Steven – Page 3 – routingloop.net

IPv4 Fragmentation

One of the attributes that helped IPv4 stand the test of time is the ability to slice and dice big datagrams into multiple smaller ones on the fly. This provides tremendous flexibility for different data links and their maximum packet size, reduced MTUs inside of tunnels, and the like. The process of chopping up IP

Exploring CEF

25FebruaryStevenRouting

There are a few fundamental actions that a router must perform to take a packet coming in one interface and forward it out of another. The router must perform lookups to determine the exit interface and next hop information, it must rewrite the data link layer header and trailer for the egress link, and if

Joining MST and PVST Domains

16FebruaryStevenHome Lab, Switching

Multiple Spanning Tree Protocol (MST) allows network operators to map one or more VLANs to a Spanning Tree Protocol instance. For example, VLANs 1,2,3,4, and 5 can be mapped to STP instance 1 while VLANs 6,7,8,9 and 10 are mapped to STP instance 2. Consolidating multiple VLANs into a single STP instance requires less overhead

VTP and DTP Interaction

16FebruaryStevenSwitching

VLAN Trunking Protocol (VTP) and Dynamic Trunking Protocol (DTP) are Cisco proprietary features aimed at reducing administrative burden and ensuring consistency between network devices. VTP is primarily used to propagate VLAN information between switches, allowing an administrator to configure VLANs on a single switch and let them be advertised to the rest of the switch

VLAN Mismatches and STP

10FebruaryStevenSwitching

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

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