Author Archives: Vyacheslav

Configuring DHCP+TFTP for DOCSIS

Recently, it was necessary to configure the issuance of IP addresses to several old DOCSIS modems and the host located after the modem.
At hand was the Arris Cadant C3 and Thomson TCM-420 modems.

First of all, let’s start a DHCP server that will issue IP addresses to modems, for example, as I described in this article – Installing and configuring isc-dhcp-server.
And also we will launch a TFTP server on which there will be files for modems, for example, as I described in the article – Installing and Configuring a TFTP Server

Continue reading “Configuring DHCP+TFTP for DOCSIS”

Configuring ports in Cisco switches

For the test I will configure ports on Cisco Catalyst 6509-E.

I’ll give an example of setting Access of the port (the traffic goes only over one specified vlan without a tag):

interface GigabitEthernet1/1
description TEXT
switchport
switchport access vlan 226
switchport mode access
no shutdown

Now I’ll give an example of setting Trunk of the port (traffic goes through one or several vlan with a tag only):

interface GigabitEthernet1/2
description TEXT
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 207,228
switchport mode trunk
no shutdown

And the third option, Hybrid port (traffic goes only on one vlan without a tag and on one or several vlan with a tag):

interface GigabitEthernet1/3
description TEXT
switchport
switchport trunk encapsulation dot1q
switchport trunk native vlan 226
switchport trunk allowed vlan 207,226
switchport mode trunk
no shutdown

To specify parameters for several ports at once:

interface range GigabitEthernet1/1-24

We prohibit automatic switching of the port to access or trunk mode:

interface range GigabitEthernet1/1-24
switchport nonegotiate
exit

See also my article – Configuring link aggregation on the Cisco Catalyst 6500

Minicom error solution “Device /dev/ttyS0 is locked”

Sometimes if the connection to Linux from which the connection was made via Minicom is lost, then the next time you start Minicom, you could see an error:

Device /dev/ttyS0 is locked.

The name /dev/ttyS0 may be different, depending on what COM port you have.
To avoid such an error, it is necessary to correctly shut down Minicom using the CTRL-A keys and then the Q key.

You can solve the problem by killing the process as a root user with the command:

killall -9 minicom

In Ubuntu, use the command:

sudo killall -9 minicom

Or you can simply delete the file «LCK..ttyS0» in the /var/lock/ directory.

After that, the error will not be displayed.

Script backup configuration DOCSIS ARRIS Cadant C3 CMTS

Actually, this is my script:

#!/bin/bash
# Backup DOCSIS CADANTS config
(
sleep 5
echo "user"
sleep 5
echo "password"
sleep 5
echo "enable"
sleep 2
echo "password"
sleep 2
echo "copy startup-configuration tftp://192.168.0.1/cadant1.xml"
sleep 5
echo "exit"
) | telnet 192.168.0.50
mv /srv/tftp/cadant1.xml /backups/devices/docsis/`date +%Y-%m-%d`_cadant1.xml

Where 192.168.0.50 – cadant, 192.168.0.1 – tftp server.

You can add the script to /etc/crontab for automatic execution (for example, every day at one in the morning):

0 1 * * * root /path/to/script/backup_cadants.sh > /dev/null 2>&1

Blocking third-party DHCP on Cisco via DHCP Snooping

On the test, I configure DHCP Snooping on the Cisco Catalyst 6509-E to block third-party DHCP servers, on the other Cisco switches, the configuration is basically the same.

After connecting to the device immediately go to the configuration mode:

enable
configure

Continue reading “Blocking third-party DHCP on Cisco via DHCP Snooping”

How to catch broadcast flooding on MikroTik devices

It took somehow in one network to determine where the jumps of broadcast traffic are coming from, because of which the CPU usage was increasing on devices and there were interruptions with the Internet.
The network equipment was used from MikroTik.

Having connected to MikroTik with the following command, let’s look at the traffic statistics on ports, namely the broadcast traffic “Rx Broadcast” coming to the port, since this is the packet counter, then the figure should grow if the flood comes, if it does not change, then all is well:

interface ethernet print stats interval=1

Here is an example of viewing the statistics of a specific port (where ether2 is the name of the interface, it may be different depending on how it was called in the configuration):

interface ethernet print stats from ether2 interval=1

See the list of ports/interfaces with the command:

interface print

In this way, by the chain we will reach the final port from which there is a broadcast flood and, if necessary, turn it off by the command (where NUMBER is the number of the port in order in the table which can be viewed by the command above):

interface disable NUMBER

To enable the port:

interface enable NUMBER

Via WEB or Winbox, you can see the statistics by opening the Interfaces menu on the left and in the Interface tab, let’s look at each interface.

Example of resetting port statistics:

interface ethernet reset-counters ether2
interface ethernet reset-counters ether2,ether3,ether4,ether5

On CRS models MikroTik, you can enable broadcast traffic control, for example, 100 packets per second on an ether3 port (similarly for other ports):

interface ethernet switch ingress-port-policer add port=ether3 rate=100 meter-unit=packet packet-types=broadcast

In the future, you can watch the network for example through the system Zabbix, in which you can configure the display of broadcast packet schedules and if the packet counter starts to grow, the system will notify you.

Speed limit on MikroTik through Queues

It was necessary somehow on the sector antenna to limit traffic for fans to shake torrents. Point set up and described in this article – MikroTik RB912UAG-2HPnD (BaseBox 2) + Ubiquiti Sector. In my case, the speed adjusts the billing, but I wanted to limit the test for the means of MikroTik.

Continue reading “Speed limit on MikroTik through Queues”