Category Archives: Cisco

The solution to the error “IP overlaps with VlanXXX. VlanXXX: incorrect IP address assignment”

It was necessary to replace the L3 switch of HP with Cisco once and after a similar switch configuration Cisco noticed an error:

172.16.63.0 overlaps with Vlan111
Vlan121: incorrect IP address assignment

As it turned out the network Vlan111 172.16.0.0/18 was ending at 172.16.63.254, it crossed with Vlan121 172.16.63.0/24.

The HP 5800 switch was configured before that and he did not say anything about it, and Cisco refused to accept the command.

Therefore, since IP addresses were used little in the Vlan111 172.16.0.0/18 network, the problem was solved by reducing the mask to 172.16.0.0/19.

After that, the IP address was successfully registered to the Vlan121 interface.

Done.

Connecting SFP-RJ45 Modules to the Cisco 6500

It took a couple of days ago to the Cisco Catalyst 6509-E in which there were modules only with SFP ports to connect a few links with RJ45.

Since RJ45 links are small, it was more economical to use SFP-RJ45 modules, so they were ordered.
I connected them to the ports WS-X6724-SFP, but nothing was displayed in the logs.

Let’s write commands so that Cisco does not disable ports when inserting unsupported modules:

service unsupported-transceiver
no errdisable detect cause sfp-config-mismatch
no errdisable detect cause gbic-invalid

I note that the ports WS-X6724-SFP in my case work only at 1Gb speed, so the link will not naturally rise to 100Mb or 10MB, although the Foxgate SFP-RJ45 modules that we had and support 10/100/1000.

In confirmation of this I checked the commands:

configure t
interface gigabitEthernet 1/1
speed ?

What was the opportunity to specify the speed of the port only in 1000.

See also:
Configure Cisco Catalyst 6509-E

Configuring DHCP relay on Cisco

On the test, I’ll take the Cisco Catalyst 6509-E switch and configure it to forward DHCP packets to the DHCP server.
The switch is configured as L3 with assigned IP addresses in each VLAN.

Connect to the switch through the console or telnet and go to the configuration mode:

enable
configure t

Let’s assume the DHCP server address is 192.168.11.1 and we want to configure the transfer of DHCP broadcast packets to it on VLAN 100, for this we execute the commands:

interface Vlan100
ip helper-address 192.168.11.1
exit

Exit the configuration mode and save the configuration:

exit
write

Done.