Dot1Q Tunnel / q-in-q Tunneling

Dot1Q tunnel or q-in-q tunneling is a technique generally used by service providers to extend customer's VLANs to different locations. It is done by encapsulating the customer VLAN inside another 802.1q encapsulation and because of this we must increase the system mtu on the service provider switches to something greater than 1500. In this blogtorial, we will take a look at a very simple topology with easy to follow configurations.

Here is the topology. 

Our objective: 

VLAN 10 in a Customer Site in California (SW1) needs to be extended to a Customer Site in Illinois (SW4).

Troubleshooting - Loop-back detected - %PM-4-ERR_DISABLE

Recently, I was tasked with troubleshooting a weird problem where one of the interfaces was err-disabled. Fun Monday mornings :) and sifting through the logs I came up on this error message. 

%PM-4-ERR_DISABLE: loopback error detected on Fa1/0/20, putting Fa1/0/20 in err-disable state

%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on FastEthernet1/0/20.

Turns out that this was because of an interface somewhere in our network was configured incorrectly which then created a loop in our network. The STP topology in our network corrected/recovered itself within a few seconds and fa1/0/20 should have "err-disabled recovered", however "errdisable recovery cause loopback" was not configured. 

Basically this error message states that the keepalives which are on by default are looping back to the source interface.  

For example, if a keepalive was sent on fa1/0/20 and if that keepalive packet loops around the network and shows up at the source interface then that must mean that there is a loop in our network and the interface which sent out the keepalive will be err-disable.  

Notice that the keepalive are set to 10sec by default on all interfaces. 

Workarounds: 

• Fix the underlying misconfigured interface downstream and/or upstream.
• Disable keepalives on interfaces. 

Many more articles to come so stay tuned. "Join this site" on the right / click +1 below ------->

Enterprise Campus Design - Part 1

In this blogtorial we are going to dive into how to effectively and efficiently design an enterprise network. As I have always said "Networking is an art not science" as there are numerous ways to design and I am merely posting one of many. 

In this topology we will sink our hands into: 

• STP (802.1d)
• HSRP
• OSPF 
• OSPF route summarization
• VTP
• Different layers/modules of Enterprise Campus Design

Check out the topology below and let's get started. 

Relevant configurations are posted below.