Duplex defines how traffic will be sent and can be related to a 2-way radio vs. A telephone.

With a 2-way radio, more so when there are more than 2 people on the same radio frequency, only 1 person may speak at the same time. If 2 people were to push the PTT (push to talk) button at the same time, they would hear a squelch. Both people would hear the squelch, release the PTT button and try again at a random time interval. Hopefully only 1 person would press the button this time and would be able to speak on the radio frequency, of course if both people pressed the button at the same time again, they would have to repeat the procedure until they managed to be the only person communicating.

In terms of Duplex, a 2-way radio would be the same as ‘Half-Duplex’ were only 1 person may communicate at the same time, if computers tried to talk at the same time, they would have a ‘collision’. Like the people, the 2 computers notice the collision and wait a random time frame before transmitting the packet again.

Full duplex on the other hand is like a telephone. Both parties in a telephone call can talk at the same time. Granted most humans cannot hear and talk at the same time on the phone, but the telephone is able to transmit both flows of traffic from both sides of the telephone line at the same time. If 2 computers wished to send data to each other, and full duplex was available, they would be able to transmit and receive at the same time.

NOTE: A hub can ONLY run at half-duplex. It is not possible for a hub to run at full duplex. A Switch however can be configured for either.

AOIP.ORG_Switch# show interface f0/7

In the live demo below you will notice I have highlighted the current status of the interface (Auto-duplex, Auto-speed)

AOIP.ORG_Switch# conf t

AOIP.ORG_Switch(config)# interface fastethernet 0/7

Enter the interface you wish to configure

AOIP.ORG_Switch(config-if)# duplex full

Set the Duplex to Full, Half or Auto. Auto doesn’t always end up with the best result, so forcing the duplex to the correct value to recommended

AOIP.ORG_Switch(config-if)# speed 100

Set the speed of the interface. Depending on the interface will depend on the possible options. Ethernet can only be set at 10Mbps, FastEthernet can be set at either 10Mbps or 100Mbps, and GigabitEthernet can be set to 10/100/1000 mbps

AOIP.ORG_Switch(config-if)# no shut

Activate the interface

AOIP.ORG_Switch(config-if)# do show interface f0/7

Confirm the new setting have taken effect.

Related posts:

1. Configuring an IP address and Default-Gateway on a Cisco Switch
2. Port Security on a Cisco Switch
3. Configuring a Trunk port on a Cisco Switch

In this tutorial I’m going to explain how to use this command, and different options available using it.

Below is the breakdown of the commands I used in the live demo, and an explanation of each.

AOIP.ORG_Switch# terminal monitor

Since I was connected to my switch via telnet, and I knew there were going to be messages from the switch, I needed to configure Terminal Monitor so I would have these messages sent to my telnet session. By default when connected to a Cisco device via telnet or ssh, no messages will be displayed to your terminal.

AOIP.ORG_Switch(config)# interface fa0/6

Enter the interface that I wish to configure the port security on

AOIP.ORG_Switch(config-if)# switchport mode access

In order for port security to be used, the port MUST be an access port, this command defines that

AOIP.ORG_Switch(config-if)# switchport port-security

This enables the port security feature, and allows me to define the commands below.

AOIP.ORG_Switch(config-if)# switchport port-security maximum 1

I have chosen to only allow 1 mac-address to be learned on this port. At any point if more than 1 mac address was to be discovered, the violation action I define will come into effect.

AOIP.ORG_Switch(config-if)# switchport port-security mac-address aaaa.bbbb.cccc

I have further secured the switch port by defining what mac address is allowed to be learned on this port. If a machine is plugged into this port that does NOT have this mac-address, the violation action will take effect.

AOIP.ORG_Switch(config-if)# switchport port-security violation shutdown

I have 3 choices when defining the violation action

                1/ protect – The switch will drop packets until the violation has been removed

                2/ restrict – This is the same as protect, however it also causes the Security/Violation counter to increment

                3/ shutdown – This will put the interface into a error-disabled state and send an SNMP trap notification

I have chosen the more harsh of the options, and the port will be shut if any of my conditions (more than 1 mac address is learned on the port, and if that one mac address is not aaaa.bbbb.cccc)

AOIP.ORG_Switch(config-if)# exit

AOIP.ORG_Switch(config)# exit

AOIP.ORG_Switch# show port-security interface f 0/6

The first time I ran this command in the live demo, you will notice the configuration on the port that I had just made, however there are no violations recorded. Shortly afterwards, I plugged a device into port f0/6 that DID NOT have the mac address aaaa.bbbb.cccc which caused a violation. You will notice I received error messages on screen (thanks to term mon), and when I run the show port-security command again, you will notice the violation count has incremented.

Additional commands I could have used are.

AOIP.ORG_Switch(config-if)# switchport port-security aging time 5

If you have configured the switch to allow 5 mac addresses to be learned dynamically, those addresses will be kept in the database until the aging time has expired. This command will set the aging time to 5 minutes, which overrides my switches default value of 20 minutes.

AOIP.ORG_Switch(config-if)# no switchport port-security aging

This will DISABLE the aging time.

In order to activate a port that has been put into ‘error-disabled’ state. Shut the port, and no shut it afterwards. If the violation has not been removed, the port will revert back to error-disabled.

Related posts:

1. Configuring a Trunk port on a Cisco Switch
2. Configuring an Access port on a Cisco switch
3. Configuring an IP address and Default-Gateway on a Cisco Switch

On a router we configure the IP address on the physical interface, however on a switch the physical interfaces are running at Layer 2 and hence don’t have IP addresses configured on them.

Even though a switch does not need an IP address to be able to switch packets, in order for you to connect to the switch via telnet or SSH you need to have a management IP address configured. Similarly, if you are connecting to the switch from a different subnet, the switch will require a default-gateway in order to have the packets routed back to you.

Since the IP address is not bound to any physical interface on the switch, we need to bind it to a logical interface. On a switch the logical interface is known as a ‘vlan interface’ (VLAN – Virtual LAN). This is similar to a loopback interface found on a Cisco router.

Here is the breakdown of the commands used in the live demo below.

AOIP.ORG_Switch# conf t

AOIP.ORG_Switch(config)# interface vlan 1

This enters the logical interface ‘vlan 1’. Vlan 1 is the native vlan and used for management purposes.

AOIP.ORG_Switch(config-if)# ip address 192.168.1.11 255.255.255.0

Assigns the ip address 192.168.1.11 to the interface

AOIP.ORG_Switch(config-if)# no shut

Activates the interface

AOIP.ORG_Switch(config-if)# exit

AOIP.ORG_Switch(config)# ip default-gateway 192.168.1.1

Defines a default-gateway for this switch.

Related posts:

1. Configuring a Trunk port on a Cisco Switch
2. Configuring an Access port on a Cisco switch
3. Port Security on a Cisco Switch

                1/ Copy and paste the backup configuration directly to the device

                2/ Copy the configuration from a TFTP server to the device.

In both cases, make sure you save your configuration afterwards using either “copy run start” or “write”.

Below is an example of restoration from a TFTP server. You will notice the router name is “Router” – the default hostname for a router without configuration. After the restoration is completed, you will notice the hostname goes back to the configured value.

Related posts:

1. Upgrading the IOS on a Cisco Router
2. Backing Up the Running-Config and IOS
3. Navigating a Cisco device

The other major reason for IOS upgrades is to fix bugs that Cisco may have found in the software.

The 2 most common ways to do an upgrade are

                1/ Take the Flash card out the router, plug it into a card reader and paste the new file on the card. Replace the card in the router and reboot.

                2/ Copy the file over the network using a protocol like TFTP.

When copying the IOS over the network, the router will check to confirm it has enough storage space to accommodate the file. It will often ask to erase the flash before copying the new image. Before erasing the flash on the router, make sure there are no other files on the flash that you might need. If there are files that you will require, make sure you have made a backup of them before erasing!

Here are the steps for upgrading via the network.

1/ Run your TFTP server and confirm it’s hosting the IOS that you wish to upgrade to

2/ Configure the following on the router

 AOIP.ORG# copy tftp: flash:

Address or name of remote host []? 192.168.1.1

Source filename []? c1841-ipbase-mz.124-5b.bin

Destination filename [c1841-ipbase-mz.124-5b.bin]?

%Warning: There is a file already existing with this name

Do you want to over write? [confirm]

In the example above, I had chosen to upgrade to the same image that was already loaded on the router. Although not a practical solution, it does show the commands to upgrade.

Below you will see the different output when I chose a different file.

AOIP.ORG# copy tftp: flash:

Address or name of remote host [192.168.1.1]?

Source filename [c1841-ipbase-mz.124-5b.bin]? c1841.bin

Destination filename [c1841.bin]?

Accessing tftp://192.168.1.1/c1841.bin…

Loading c1841.bin from 192.168.1.1 (via FastEthernet0/0): !

%Error copying tftp://192.168.1.1/c1841.bin (Not enough space on device)

Again, I have received an error message, this time due to lack of free space on my flash card on the router. I would then have a chance to confirm there are no files on the flash that I need to keep before erasing the flash. If there are files I need to retain, make sure you back them up before erasing!

Make sure that you have already confirmed that the router physically has enough flash to be able to support the image you wish to install. You can see how much Flash is available and installed by using the ‘show flash:’ and ‘show version’ commands

Depending on the router model, you can erase the contents of the flash by typing

AOIP.ORG# erase flash:

OR

AOIP.ORG# erase nvram:

After the flash has been erased, you would then be able to copy the file from the tftp server.

Below is a live demo of an upgrade.

Related posts:

1. Backing Up the Running-Config and IOS
2. Restoring Cisco Router Configuration
3. Configuring DHCP on a Cisco router

It’s a logical statement but sadly one that many people only truly appreciate when it’s too late!

Two of the most important files on a Cisco router are the Running-Config which stores the current configuration of your router, and the IOS (The operating system) that your router needs to be able to boot.

It’s vitally important that a copy of both of these files is available at all times, and one of the easiest and quickest ways to keep these files from all your devices together is to copy them to a TFTP server. I use PumKIN as my TFTP server as it’s free, easy to use and does the job perfectly.

At the end of this post you will see a Live demo of how to copy the IOS and Running-config to a TFTP server running an IP address of 192.168.1.1.

Here is a breakdown of the configuration

AOIP.ORG# show flash

This will show you all the files that are currently stored in the Flash memory of the router. The file with a .bin extension is the IOS that we want to backup. You can copy this file name for use in the next step.

AOIP.ORG# copy flash: tftp:

This instructs the router I would like to copy data from the flash of the router to a tftp server.

Source filename []? c1841-ipbase-mz.124-5b.bin

You are prompted for the file name you wish to copy, you can paste the .bin file you have just copied.

Address or name of remote host []? 192.168.1.1

Specify the IP address of the TFTP server

Destination filename [c1841-ipbase-mz.124-5b.bin]?

The router will automatically prompt you to save the file with the same name as the source. You can change it here, or just press <Enter> to accept the file name.

At this point your router will start to copy the file. Once the file has finished copying, it will show you the details. In my case my router displays “13966524 bytes copied in 50.068 secs (278951 bytes/sec)”

You can backup your Running-Configuration very similarly, however replace flash: with running-config as seen below

AOIP.ORG# copy run tftp:

Address or name of remote host []? 192.168.1.1

Destination filename [aoip.org-confg]?

!!

784 bytes copied in 1.180 secs (664 bytes/sec)

Related posts:

1. Upgrading the IOS on a Cisco Router
2. Restoring Cisco Router Configuration
3. Navigating a Cisco device

In order to understand clock rate we first need to understand how the cabling works on routers. When connecting two routers together with a serial cable, one of the routers needs to host the DCE (Data Communications Equipment) side of the cable, and the other will host the DTE (Date Terminal Equipment) side. Most serial cables are marked on the connector if it’s the DTE or DCE.

So what’s the difference between the 2 sides? The DCE side of the cable is the side that sets the speed of the link (also known as clocking). Based on this, it’s safe to assume that the cable coming out of your router and going to your service provider is the DTE side, since you service provider sets the speed of the line based on the subscription you have purchased. The DTE side of the cable is where the communications terminate ie: your router terminates the connection from the service provider.

From a configuration point of view the DCE side of the cable is able to use the clock rate command to set the speed of the line. If the command is not used the interface will run at the maximum speed supported by the interface. If you have only subscribed for a 64k line, then the clock rate would have been set on the DCE side of the cable using the command ‘clock rate 64000’ under the interface.

AOIP.ORG# conf t

AOIP.ORG(config)# interface serial 1/0

AOIP.ORG(config-if)# clock rate 64000 (represented in BITS per second)

If you try use the clock rate command on the DTE interface you will receive the following error message “This command applies only to DCE interfaces”

To identify which end of the cable has been plugged into a Cisco router, you can also use the command “show controller” – see below diagram.

Cisco Show Controller Command

The bandwidth command however does not adjust the speed of the line at all, however it should be configured on ALL DTE and DCE interfaces because it is used by;

1/ Routing protocols – to calculate the cost of a path

2/ QOS (Quality of Service) – to identify how much bandwidth is availble to prioritize,

If no bandwidth command has been configured on the interface, Routing protocols and QOS will assume the line is running at the maximum speed supported by the interface which can result in incorrect routing and incorrect prioritization of packets.

The bandwidth command is issued under the interface, as is represented in Kilobits per second

AOIP.ORG# conf t

AOIP.ORG(config)# interface serial 1/0

AOIP.ORG(config-if)# bandwidth 64 (represented in kilobits per second)

Related posts:

1. Interface Status
2. Configuring EIGRP on a Cisco Router
3. Cisco Administrative Distance

RIPv1 is a classfull routing protocol and does not support VLSM’s or CIDR values and sends it’s updates as a broadcast message with a destination address of 255.255.255.255

RIPv2 supports classless address and supports VLSM and CIDR values. It sends it’s updates as a multicast messages and includes the subnetmask value in its updates.

In both cases RIP is a distance vector routing protocol and uses hop count as it’s metric value.

RIP sends its entire routing table information to its neighbouring routers every 30 seconds (default Update Timer). Since the updates are sent every 30 seconds no acknowledgement is needed from the receiving router to confirm it received the update and if it does happen to miss one it will receive the next one in 30 seconds.

If RIP does not receive an update for a route that has been learned from a neighbour within the Invalid Timer, which has a default value of 180 seconds (6 updates), the route will be marked as invalid and no packets destined for that network will be forwarded. The entry will however still remain in the routing table until the Flush timer has been reached (240 seconds by default), at which point the route will be removed from the routing table.

Cisco also includes a Holddown Timer, which allows the router to identify if a metric value has changed for a route that has already been learnt. This often happens when a link has failed and the router receives an old update from a router through another path. The router will then not accept any updates regarding that route until the holddown has expired (240 seconds by default) which will prevent old out of date information propagating the network.

To identify the default timers, you can issue the ‘show ip protocols’ commands from enable mode on a Cisco router (See diagram below)

 In the live demo below, I have tried to ping the address 10.0.3.2 which is the FastEthernet 0/0 interface on R2. You can see that ping was unsuccessful since there is no route for the network 10.0.3.0 /24 – ‘sh ip route’. I then enabled RIP on my router

 AOIP.ORG(config)# router rip                                                     {enables RIP}

AOIP.ORG(config-router)# network 192.168.1.0                 {the network that I would like to participate in the routing updates}

After looking at the routing table again ‘sh ip route’ you will see there is a new entry with a ‘R’ in front of it indicating it is a RIP entry. You will also notice the entry is for a network ’10.0.0.0/8 [120/1]‘ The reason it’s showing as a class A address is RIPv1 is classfull and doesn’t understand CIDR values which I have used on this network. It assumes that since the network address starts with a 10, it must be a class A address and shows it in the routing table as such. [120] indicates the administrative distance and [1] is the metric value for this route. After the route has been added to the routing table by RIP the ping is successful.

Below is the network diagram used for the live demo, and the live demo.

Related posts:

1. Configuring EIGRP on a Cisco Router
2. Configuring OSPF on a Cisco Router
3. Cisco Administrative Distance

We have seen in previous posts on this website (Creating a Static Route and Default Gateway of Last Resort) how we make routes ourselves and how to teach a router how to send packets to networks that are not directly connected to it. Using static routes can be useful in some cases however in large networks it would take us a long time to manually teach every router about every network that we have.

The second problem with creating static routes is we are unable to monitor the links and whether they are operational or not. So a router will always send  a packet along the path we have told it without checking if the path is the best option or even a valid option.

For these reasons a routing protocol is a better solution.

Once we enable the same routing protocol on multiple routers, they are able to communicate with each other and share information about all the networks they are directly connected to and they are able to monitor if a network goes down due to a link failure. Since all routers need to run the same routing protocol to communicate with each other, you can almost think of this as a language.

If a router receives information about a network from multiple routers, it is able to discover there is more than one path to that network and then looks at a value called the ‘Cost’ or ‘Metric’ to work out which path is the best to use.

Each routing protocol has a different approach in sending information and what type of information is sent. Routing protocols such as RIP send information at set periodic intervals and only report how many hops it is to the destination network. In comparison, routing protocols like OSPF and EIGRP send information only when something has changed or there is something new to report, and they send information about the link speeds, their reliability and delay. This makes routing protocols like OSPF and EIGRP far more reliable than RIP. For this reason each routing protocol also has a ‘trust’ factor known as the administrative distance which the router uses to determine which routing protocol it will trust when making a decision as to which path is the best.

Routing protocols are often separated into “Distance Vector” or “Link-State”.

Distance Vector routing protocols, such as RIP and IGRP, derive their cost of the route based on how many devices are along the path – Hop Count.

Link-State protocols, such as OSPF and IS-IS, also look at information about the links to identify which path will be the fastest, even if this means sending the packet the longest distance.

EIGRP is often placed under distance vector routing protocols but since it also has a lot of the traits of a link-state protocol it falls into its own category – “Hybrid”

Related posts:

1. Cisco Administrative Distance
2. Configuring OSPF on a Cisco Router
3. Static Routing

Learning routes can take place through the use of routing protocols, however in this article I’m going to cover how to configure static routes to tell the router how to get to the destination network.

In the live demo you will notice that all the configuration is done on a router named AOIP.ORG and in the diagram below you will see that AOIP.ORG is only connected to the subnet 192.168.1.0 /24 and the loopback interface which has an IP address of 5.5.5.5 /24. If our router is to send a packet to any other network, it will have to have routing configured on it. If however it wants to send a packet to the network 192.168.1.0 or 5.5.5.0 it could do this without creating routes as both networks are directly connected.

To illustrate this point, I tried to ping the IP address 10.0.1.2 which is connected to R2, and is not directly connected to AOIP.ORG. You will see that the ping was unsuccessful since AOIP.ORG does not have a route to the network 10.0.1.0 and it’s not connected directly to it either.

After adding a static route of ‘ip route 10.0.1.0 255.255.255.0 192.168.1.1′ the ping works perfectly.

The ‘ip route’ command is broken up as follows

‘ip route {destination network} {subnetmask to match} {next-hop-address} {administrative distance}‘

The {destination network} is the network I would like to route to. (10.0.1.0)

The {subnetmask to match} entry identifies how much of the destination network needs to match. So in our case we are telling it to check the first 3 octets for a match ie: 10.0.1 and not worry about the value in the last octet. (255.255.255.0)

Note: I could have created a {destination network} of 10.0.0.0 and a {subnetmask} to match of 255.0.0.0. This would have told the router that if a packet was going to ANY network starting with IP address ’10′ it could use this route. I have been as specific as possible and hence told the router to match ’10.0.1′

The {next-hop-address} is the next router that the packet will be sent to and must be directly connected to the router you issue this command on (192.168.1.1)

The {administrative distance} I have not used in our example, but this is used to indicate how trust worthy the route is. If there are multiple options for the router to choose from, it will choose the most trust worthy, the lowest number.

Note: Routing has already been configured on all the other routers in the example below.

Related posts:

1. Default Gateway of Last Resort
2. Uni-directional routing issues
3. What is a Routing Protocol?