CCNA Question of the Week 2

In the following image, you'll see a network topology.  In this topology, the routers are running the RIP routing protocol.  As is traditional with these questions, I'm going to strip out all the irrelevant information.  We're not going to see any router configuration, IP addressing,...

Netflow Collectors

One of the big topics currently in Cisco's security track is Netflow.  According to Cisco, "NetFlow provides valuable information about network users and applications, peak usage times, and traffic routing."  With all of it's known, and yet to be discovered uses, it's no doubt that NetFlow will continue to be a big part of Cisco's security exams for the foreseeable future, as well as potentially...

Symmetric Traffic and IPS

A well known problem for network and security professionals in the enterprise is asymmetric routing.  At it's simplest, this is where traffic flows outbound through Router A, while the return traffic returns through Router B, or through both Routers A and B.   If you're using a reflexive ACL, for example, this will lead to some, if not all of the return traffic being blocked as...

The Accuracy of Sampled Netflow

To alleviate the fear of overburdening the CPU due to the collection of NetFlow statistics, Cisco gives us the option of using Sampled NetFlow. Sampled NetFlow allows you to sample 1 out of 10 packets, 1 out of 100 packets, or however much of a subset of the total number of packets. The theory is that...

IOS Zone Based Firewall

One of the most commonly covered security features when it comes to Cisco security is the ZBF.  It wouldn't be much of a network security blog without at least one post on this topic, so here's my take. With IOS version 12.4(6)T, Cisco introduced the Zone Based Firewall (ZBF), sometimes referred to as the Zone Policy Based Firewall.  With this, the Classic IOS Firewall or Context-Based...

ACLs by Country

Have you ever wanted to create an ACL by country?  There's a number of different ways you can go about it.  Certain models of firewalls have this functionality built in.  IOS based routers and ASA firewalls have no such capability, so we'll have to do this a bit more manually.  I'll present two methods. The first method is detailed here.  In this post, wget goes out...

Resequencing an ACL

Here's a quick post on a very useful command when working with ACLs.  I first heard about it while watching a CBT Nugget video, and I can say that it was definitely not covered in the NetAcad curriculum when I went through the classes, because I remember bringing it up to the instructor and it was news to him. So let's begin by setting the scenerio.  You have the following ACL: show ip...

Reflexive ACLs on IOS Routers

In a nutshell, reflexive ACLs allow packets to be evaluated based on upper layer session information. You use reflexive ACLs in order to permit the return traffic from an established session, but deny all other traffic in that direction.  For example, you open up a browser and establish an HTTPS session with www.awesomewebsite.com.  Now obviously, you want the return traffic from the server...

Setting up the 2511 Part 2

There is one more feature of the Cisco access servers that I do not see mentioned very often. Maybe it's been talked to death and I just missed it. But either way, I think is hugely important. That is the ability to telnet through it to the connected devices without ever actually appearing to touch the access server itself. In this post, I'll go over that briefly. To recap, in the previous post,...

Setting up the Cisco 2511

Racked up in my lab, I have more routers, switches, firewalls and access points than I care to count.  These devices are all in the basement, which is a place I don't necessarily want to set up shop when I'm labbing due do a number of reasons such as the temperature and the constant hum of the dehumidifier.  So when I'm upstairs, any oopsie that causes loss of connectivity would require...

Building an ACL

The different types of ACLs are first identified by the line number used. Standard IP ACLs use numbers in the range of 1 – 99 and 1300 – 1999. Extended ACLs use numbers in the range 100 - 199 and 2000 – 2699. Other types of ACLs which filter traffic utilizing other protocols such as Appletalk, DECNet, IPX, and XNS use other number ranges, however those are rarely used today. Named ACLs of course do...