Cross-segment data communication flow

The process of verifying data communication through GNS3 and wireshask packet capture software

Experimental diagram:

Content of the experiment:

The process of data communication when router R1 ping router R3.

Steps:

① can view the MAC address of the interface through the show Int f0and0 command.

The MAC address of f0max 0 of R1 is: cc11.20f4.0000-MAC1

The MAC address of f0max 0 of R2 is: cc12.20f4.0000-MAC2

The MAC address of f1max 0 of R2 is: cc12.20f4.0010-MAC3

The MAC address of f1max 0 of R3 is: cc13.20f4.0010-MAC4

② analyzes pingR3---- by grabbing packets on R1.

Ping only once, command R1#ping 3.3.3.3 repeat 1

Grab packets between R1 and R2, link L12, and L23 between R2 and R3, and verify.

ICMP request package for L12:

ICMP reply package:

ICMP request package for L23:

You can know from the packet that:

In cross-segment data communication, the information of the transport layer and the source and destination IP address of the network layer remain unchanged, but the TTL value of the IP header will be reduced by 1 hop by 1 until the destination ip is reached. The checksum value also changes.

The link layer encapsulated MAC address in the packet changes, as shown in the lab:

The mac package of R1Musi-> R3 is as follows:

L12: MAC1 MAC2

L23: MAC3 MAC4

Summary:

The flow of data communication across network segments:

Transport layer: unchanged

Network layer: the IP address remains unchanged, the lifetime TTL will be reduced by 1 hop by hop (anti-ring function), and the checksum will also change.

Link layer: MAC source and destination addresses are sent and rewritten across network segments