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VLAN Mismatches and STP

Lately I’ve been revisiting Ethernet switching and Spanning Tree related topics in much more depth than before. While working through a lab I stumbled upon something I found interesting. I didn’t plan on writing tonight, so I’ll try to make this brief. The focus is the link from SW1 G0/2 to SW2 G0/3. SW1 is

Spanning Tree BackboneFast

8FebruaryStevenHome Lab, Switching

To close out this 3-part series on Spanning Tree convergence optimizations we’ll be diving into Spanning Tree BackboneFast. This feature allows “classic” STP 802.1D bridges to converge faster when a remote bridge loses connectivity to the root bridge, i.e. this allows faster convergence to indirect link failure. Demo Network To demonstrate convergence with and without

Spaghetti

8FebruaryStevenHome Lab

Spanning Tree Uplink Fast

31JanuaryStevenSwitching

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

31JanuaryStevenSwitching

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

30JanuaryStevenSwitching

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

18JanuaryStevenPython

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

PIM RP Redundancy that (kinda) Works

10JanuarySteven1 CommentHome Lab, Routing

I spent a few minutes thinking about anycast RP for PIM sparse-mode deployment and thought of a way to provide RP redundancy without auto-RP or BSR to dynamically learn RPs, and without using MSDP to synchronize active source information between anycast RPs. There is a major pitfall to this approach but it can work. Demo

ToS Byte Reference Sheet

10JanuaryStevenENSLD

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

31DecemberStevenHome Lab, Switching

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