Monthly Archives: October 2012

Keeping Time on Cisco IOS

When doing any type of troubleshooting I am big on log files. To get the most out of your log files, however, you need the time stamps to be accurate. For time stamps to be accurate you need to ensure your systems are all running with exactly the same time Continue reading

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Cisco Switch Remote SPAN

Capturing network traffic can be a very effective way of solving complicate issues particularly when log files are not yielding useful results. One issue typically is the ability to actually physically perform the capture. To overcome these limitations a Remote SPAN or RSPAN instance can be used. Continue reading

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Prevent IP Access by Routing to Null

The goal of a router is to send network traffic to its destination based on its routing table. Some times its necessary to prevent traffic form flowing through a router. This can typically be achieved with access control lists, but a cost. ACLs, particularly long and complicated ones, come with the cost of performance. When preventing access to an entire subnet it is possible to simply route traffic into a “black hole” where it will be discarded. This is achieved without using an ACL. Continue reading

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Introduction to Virtual Routing and Forwarding (VRFs)

If you have listened to anything IT in the past five plus years you have probably heard of and experienced the “virtualization” buzz word. Typically, it’s referencing host or server virtualization as this is the most common and well known. There are, however, other instances where virtualization of sorts can be used to break hardware into isolated network building blocks. Virtual Routing and Forwarding instances (VRFs) on Cisco routing devices are one such example. Continue reading

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