Awesome IPv6 Calculator!!!

Here is a link to a great IPv6 calculator:

What I really like about this calculator is the fact that when you type in a full blown IPv6 address it shows you the compressed version of the IPv6 address provided. Really cool. Download & enjoy 🙂

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DHCP Server Redundancy using Cisco Routers!!!

In order to build resiliency in your network you absolutely have to have redundant DHCP LAN servers. You can use your Cisco routers or switches as DHCP servers to provide this capability. Here is a simple topology:

(Click on the Picture Below to Enlarge)

In the above diagram we have two core (CORE_1 and CORE_2) routers acting as a single gateway via HSRP. We have our DHCP ranges defined as follows… CORE_1 is serving IPs in the following range –> through and CORE_2 is serving IPs in the following range –> through Remember we can’t use (Network Address) and (Broadcast Address).

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Influencing Inbound vs. Outbound Traffic flow in BGP

There are multiple ways of influencing Inbound vs. Outbound Traffic flow in BGP. The most common ones are:

Inbound Traffic Flow can be influenced by manipulating the following attributes –>

  • AS-Path Prepending
  • MED

Outbound Traffic Flow can be influenced by manipulating the following attributes –>

  • Weight
  • Local Preference

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BGP Route Selection Order!!!

Yes, I am still ALIVE. Sorry folks for not being able to post stuff on a more consistent basis. I have been extremely busy lately. However I don’t like excuses… so time to get to WORK 🙂

Here are the most common BGP route selection path attributes that are taken into consideration when a BGP speaking router has multiple routes in its BGP routing table & it has to make a decision which route to mark as best & put in its routing table. Here is the exact order:

  1. Prefer highest Weight
  2. Prefer highest Local Preference
  3. Prefer routes that the router originated (with the Network or Redistribute command)
  4. Prefer shortest AS-Path
  5. Prefer lowest origin code (i<?)
  6. Prefer lowest MED (metric)

Please note that there are more path attributes than the ones listed above. But the ones mentioned in this post are the most important path attributes used most frequently. For a complete list please visit the following link: BGP Best Path Selection Algorithm

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No Multicast Traffic support from ISP?

If your ISP does not carry Multicast Traffic. You can create a GRE Tunnel to bypass the Provider Cloud by essentially creating a Point-2-Point logical link between two end-points. Here is a link do a Cisco doc with more info:

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(ISC)2’s recorded video conferences!

Found a very cool link regarding (ISC)2 recorded web-casts.

CISSP certified folks can also earn CPE credits just for watching these videos. Pretty awesome deal, huh!

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How to assign a static IP in Ubuntu OS?

First of all, you want to run the following command in CLI mode: ifconfig. “ifconfig” will show you all the interfaces on your server. You are expected to see a loopback interface & either a single or multiple Ethernet interfaces depending on the server specs. By default, the Ethernet interface or interfaces are set to DHCP. Once you have figured out which Ethernet interface you want to change the IP address on. Go ahead & enter the VI editor mode by typing in the following CLI  mode command:

sudo vi /etc/network/interfaces

Now edit the interface as follows (the below-mentioned is just an example & you would obviously have to replace these parameters based on your local subnet 🙂):

auto eth0
iface eth0 inet static

Once you are done making the changes. Type in “:wq” to save (write) the file & quit the VI editor. Now verify the change by typing in “ifconfig“.

In some cases you may have to re-initialize the networking module. You can do so by typing in the following command:

sudo /etc/init.d/networking restart

Congrats!!! You have now successfully assigned a static IP to an Ethernet interface.

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Route filtering in Cisco!!!

Route filtering is a mechanism whereby you can filter routes based on various types of criteria. This topic is a big part of advanced routing technologies. You definitely have to master this skill in order to be a routing super-hero 🙂

Here is a list of all the route filtering methods that can be applied on a Cisco router:

  • Access List
  • Distribute List
  • Filter List
  • Prefix List
  • Route Map


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Harvard Business School – Leadership Excercise!!!

I was reading an article on Harvard Business School’s website regarding an excellent Leadership Exercise. The whole point of this excercise is to come up with Six Words that define your Career/Personality. Here is what I came up with:

Seek Knowledge. Take Chances. Inspire Others.

Now the question is how would you define yourself in Six Words?


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Cisco SDM – How do I Download + Configure it?

Cisco SDM (Security Device Manager) is a GUI (Graphical User Interface) based Software for Configuring/Managing Cisco Routers. Its a great tool for Small Businesses and for IT Professionals alike since it allows non-Cisco folks to configure Cisco Routers even if they don’t have an intimate know-how of the CLI (Command Line Interface).

The reason Cisco is getting into GUI style of configuration is because its competitors like Juniper, Checkpoint etc. have gained a decent market share over the past few years by being leaders in GUI based Networking products. Although, in the background, its all CLI that does the magic but the market/customers demand ease of use & that is what GUI based Softwares provide.

Here is the link for downloading SDM:

Once downloaded & installed. Here is what you need to do – bare minimum – on your router for the SDM to be able to communicate with the Router. Its a 4 Step Configuration Process –>

  1. Assign IP to the Router’s Ethernet Interface & PC in the same Subnet, of course :-). Also verify Layer 3 connectivity by pinging.
  2. Create a User Account on your Router (via CLI):user
  3. Enable HTTP &/or HTTPS on your Router:enable http
  4. Enable local authentication for HTTP/HTTPS on your Router:enable authentication