Archive for the 'Routing' Category

Static NAT overloaded???

Wednesday, July 27th, 2011

So we have already looked at all 3 possible NAT configurations, however there is one more trick that is always useful to know. How to overload a Static NAT. Let’s assume the following for this example We have 2 public IP addresses ( & The IP address on the outside interface has been configured [...]

NAT (Network Address Translation)

Monday, September 21st, 2009

NAT or Network Address Translation is a key function required in every organisations network. Since all organisation use RFC 1918 IP addressing, and these IP addresses are not allowed to exist on the internet, before we send packets to the internet we need to translate the internal IP address into a useable public IP address. [...]

Floating Static Routes

Wednesday, August 26th, 2009

In order to have a fully fault tolerant network, a backup solution for WAN links is vital. There are many options for configuring a backup line incase the primary line fails and in this tutorial we are going to look at using floating static routes to achieve a dial-up connection to act as our backup [...]

Configuring Basic ISDN with Interesting Traffic

Tuesday, August 25th, 2009

When configuring ISDN with interesting traffic, it’s important to first understand how Cisco defines ‘interesting’ and what this means in terms of the connection been formed. Interesting traffic is traffic that we define in the form of an access-list that is allowed to cause the ISDN to dial. This does NOT mean it is the [...]

ISDN Switch-type

Monday, August 24th, 2009

In order for any ISDN lines to establish Layer-1 connectivity the switch-type has to be defined correctly. The switch-type is very much country dependant so it’s also important to memorize the switch-type that applies to where you do most of your installations.   If Layer-1 is showing ‘Deactivated’ when using the show isdn status command, [...]

Inter-VLAN Routing (Router on a Stick)

Wednesday, August 12th, 2009

In order for 2 VLANS to be able to communicate, the traffic must be routed. This can be done either by a multi-layer switch which will have routing capabilities, or the packets can be routed by a router.   As an example, I have 2 VLANS, VLAN 10 and VLAN 20 which have subnets [...]

Speed and Duplex configuration

Monday, August 10th, 2009

Although the configuration of both of these functions is incredible easy, the concept behind them is somewhat important.   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, [...]

Clock rate and Bandwidth commands

Thursday, July 16th, 2009

Two commands which are often confused or misunderstood, are the clock rate and bandwidth commands. 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 [...]

Uni-directional routing issues

Monday, July 13th, 2009

It is important to understand that any TCP packet that is routed over a network requires bi-directional support. This means that even though I may have a route on my router to your network, if you don’t have a route back to me, the transmission of packets will fail. TCP uses acknowledgments to confirm the [...]

Configuring OSPF on a Cisco Router

Friday, July 10th, 2009

OSPF (Open Shortest Path First) is an industry open standard supported by almost every router on the planet.   OSPF is a classless routing protocol so subnetmask values are sent in the update and it supports CIDR. OSPF does not send periodic updates and is designed to only send updates when something has changed or [...]