configuring cisco catalyst switch VTP modes

VTP (Vlan Trunking Protocol) modes are different ways a switch can be set to share or interact with VTP advertisements. The three modes are Server, client and Transparent.

Server mode is the default setting for Cisco catalyst switches. Within any VTP domain, there must contain at least one switch configured in server mode. When in this mode, the switch can be used to add, modify or delete VLAN related infrormation. These changes are then advertised to other switches in the same domain that are configured as either servers or clients. The receiving switches then compare the revision number from the received update and if the revision number is higher, then it changes its configuration

Client mode is simply a receiver of VTP information configured from a server. when in client mode, no changes can be mad to the switch.

Transparent mode is where a switch will not listen to VTP information being advertised but will pass on the information to other switches. It acts as a standalone device and any VLAN information can be configured.

Configuring the modes:

First we get into the vlan database.

SWITCH#vlan database

We can then type in the vtp command followed by a question mark to see the different options.


SWITCH(vlan)#vtp ?
client Set the device to client mode.
domain Set the name of the VTP administrative domain.
password Set the password for the VTP administrative domain.
pruning Set the administrative domain to permit pruning.
server Set the device to server mode.
transparent Set the device to transparent mode.
v2-mode Set the administrative domain to V2 mode.

for now we shall only concentrate on the three vtp mode options: Server, client and transparent.

To configure as a Server simply type in the vtp server command and you get a notification that the device is switching into server mode.

SWITCH(vlan)#vtp server
Setting device to VTP SERVER mode.

The same goes for configuring as a vtp client.

SWITCH(vlan)#vtp client
Setting device to VTP CLIENT mode.

as well as transparent mode.

SWITCH(vlan)#vtp transparent
Setting device to VTP TRANSPARENT mode

To save and exit into the vlan database, simply type in "exit".

SWITCH(vlan)#exit
APPLY completed.
Exiting....

To verify the VTP mode configured, we can use the "show vtp status" command.

SWITCH#sh vtp status
VTP Version : 2
Configuration Revision : 0
Maximum VLANs supported locally : 256
Number of existing VLANs : 7
VTP Operating Mode : Transparent
VTP Domain Name :
VTP Pruning Mode : Disabled
VTP V2 Mode : Disabled
VTP Traps Generation : Disabled
MD5 digest : 0x1A 0x47 0x70 0xB8 0xD1 0x2F 0x7E 0x32
Configuration last modified by 10.1.35.2 at 3-1-02 00:48:56


In this instance, the switch is in Transparent VTP operating mode.

Configuring NAT Part 2: Static Translation

This article is a continuation of Part 1, so if you haven't read the part 1, I highly recommend that you go back and read Part 1. In this article, I will be assuming that you know how to configure the basic dynamic NAT configuration. However, if you are familiar with NAT and just want to read up on configuring a Static NAT, you can continue on without reverting to the previous NAT blog.

Let's do a concise review of Dynamic NAT configuration before moving on.

Step 1: Determine inside and outside networks.

• GatewayRouter(config)#int s0/0
  GatewayRouter(config-if)#ip nat outside
  GatewayRouter(config-if)#int fa1/0
  GatewayRouter(config-if)#ip nat inside
  

Step 2: Ensure that the connectivity exists between the routers.

• GatewayRouter(config)#ip route 0.0.0.0 0.0.0.0 s0/0
  

Step 3: Configure a pool private IP addresses.

• GatewayRouter(config)#access-list 1 permit 10.0.0.0 0.0.0.255
  

Step 4: Configure a pool public IP addresses.

• GatewayRouter(config)#ip nat pool OUTSIDE 216.116.120.250 216.116.120.254 net 255.255.255.248
  

Step 5: Configure Network Address Translation.

• GatewayRouter(config)#ip nat inside source list 1 pool OUTSIDE
  

Step 6: Verification.

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What is Static NAT

A type of Network Address Translation where an one-to-one map exists between a public IP address and an internal, private IP address. This method is manual and static; as a result, it is more time consuming and does not automatically react to changes in networks.

So, what is its purpose? When you publish your webservers or Front-end Exchange servers, you will need to associate those servers with a public IP address so that the clients and users can access the servers from external networks. Likewise, the above servers will also require a private IP address for internal users to access them. Instead of using two NIC's to resolve the above issue, it's recommended to use Static NAT to more securely kill two birds with one stone. Here are the general steps when configuring a Static NAT:

• Step 1: Determine inside and outside networks. Label each interface as “inside” or “outside” using the ip nat inside and ip nat outside commands at the interface or subinterface configuration mode.


• Step 2: Ensure that the connectivity exists between the routers. By using either static routes or dynamic routing protocols, make sure that your inside global network can connect to the outside network.

• Step 3: Configure Static Network Address Translation. Define components that will be translated.


• Step 4: Verify your work

In the below network diagram, let's configure a static NAT for the front-end Exchange Server.

Step 1: Determine inside and outside networks. Label each interface as “inside” or “outside” using the ip nat inside and ip nat outside commands at the interface or subinterface configuration mode.

GatewayRouter#conf t
GatewayRouter(config)#int s0/0
GatewayRouter(config-if)#ip nat outside
GatewayRouter(config-if)#int fa1/0
GatewayRouter(config-if)#ip nat inside

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Step 2: Ensure that the connectivity exists between the routers. By using either static routes or dynamic routing protocols, make sure that your inside global network can connect to the outside network.

GatewayRouter(config)#ip route 0.0.0.0 0.0.0.0 s0/0

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Step 3: Configure Network Address Translation. Define components that will be translated.

GatewayRouter(config)#ip nat inside source static 10.1.0.25 216.116.120.252

Note that for Static NAT, you do not need to configure a pool or an access-list.

Configuring NAT Part 1: Dynamic Translation

NAT was introduced to help resolve the issue of IP Address shortages. With a fast growing number of devices that require an IP Address, many experts predicted that the pool of IP addresses will run out soon. Before a well thought out, long-term solution can be implemented, a few short-term solutions were introduced. Amongst ingenious solutions such as CIDR and Private IP addresses, NAT was introduced.

The term NAT is used by many vendors, and it may differ slightly depending on which equipment you are using to configure NAT. In this blog, we will be exploring NAT used by Cisco IOS.

Here are general steps when configuring a NAT:

• Step 1: Determine inside and outside networks. Label each interface as “inside” or “outside” using the ip nat inside and ip nat outside commands at the interface or subinterface configuration mode.


• Step 2: Ensure that the connectivity exists between the routers. By using either static routes or dynamic routing protocols, make sure that your inside global network can connect to the outside network.


• Step 3: Configure a pool private IP addresses that will be allowed to access the external network


• Step 4: Configure a pool public IP addresses that will be used by your internal network to access the external network


• Step 5: Configure Network Address Translation. Define components that will be translated.


• Step 6: Verify your work

Let's configure the Gateway Router from the below diagram so that the PC's in the internal network can communicate with the devices in external networks.

Step 0: Configure base configuration and assign IP addresses according to above diagram

Router>enable
Router#configure terminal
Router(config)#hostname GatewayRouter
GatewayRouter(config)#interface s0/0
GatewayRouter(config-if)#ip add 216.116.120.250 255.255.255.248
GatewayRouter(config-if)#no shutdown
GatewayRouter(config-if)#int fa1/0
GatewayRouter(config-if)#ip add 10.0.0.1 255.255.255.0
GatewayRouter(config-if)#no shut

Always verify that IP addresses are inputted properly and the Status and Protocol are both up.

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Step 1: Determine inside and outside networks. Label each interface as “inside” or “outside” using the ip nat inside and ip nat outside commands at the interface or subinterface configuration mode.

GatewayRouter#conf t
GatewayRouter(config)#int s0/0
GatewayRouter(config-if)#ip nat outside
GatewayRouter(config-if)#int fa1/0
GatewayRouter(config-if)#ip nat inside

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Step 2: Ensure that the connectivity exists between the routers. By using either static routes or dynamic routing protocols, make sure that your inside global network can connect to the outside network.

GatewayRouter(config)#ip route 0.0.0.0 0.0.0.0 s0/0

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Step 3: Configure a pool private IP addresses that will be allowed to access the external network

You need to specify the source addresses that will be translated. In this exercise, you will be using an access-list to specify a pool of IP addresses. We will be allowing all internal IP addresses to pass through the gateway router.

GatewayRouter(config)#access-list 1 permit 10.0.0.0 0.0.0.255

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Step 4: Configure a pool public IP addresses that will be used by your internal network to access the external network

Next step is to specify the pool of IP addresses that will be used as Inside Global IP Addresses. Ensure that the pool name specified in the previous command matches the pool name you will be creating in the below command line. Please note that pool name is case-sensitive.

GatewayRouter(config)#ip nat pool OUTSIDE 216.116.120.250 216.116.120.254 net 255.255.255.248

Note that the first IP address marks the starting point of the Inside Global IP Address and the second IP address marks the last Inside Global IP Address to be used for NAT.

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Step 5: Configure Network Address Translation. Define components that will be translated.

GatewayRouter(config)#ip nat inside source list 1 pool OUTSIDE

As you can see there are many options to choose from. Let’s go over the options that we chose.

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Step 6: Verification

The last step is to verify that the Network Address Translation works. You can login to one of your PCs and ping out to the internet. If you have a DNS set up, you can ping a known websites such as google.com or yahoo.com. If your internet network does not have a DNS setup, try to ping an external DNS such as 4.2.2.3. Once you have successfully pinged an external entity, you can revert to GatewayRouter to verify the translation.

Translation is successful. Please note that port number from the output will vary, but the port numbers from Inside global and Inside local will generally match.

How Are IP Addresses and Subnet Masks Related?

One of the topics that I (along with many others) had trouble understanding was how to differentiate the network side of an IP address with the host side of an IP address. For example, our computer has an IP address 192.168.1.1 with a subnet mask of 255.255.255.0. The subnet mask is key in determining the difference between your network address and host address.

Now let’s break down our computer’s IP. Every IP consists of four, 8-bit octets that range in decimal value from 0 to 255. For instance, 192 (our IP’s 1st octet) in decimal translates to 11000000 in binary (for more information on binary to decimal translation, see this article: http://www.wikihow.com/Convert-from-Binary-to-Decimal). This next step is key. ANY BINARY ‘1’ IN OUR SUBNET DEFINES THE NETWORK, and ANY ‘0’ IN OUR SUBNET DEFINES OUR HOST. Because our subnet mask’s 1ST, 2nd, and 3rd octets are 255 (or 11111111 in binary), this means that our network address that our IP exists in is 192.168.1.0. The 4th octet, thus, defines the host number in this network. So in this one particular network, 192.168.1.0, we can have up to 254 host computers. You may wonder why not 255?? This is because the host address 255 (192.168.1.255) is reserved for the network’s broadcast address. Therefore, we can only host 254 addresses on our network (1-254).

Let’s try a little harder example this time. Say we go to the command line on our computer and do an “ipconfig” command to display our IP and subnet mask. The output displays our IP address being 192.168.1.193 with a subnet mask of 255.255.255.192. The first thing to do is break the IP address into each octet. Luckily for us, the 1st three octets are all our network address as defined by the subnet being all 1’s (255 = 11111111 in binary). Now all we need to worry about is our last octet! The 192 in our subnet mask translates to 11000000 in binary (All ‘0s’ being possible host IP addresses). Because our network is defined by 1’s in the subnet, the first 2 bits of the last octet of our IP are still part of the network address. So, if we translate 193 to binary, we get 11000001. 193 ends up being the first host in the network 192.168.1.192! So in this case, our network address is 192.168.1.192 in which hosts in this network will range from 192.168.1.193 – 192.168.1.254!

Fortunately for us, IPv6 has been created which voids the need for differentiating the network address from the host address using a subnet mask. I will post this topic in a later article, but for now, IPv4 takes practice, practice, and even more practice to understand the concept of the relationship between your IP address and subnet mask. Try it yourself on your computer at home and let me know how things go!

Using Question Mark For Secret Password With Cisco Routers and Switches

As you know a question mark in Cisco IOS (Internetwork Operating System) is used to display available options in the context you actually are. What if you want to use a question mark as a one of the letters of your password? We will assume the password you want to use is "qm?".

If you use it here is what will show:

Router(config)#enable secret ?
0 Specifies an UNENCRYPTED password will follow
5 Specifies an ENCRYPTED secret will follow
LINE The UNENCRYPTED (cleartext) 'enable' secret
level Set exec level password

Even if you type the beginning of it:

Router(config)#enable secret qm?
LINE
Router(config)#enable secret qm

It still comes back to just "qm". So how can we use it? You need to press CTRL+v prior to pressing "?":

Router(config)#enable secret qm?

Whoala! The magic combination was CTRL+v.
By the way you may have the idea to cut and paste a "?" from the notepad into terminal try it... and let us know.