Maximum Segment Size (MSS) and MSS clamping are concepts that can be confusing. How do they relate to the MTU (Maximum Transmission Unit)? Before we setup a lab to demonstrate these concepts, let’s give some background. Note that this entire
Not All OSPF Inter-area Traffic Traverses Interfaces In Area 0
Everyone knows that OSPF is a link state protocol. Those that study also discover that OSPF behaves like distance vector between areas as Type-1- and Type-2 LSAs are not flooded between areas, but rather summarized in Type-3 LSAs. This means
Ethernet History Deepdive – Why Do We Have Different Frame Types?
In my previous post Encapsulation of PDUs On Trunk Ports, I showed what happens to PDUs when you change the configuration of a trunk. You may have noticed that there are typically three different types of Ethernet encapsulations that we
Why Are OSPF Type 5 LSAs Flooded?
I recently saw a great question on Reddit, on why Type-5 (AS-external) LSAs are flooded, in comparison to Type-3 (Summary) that are regenerated at the ABR. To investigate this, we’ll use the following simple topology where R2 and R3 are
Some History on VLAN 1 in Cisco Switches
Over the years, there has been a lot of discussion on if VLAN 1 in Cisco switches is special or not. Does it have any characteristics that other VLANs don’t? I covered some of this in the Is VLAN 1
Encapsulation of PDUs On Trunk Ports
When I studied for my CCIE almost 15 years ago, I recall that I was fascinated by how different PDUs such as CDP, DTP, STP would have different encapsulations on a trunk depending on the configuration of it. What happens
Adding Arista Switch to CML
I wanted to add Arista switches to CML to do some STP interopability testing. However, the process of adding them is not well described. I had to refer to some Youtube videos to understand what to do. This is what
Detecting Mismatched Native VLANs
Many people have seen the message logged to their switch about a mismatched native VLAN on a trunk, but how is it detected? There are two methods of detecting mismatched native VLAN on a trunk link: To demonstrate how this
802.1Q-Tagged Frames Through Unmanaged Switch – Forwarded or Dropped?
As a follow-up to the post yesterday on native VLANs, there was a question on what would happen to 802.1Q-tagged frames traversing an unmanaged switch. Unmanaged in this case being a switch that does not support VLANs. While this might
Why Do We Have Native VLANs?
Recently, my friend Andy Lapteff asked an excellent question. Why do we have native VLANs? As in, why allow untagged frames on a trunk link? There was a time where we didn’t have VLANs. At first there was hubs, then