Network Address Translation (NAT) is an operation by which source and/or destination IP addresses within a packet are replaced with different IP addresses. NAT conserves available IP address space by allowing many private IP addresses to be represented by some smaller number of public IP addresses. Private IP addresses are defined in RFC 1918 and […]
Configuring GRE Tunnel Through a Cisco ASA Firewall
In this configuration tutorial I will show you how to configure a GRE tunnel between two Cisco IOS routers. One of the routers is located behind a Cisco ASA 5500 Firewall, so I will show you also how to pass GRE traffic through a Cisco ASA as well. As you might know already, GRE tunnel […]
Configuring EasyVPN Between Cisco Routers
Basically Cisco Easy VPN is usually used for Cisco easy VPN Client termination. In this post however, let’s consider the configuration of Cisco Easy VPN between two Cisco routers. Just keep in mind that an EasyVPN scenario involves a Server and Remote Clients. The basic configuration is performed on the Server and the configuration done […]
Using USB Flash Drives in Cisco routers
A few days ago I have written a post about Booting a Cisco Router from USB Flash. Related to that post and since I got some questions about the subject, it seems appropriate to add some more information about how USB Drives can be managed and used in Cisco IOS routers. Some things to consider: […]
Booting a Cisco IOS Router from a USB Flash Drive in Rommon Mode
Cisco routers typically store a copy of the device’s operating system (Cisco IOS) in their flash memory, and load this operating system image into RAM during the boot-up process. The flash memory of Cisco routers is usually internal or can be a removable flash card on higher end routers. However, it may happen that for […]
How to Configure Static Routing on Cisco Routers
Cisco IOS Routers support both static and dynamic routes. In small networks (2 to 5 routers) I would suggest to configure only static routes, especially if the network is not going to change much over time. Of course dynamic routing (using dynamic routing protocols such as RIP, OSPF, EIGRP) is much more flexible and scalable […]
Configuring Cisco Router Interfaces
One of the main elements to configure on a Cisco router is its interfaces. The router is usually equipped with Ethernet interfaces (for LAN connectivity) or Serial/ATM/T1/E1 interfaces for WAN connectivity. Below you will find three typical router interface configuration scenarios to get an idea about this important setup. Configuring Ethernet or Fast Ethernet Interfaces […]
Using TCP Intercept to mitigate DoS SYN Attacks
The most common attack against Service Provider IP Networks is Denial of Service. Usually these attacks take the form of “Many-to-One Attacks” where multiple attacking sources send flooding traffic towards a single destination. These attacks are sometimes called Distributed Denial of Service. Usually the attacking hosts are “zombie” computers which are compromised by hackers and […]
Routing Redistribution on Cisco Routers
In order for two Layer 3 devices (routers or layer 3 switches) to be able to exchange routing information, it is necessary to use the same routing protocol, such as RIP, EIGRP, OSPF, BGP, etc. Different routing protocols, or protocols configured differently (eg different EIGRP autonomous system) do not exchange information. However, when a device […]
DoS Protection on Cisco 7600 Routers
The Cisco 7600 router is in my opinion one of the most versatile High End routing machines on the planet!! It is one of my favorite networking devices. If you take a look at Cisco website under the Routers Product Category, you will notice that the 7600 can be used in Data Centers, in Service […]
Cisco Router Password Recovery-How to recover cisco router lost password
One of the most frustrating moments as a network administrator is when you desperately want to log on to a Cisco router and you try all possible passwords you have with no success. Fortunately there is a Cisco password recovery procedure for IOS routers which you can follow and bypass password control in order to […]
BlackHat 2009 Router Exploitation Presentation
I was reading an interesting presentation the other day, taken from BlackHat USA 2009 Briefings from Felix Lindner, and thought about sharing the main points here. The guy presents a reasonable analysis of the current situation regarding Router security and exploitation. As we all know, routers are exploitable of course, but not as easy as […]
How to Configure DHCP on Cisco 851 or 871 Router
DHCP stands for Dynamic Host Configuration Protocol. Basically it’s a mechanism which assigns IP addresses to computers dynamically. Usually DHCP is a service running on a server machine in the network in order to assign dynamic IP addresses to hosts. All Cisco 800 series models have the ability to work as DHCP servers, thus assigning […]