Category Archives: Switching

Initial Configuration of IOS Devices

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In this post we will take a look at the initial configuration that should be performed on a new device before the actual network configuration is performed.

Most Cisco IOS devices come from the factory with a very basic default configuration, and require configuration before the are deployed. The majority come with no IP addresses, and routers even have all their interfaces shut down by default.

To begin the configuration, the first step is to console into the device, using either a serial port and the Cisco rollover serial cable, or the mini USB port on the newer devices. Console settings should be 9600 baud, 8 data bits, no parity, 1 stop bit, no flow control. You need terminal emulation software, such as PuTTY or SecureCRT.

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Configuring Router on a Stick

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Router on a stick (or One Armed Router) is a common name for a configuration used for routing between VLANs on a single Ethernet (including Fast/Gig/10Gig) interface. This configuration uses sub-interfaces on the separate VLANs and an 802.1q or ISL trunk to perform the routing. There are a few reasons you would use this configuration, and several not to. Probably the most common reason is cost. Most branches will have a router for WAN or internet access, using the same router for routing between VLANs saves the cost of a Layer 3 switch. You may also wish to make use of some of the more advanced features on a router, such as firewalling.  Another thing that cuts both ways is that you gain some simplicity by minimizing the number of routing devices, but you add complexity to the configuration of the router. The main drawback is performance. You are pushing all your traffic between two VLANs through a single interface, which could become a network bottleneck. You have to determine the potential impact to your network. The topology and network usage will largely determine how big an impact this will have. If you have all your PCs on one VLAN, and a couple servers that you are doing large file transfers with on another, and your printers on their own VLAN, you are probably going to see some serious issues. Conversely, if you have a single data VLAN, a wireless VLAN, and a voice VLAN, with the majority of the traffic from each going back to a central site over a WAN connection, this may be a very legitimate production use of router on a stick.

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Implementing SPAN

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Switched Port Analyzer (SPAN) is a means of redirecting traffic from one switch port to another for analysis. An example would be capturing the traffic to a host with a PC running a program like Wireshark. Setting up SPAN is a relatively simple operation, consisting of creating a monitoring session by specifying a source and destination. Multiple SPAN operations can be active on a switch at any given time, depending on the hardware platform.

To specify a source,  the port with the host to be monitored, issue the following command:

monitor session <session number> source interface <interface name> [rx|tx|both]

The session number is a locally significant value, used to match the source to the destination. This value must match in both commands. The RX, TX, or Both keyword limit the traffic captured to received or transmitted traffic only, or both directions. If no option is specified, bidirectional traffic will be captured.

To specify the destination, the port with the traffic analyzer, issue the following command:

monitor session <session number> destination interface <interface name>

Once both commands are configured, all traffic to and from the source port will be mirrored to the destination port, and can be captured with some sort of traffic analyzer. By default the destination port will not pass other traffic while in SPAN destination mode.

Here is an example of the configuration, as well as verification with the “show monitor session” command.

Switch(config)#monitor session 1 source interface fastEthernet 0/24
Switch(config)#monitor session 1 destination interface fastEthernet 0/23
Switch(config)#end
Switch#sh monitor session 1
Session 1
———
Source Ports:
RX Only:       None
TX Only:       None
Both:          Fa0/24
Destination Ports: Fa0/23