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Entries in best current practice (7)


Cisco 'ip helper-address' and Windows DHCP Servers

All ip helper-address lines configured in your VLAN take the DHCP broadcast from the client, add the router’s (gateway) address into the UDP packet, then unicasts to the DHCP servers. [I’m sure the packet rewrite is only done once, then a copy sent to each DHCP server.] All the listed servers configured receive the DHCPDiscover packet by the router relay.

The redundancy of your DHCP servers not only depends on your OS, but the specific version! For Windows, your options range from a true split-scope in Windows 2008 R2 to active-failover redundancy in Windows 2012. For not-so-robust DHCP servers (i.e., Windows 2003), you can manually configure a split-scope. Common recommendation is the 80/20 rule with 80% of the leases configured on what you (and you alone) consider your primary DHCP server and 20% on the secondary. Exclusions get added to each DHCP server as they have overlapping scopes.

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What factors drive a Cisco IOS upgrade?

In order of preference/priority, what factors do you consider in driving an upgrade (or downgrade) with Cisco IOS? If no compelling factors exist, how long would you allow a particular version of IOS to stay running? I’ve seen some switches with uptimes > 5 years.  And when upgrading, how is the specific IOS release identified as the upgrade target?

In order of preference/priority, best practice tends to dictate an upgrade based on these factors:

  • Vulnerabilities, vulnerabilities, vulnerabilities!
  • Bugs
  • Attaining new features not currently available— new cards/modules have a “first sup ported in” IOS version which could be higher than what you have running
  • Migrating away from retired release trains
  • Matching versions on more recently deployed and similar hardware

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Cisco WLC (Wireless LAN Controller) Series - IP & VLAN Planning

In part III of this Cisco WLC Series, we’ll discuss IP and VLAN Planning, an unexceptional area in network design that often doesn’t get the level of treatement it deserves, in the context of wireless. 

Any good IP and VLAN design needs to accomplish three goals.

  1. Able to support current needs
  2. Permits some degree of flexibility 
  3. Creates a framework for security

These goals are no different for wireless.  In the corporate network I support, the IP and VLAN Plan was established many years before wireless was given a serious seat at the table.  Only by meeting the second goal that allows for flexibility, wireless was able to be overlayed with the wired network with enough degrees of separation to continue to meet the third goal of maintaining a security framework.  Some of the decisions that go into a sustainable IP and VLAN Plan don’t require more than a few extra cycles of thought that readily pays for itself in unforseen ways.

To jump to the beginning of this series, see the Introduction.

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Cisco WLC (Wireless LAN Controller) Series - Building Blocks

In this section on the Cisco WLC Series, I’ll attempt to show the relationships between the basic building blocks in a concise and meaningful way.  We’ll cover:

  1. Ports
  2. Interfaces (Service, Management, Dynamic)
  3. Interface Groups
  4. WLANs

To jump to the beginning of this series, see the Introduction.

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Cisco WLC (Wireless LAN Controller) Series - Introduction

After many late nights and pouring over countless sources of documentation, including the voluminous docs found at, all the concepts and design elements that comprise a Cisco WLC (Wireless LAN Controller) architecture have finally “clicked”.  I’ve somehow managed to escape enterprise wireless until now.  Learning a new platform is simultaneously exciting and challenging, particularly on day 1.  For any seasoned network engineer, jumping into the depths of enterprise wireless for the first time presents a steep learning curve that quickly needs straightening out.

Image (c) 2012 - used with permission

Tackling a new GUI/CLI is initially a series of exercises and tests in attaining enlightenment of not only the overarching architecture of the platform, but in understanding subtle nuances and hierarchical relationships.  Sample walk-thrus of configuration rarely explore these nuances in any meaningful way; that takes frequent help-searches and reference materials.  The process of reaching your “light bulb” moment is akin to having a pile of multicolored marbles that you can finally begin to order in a meaningful way.

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