Description of the problem:
When the landlord used GNS3 to do DHCP experiments, he found that the router was used to simulate a PC. After configuring the DHCP server, some PCs under the vlan could not obtain ip addresses; the DHCP configuration, vlan, trunk allow vlan, and configuration were all correct . Repeated checks are fruitless, the following is the experimental topology:
Finally, I checked the other side carefully and found that it was a problem with the static mac-address table under Layer 3 switching. I used clear mac-address-table static to get the address perfectly.
I have sorted out a few troubleshooting ideas, and you are welcome to refer to and actively add:
The solution to the problem that the PC cannot obtain an address from DHCP on GNS3
1. First check whether the DHCP pool address pool is established correctly, whether the default-router is specified, and then check whether ip help-address is specified under SVI
2. Then check the second layer , Does the vlan corresponding to the Layer 3 switch exist (the switch connected to dhcp)
3. Is the vlan allowed on the trunk line of the Layer 3 switch? Is the vlan assigned to the Access port of the access layer?
4. If a router is used to simulate a PC, is there no ip routing ?
5. After checking the above 4 items, try to clear mac-address-table static on the Layer 3 switch. Then observe the principle of clear mac-address-table static on
the PC.
:
The mac address of the original PC is cached in the mac-address table of Layer 3 switching. At this time, a new vlan is added. A new DHCP is deployed. A large number of mac addresses are cached in its mac table. It is necessary to manually release the PC to obtain the address. ;The principle is the same as IE browser cache (guess)