong to the same network segment different VLAN host is not normal communication; and the origin of Trunk link is to communicate with each other between different hosts of the same VLAN switch, then we will consider such a question: after VLAN division in the LAN comprising a host of different VLAN how does communicate between each other it?
This requires three-tier exchange technology to help us solve this problem.
Speaking of three-tier exchange would have to talk about its predecessor: single-arm routing - single-arm routing (router-on-a-stick) refers to an interface on a router by configuring sub-interfaces (or "logical interface" , there is no real physical interface) way, interoperability between different VLAN (virtual local area network) to achieve the original isolation.
So what is it the principle of the single-arm routing?
Concept of logical sub-interface: physical interface of the router may be divided into a plurality of logical interfaces (or that a virtual interface), which is divided into the logical interface is called the image of the sub-interfaces. It is noted that these individual logical sub-interface can not be turned on or off, i.e., when the physical interface is turned on or off, all of the sub-interfaces of the interface will also be turned on or off.
II below with a specific routing topology-arm will be described:
(1) configure the switch: VLAN classification and Trunk link structures
(2) the router configuration
(3) pc configuration IP address, use wireshark capture software analysis and test results are given first and then look Ethereal to analyze the process
Ethereal result (selected data wherein the first ping packet (ICMP protocol based) to capture information about the process):
I chose the last ping the above results ping packets, i.e., the cut out layer by layer sequence number process about 100-103
(1) First, the source IP address of PC1, PC2 destination IP address, when the first sent through the switch, then the switch at this time is found from the tag will vlan10 over a label, VLAN-ID is 10, the image above ID information (gray) is indeed 10, and send data to Rl;
(2) At this time R1 receives routing table, and then found the directly connected segments, then forwards, in fact, come back, but this time the VLAN-ID is converted into 20, because the router according to their logical subinterface and transmitting the packet PC1 is to be sent may be determined to which the VLAN-ID. After it is converted to the above figure 20. Back ID Trunk link from the switch, the switch receives information sent to know VLAN20, and then releasing the tag ID
(3) When PC2 receive a message, to respond with, the source IP address of PC2, PC1 destination IP address, and passes through the switch, it is again to tag VLAN20, i.e. ID of the image above 20
(4) after receiving the message sent by the switch Trunk link to the router, the router begins the matching routing table, the VLAN ID convert forward the information out, so at this time the ID 10.
Third, the summary: The above experiment focused on understanding what is logical sub-interfaces, and then to understand the principle of packet capture is through single-arm routing, in fact, single-arm routing, the switch is mainly responsible for packaging and removal of labels, and the router is mainly responsible for routing and forwarding and VLAN translation.
In fact, there is a single arm routing bottleneck, you can see the next blog post, we will enter the world of three-tier exchange!