Spanning Tree Uplink Fast

In the previous article we looked at the PortFast feature to expedite STP edge port forwarding. The next two articles will focus on STP enabled links connecting switches together. As you guessed by the title, this one is about a feature called Uplink Fast. What problem are we solving with this feature? Consider the topology

Spanning-Tree PortFast

Imagine the scenario: It’s Monday morning and you’re running a few minutes late getting into the office. You have an important meeting starting as you’re taking out your laptop, and letting it boot up while you plug in the charger and that cruddy old network cable coming out of the floor. As the network cable

STP Bridge Priority 4096

Have you ever wondered why per-VLAN spanning tree only accepts bridge priority values in increments of 4096? I have. I spun up a lab and Wireshark to figure it out. In Cisco IOS, if you attempt to configure a bridge priority value that is not an increment of 4096 the CLI will print a helpful

Multicast Calculator

Last week I was diving into IP multicast and decided to write a multicast calculator in Python to reinforce multicast addressing schemes. When the script is ran, it prompts the user for an IP address. If an IPv4 or IPv6 multicast address is entered, the corresponding multicast MAC address is calculated and returned. If an

ToS Byte Reference Sheet

This morning I put together a spreadsheet all about the Type of Service byte and it’s associated DSCP and ECN markings. Feel free to download it and reference or modify it as you see fit.

STP Priority

When learning protocols, I like to build out a lab, take packet captures, debugs, and show commands and try to understand and justify the output. My most recent exercise is with PVST+. I built out the 6-switch topology below in CML. One of the first things I experimented with is bridge priority configuration. VLAN 10

AS Boundary Links in IGP?

Lately I’ve been focusing on honing my BGP skills to prepare for the ENSLD exam. While this isn’t a configuration focused exam, I’ve been spending some lab time on it to reinforce design concepts. I built out a 29-node lab in Cisco modeling labs featuring eBGP, iBGP, route reflectors, BGP confederations, and eBGP third party

Information Hiding in Routing Systems

One of the most effective methods to build stable and scalable routing designs is to summarize topology and reachability information in routing protocols. Summarization can help networks converge faster and limit the number of routers that need to perform route calculations when an event such as a link flap occurs. However, there are tradeoffs to