Networking

If you work with Alcatel OmniSwitch switches, sooner or later you’ll need to drop into the command line to troubleshoot a port, configure a VLAN, check a stack, manage PoE or investigate a network problem. I’ve put together this list of the Alcatel OmniSwitch CLI commands I find most useful, based on working with OmniSwitch equipment in real-world environments. The commands in this guide have been tested on Alcatel OmniSwitch 6400 and 6850 switches. They may also work on other OmniSwitch models, but command availability and syntax can vary depending on the switch model and AOS version. Important: Always check the command against your particular OmniSwitch model and AOS version before using it on a production switch. Some commands in this guide can be disruptive or destructive. Alcatel OmniSwitch CLI Basics OmniSwitch uses a command-line interface that is slightly different from Cisco IOS, so if you’re more familiar with Cisco switches, some of the syntax may look unfamiliar initially. One of the useful things about OmniSwitch CLI is that commands are generally quite descriptive. Once you become familiar with the structure, day-to-day configuration and troubleshooting becomes relatively straightforward. This guide covers: 1. Reset an OmniSwitch to Factory Defaults Use this with extreme caution. If you need to completely reset an OmniSwitch, the configuration files can be removed from the working and certified directories before rebooting the switch. The switch will reboot and come back without the previous configuration. After the reboot, you can verify that a new configuration has been created with: or: ⚠️ Important This is a destructive operation. Make sure you have a backup of the configuration before doing this. I would strongly recommend confirming that you’re connected to the correct switch before running the commands, particularly when working remotely. 2. Save the Switch Configuration One thing worth understanding with OmniSwitch is the distinction between the Working and Certified configurations. To save the current working configuration: To copy the working configuration to the certified configuration: For a stack containing multiple switches, you can synchronise the configuration across the slots: You can then check the current configuration and synchronisation state with: A healthy switch should show something similar to: Why this matters Saving the configuration to the working configuration isn’t necessarily the same thing as having a certified configuration ready for the next reboot. For production switches, I recommend checking show running-directory after making significant configuration changes. 3. Change the Switch Name Giving switches meaningful names makes life considerably easier when you’re managing multiple devices. For example: Replace Switch1 with whatever naming convention you use. For example: This changes both the system name and the CLI prompt. 4. Configure System Location and Contact Information You can also configure basic information describing where the switch is located and who is responsible for it. Check the configuration with: This is particularly useful in larger environments where a switch may be physically located somewhere different from the person administering it. 5. Configure DNS and NTP Configure a DNS server with: Configure the DNS domain: Configure an NTP server: Enable the NTP client: For production networks, accurate time is particularly important for troubleshooting and log correlation. If you’re investigating an issue across switches, firewalls and servers, having every device using the same time source makes a huge difference. 6. Check System and Hardware Information These are some of the first commands I use when connecting to an unfamiliar OmniSwitch. General system information Chassis information Stack topology CPU health Memory health These commands are particularly useful when troubleshooting performance or unexpected behaviour. 7. VLAN Configuration Create a VLAN To create VLAN 10 and give it a name: This creates VLAN 10 with the description Guest. Delete a VLAN To remove VLAN 10: Be careful: removing a VLAN can immediately affect any ports, interfaces or services using it. Configure an Access/Untagged Port To assign ports 1 through 24 on slot 1 to VLAN 10: This makes those ports untagged/default members of VLAN 10. For example, if VLAN 10 is your guest network, this could be used to place a group of access ports into that VLAN. Configure a Tagged Port To add port 1 on slot 1 to VLAN 10 as an 802.1Q tagged member: This is useful for trunk/uplink-style connections where multiple VLANs need to traverse the same physical interface. Remove VLAN Membership To remove the default VLAN membership: To remove tagged membership: Check a VLAN To display the configuration of VLAN 10: To see the ports associated with the VLAN: These are useful commands when troubleshooting VLAN-related connectivity problems. 8. Interface and Port Configuration Show Port Status To see the status of the switch ports: This is one of the first commands I’d use when troubleshooting a device that cannot connect to the network. Show Detailed Port Information This provides more detailed information about the interfaces. Check a Specific Port OmniSwitch identifies interfaces using a slot/port format. For example: This displays information for port 5 on slot 1. 9. Configure Port Speed and Duplex The following examples use port 1/5. Disable auto-negotiation To enable it again: Set duplex Set interface speed For example: ⚠️ Be careful with manual speed/duplex In modern Ethernet networks, auto-negotiation should generally be preferred unless there is a specific reason to manually configure speed and duplex. A mismatch can result in poor performance, errors or an unreliable connection. 10. Administratively Disable or Enable a Port To shut down port 1/5: To bring it back up: This is useful when troubleshooting a device or deliberately taking a port out of service. 11. Give a Port an Alias Adding a description to a port makes switch management much easier. For example: You could also use something more descriptive: I highly recommend doing this wherever practical. When troubleshooting months later, a useful port description can save a lot of time. 12. Configure a Management Interface You can create an IP interface on a VLAN for switch management. For example: In this example: Check the configured IP interfaces with: To remove the

If you have a Cisco 887VA sitting around and a BT/Openreach FTTC connection, you can use the Cisco router’s built-in VDSL2 modem to connect directly to the fibre broadband service. Unlike a traditional ADSL connection, BT’s VDSL2 service uses PPPoE over Ethernet, with the broadband service carried over 802.1Q VLAN 101. The Cisco 887VA can handle the VDSL2 connection itself, meaning you don’t necessarily need the separate Openreach modem or a BT Home Hub. This guide shows a practical Cisco IOS configuration for connecting a Cisco 887VA directly to a BT VDSL2/FTTC service. Important: This guide is specifically for VDSL2/FTTC. It does not apply to FTTP, where the WAN connection is presented to your router over Ethernet. What you need For this configuration you will need: The exact interface names and available commands can vary between Cisco 800-series models and IOS releases, so you may need to adjust the configuration for your particular hardware. How BT VDSL works There are three important pieces to the connection: The Cisco router establishes the physical VDSL2 connection using its built-in modem. The broadband traffic is then placed into VLAN 101 on the Ethernet-based VDSL interface. PPPoE runs over that VLAN and creates the PPP session on the Cisco Dialer0 interface. BT’s current documentation confirms that its fibre broadband uses PPPoE, while VLAN 101 is commonly required when configuring third-party equipment directly against the BT/Openreach VDSL service. Configure the VDSL controller Start by configuring the VDSL controller. A typical configuration is: On some Cisco IOS versions, the controller may be named differently. For example, you may see something such as: Use: or, depending on the platform: to identify the correct controller. You can also use the following command to check the current DSL state: The important thing at this stage is that the modem successfully synchronises with the VDSL line. Configure the VDSL Ethernet interface The Cisco 887VA presents the VDSL connection as an Ethernet Layer 2 interface. First configure the physical interface: We then create an 802.1Q subinterface for VLAN 101: The important line here is: This places the PPPoE traffic into VLAN 101. The following command associates the PPPoE client with dialer pool 1: That dialer pool will later be used by Dialer0. Disable the ATM interface The 887VA supports both ADSL and VDSL technologies, so you may see an ATM0 interface in the configuration. For a VDSL2-only configuration it isn’t required: This makes it clear that the router is using the Ethernet-based VDSL interface rather than attempting to establish the broadband session over ATM. Configure the PPPoE Dialer The PPPoE session is terminated on a dialer interface. Create Dialer0: Replace: with the credentials supplied by your ISP. Why MTU 1492? A normal Ethernet frame supports a 1500-byte IP packet. PPPoE introduces an additional 8 bytes of overhead, which normally reduces the usable IP MTU to: Therefore: is a sensible starting point for a standard PPPoE connection. The MSS adjustment: helps prevent TCP packets from being too large for the PPPoE path. Some providers and configurations support larger frames, sometimes referred to as “Baby Jumbo” frames, but there is no reason to introduce that complexity unless your service and downstream equipment support it. Add the default route Once the PPPoE session comes up, traffic needs a default route towards the Internet. You can use: Alternatively, depending on the IOS configuration, ppp ipcp route default can install the default route automatically. I prefer explicitly configuring the route when building a simple configuration because it makes the routing behaviour obvious. Complete basic configuration Putting the important pieces together, a basic BT VDSL configuration looks like this: You will obviously need to adapt this configuration to your own LAN, NAT, DHCP, firewall and routing requirements. Configuring NAT If the Cisco 887VA is also going to act as your Internet router, you will normally need NAT. For example, if your LAN is: configure the LAN interface: Then mark the PPPoE interface as the outside interface: Create a NAT access list: And configure PAT: Your traffic flow is then: Checking the VDSL connection Once the configuration is applied, the first thing to establish is whether the physical VDSL connection is working. Use: You want to see that the modem has trained and is operating in VDSL2 mode. Depending on the IOS version, useful information may include: If the modem hasn’t synchronised, don’t start troubleshooting PPPoE yet. The problem is at the DSL layer. Checking the PPPoE session Once VDSL has synchronised, check the dialer: You should see the interface as up/up and an IP address assigned by the ISP. For example: You can also check the PPPoE session: And check the dialer status: If VDSL is synchronised but Dialer0 isn’t coming up, the problem is likely to be related to PPPoE, VLAN 101 or authentication rather than the physical DSL connection. Useful troubleshooting commands These commands are particularly useful when troubleshooting the connection: For a quick overview of the interfaces: You should expect to see something similar to: VDSL is up but Dialer0 is down This is one of the most common situations. If the router has successfully synchronised with the VDSL line but the PPPoE session isn’t establishing, check the following: 1. Check VLAN 101 Make sure the subinterface contains: VLAN 101 is an important part of many UK BT/Openreach FTTC configurations. 2. Check the PPPoE configuration Make sure the subinterface contains: And make sure Dialer0 contains: These three settings tie the physical VDSL interface, PPPoE client and dialer together. 3. Check your username and password Incorrect PPP credentials will prevent the PPP session from completing. Make sure you haven’t accidentally copied a space or line break into the username or password. 4. Check the PPP negotiation If necessary, PPP debugging can provide more information: Be careful with debugging on a production router because it can generate a significant amount of console/log output. Disable debugging when finished: The VDSL modem won’t synchronise If: shows that the modem is not synchronised, PPPoE configuration is not yet