Archive for category Cisco

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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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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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


Cisco Learning Network

Cisco Learning Network (CLN) is a great place for gaining/sharing in depth knowledge of Cisco Technologies. Number of well known experts around the globe are available to provide assistance to folks from Novice to Expert level. Its basically a Cisco Virtual Learning Community or Human Network 🙂

If you are interested in becoming a CLN member click on the following link: CLN

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Cisco Security Advisories and Notices

The following link provides a report on vulnerabilities found in Cisco products/softwares & mitigation techniques:


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EtherChannel MAC Flap Issue on a Cisco Switch – Solution!

Have you ever come across a following error message?

*Mar  1 00:17:16.211: %SW_MATM-4-MACFLAP_NOTIF: Host 0015.2b57.8317 in vlan 1 is flapping between port Fa0/21 and port Fa0/23

The chances are you have, specially while configuring an EtherChannel. Here are some troubleshooting steps:

1) Make sure that the EtherChannel Port is UP via “show ip int brief” command or “show etherchannel summary

2) sh mac address-table command will show you whether or not those ports belong to Po or Port-Channel interface

3) Here is how to resolve this issue:

  • Make sure that the “Channel-Protocol” is the same on both ends. PAgP or LACP or on. If one end is PAgP & the other is set to on EtherChannel won’t form. Same goes with LACP.
  • Both sides of the EtherChannel have to have the same EtherChannel Negotiation Protocol i.e. PAgP or LACP. If you don’t want to use Negotiation Protocol then both ends have to be set to “on”.
  • The above-mentioned will resolve the issue.


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