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Clear arp-cache vs clear ip arp

Cisco IOS CLI provides two different commands that seem the same on the surface but have a slightly different outcome on the wire. Based on the title you probably guessed that it’s clear arp-cache vs clear ip arp. To see the difference, I connected two routers on network 10.0.0.0/30 with a SPAN enabled switch in

TCP CUBIC vs BBR Pt. 1

1AugustStevenHome Lab

TCP congestion control algorithms have a tough problem to solve. They need to estimate the available capacity of a network path without any knowledge of network. Remember that TCP runs in the end hosts. It’s important to estimate the network capacity to achieve the maximum throughput for a single flow while sharing network resources with

Don’t Have a Bad Day

31MarchStevenHome Lab, Routing

Anycast RP with Multicast Source Discovery Protocol (MSDP) is a great way to provide fault tolerance to your multicast network. Configure another router with the RP address, bang out a few msdp commands on the RP routers and you’re all set for a basic anycast RP design. It’s important to not get tunnel vision and

EIGRP Metric Calculation

27MarchStevenHome Lab, Routing

If you’ve worked with or studied EIGRP, you’ve probably seen that scary formula with the K values that EIGRP uses to calculate routing metrics. It turns out that if you’re using the default K values so that only bandwidth and delay are being considered, the math involved isn’t that bad. In this article we’ll explore

IPv6 Next Hop

16MarchStevenHome Lab, IPv6, Routing

Over the last two weeks I’ve been focusing on strengthening my knowledge of IPv6 and it got me thinking about usage of link local addresses as the next hop for routing. It became apparent that this difference from IPv4 makes it possible to have equal cost next hops via the same output interface and for

IPv4 Fragmentation

2MarchStevenHome Lab, Routing

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