… updated on Thursday, June 1, 2023 16:32 UTC
Source IP Address in Multicast Packets
One of my readers sent me this (paraphrased) question:
What I have seen in my network are multicast packets with the IP source address set to 0.0.0.0 and source port set to 0. Is that considered acceptable? Could I use a multicast IP address as a source address?
TL&DR: **** NO!!!
It also seemed like a good question to test ChatGPT, and this time it did a pretty good job.
NTP in a Nutshell
Years ago I’ve been involved in an interesting discussion focusing on NTP authentication and whether you can actually implement it reliably on Cisco IOS. What I got out of it (apart from a working example) was the feeling that NTP and it’s implementation in Cisco IOS was under-understood and under-documented, so I wrote an article about it. Of course the web version got lost in the mists of time but I keep my archives handy.
Last weekend I migrated that article to blog.ipSpace.net. I hope you’ll still find it useful; while it’s pretty old, the fundamentals haven’t changed in the meantime.
Path Failure Detection on Multi-Homed Servers
TL&DR: Installing an Ethernet NIC with two uplinks in a server is easy1. Connecting those uplinks to two edge switches is common sense2. Detecting physical link failure is trivial in Gigabit Ethernet world. Deciding between two independent uplinks or a link aggregation group is interesting. Detecting path failure and disabling the useless uplink that causes traffic blackholing is a living hell (more details in this Design Clinic question).
Want to know more? Let’s dive into the gory details.
Goodbye Twitter. It Was Fun While It Lasted
I joined Twitter in October 2008 (after noticing everyone else was using it during a Networking Field Day event), and eventually figured out how to automate posting the links to my blog posts in case someone uses Twitter as their primary source of news – an IFTTT applet that read my RSS feed and posted links to new entries to Twitter.
This week, I got a nice email from IFTTT telling me they had to disable the post-to-Twitter applet. Twitter started charging for the API, and I was using their free service – obviously the math didn’t work out.
That left me with three options:
Worth Reading: Cargo Cult AI
Before we managed to recover from the automation cargo cults, a tsunami wave of cargo cult AI washed over us as Edlyn V. Levine explained in an ACM Queue article. Enjoy ;)
Also, a bit of a historical perspective is never a bad thing:
Impressive progress in AI, including the recent sensation of ChatGPT, has been dominated by the success of a single, decades-old machine-learning approach called a multilayer (or deep) neural network. This approach was invented in the 1940s, and essentially all of the foundational concepts of neural networks and associated methods—including convolutional neural networks and backpropagation—were in place by the 1980s.
Worth Reading: Building Trustworthy AI
Bruce Schneier wrote an excellent essay explaining why we need trustworthy AI and why we won’t get it as long the AI solutions are created by large tech companies with you are a product business model.
Network Security Vulnerabilities: the Root Causes
Sometime last autumn, I was asked to create a short “network security challenges” presentation. Eventually, I turned it into a webinar, resulting in almost four hours of content describing the interesting gotchas I encountered in the past (plus a few recent vulnerabilities like turning WiFi into a thick yellow cable).
Each webinar section started with a short “This is why we have to deal with these stupidities” introduction. You’ll find all of them collected in the Root Causes video starting the Network Security Fallacies part of the How Networks Really Work webinar.
Inter-VRF DHCP Relaying with Redundant DHCP Servers
Previous posts in this series covered numerous intricacies of DHCP relaying:
- DHCP relaying principles described the basics
- In Inter-VRFs relaying we figured out how a DHCP client reaches a DHCP server in another VRF without inter-VRF route leaking
- Relaying in VXLAN segments and relaying from EVPN VRF applied those lessons to VXLAN/EVPN environment.
- DHCP Relaying with Redundant DHCP Servers added relay- and server redundancy.
Now for the final bit of the puzzle: what if we want to do inter-VRF DHCP relaying with redundant DHCP servers?
Missing Information for the EIGRP Network Design Solutions Cisco Press Book
When I was writing the EIGRP Network Design Solutions book for Cisco Press we agreed to have a companion web page with router configurations and exercise solutions. That never happened, so I published them on my private web site which eventually disappeared.
Dealing with Cisco ACI Quirks
Sebastian described an interesting Cisco ACI quirk they had the privilege of chasing around:
We’ve encountered VM connectivity issues after VM movements from one vPC leaf pair to a different vPC leaf pair with ACI. The issue did not occur immediately (due to ACI’s bounce entries) and only sometimes, which made it very difficult to reproduce synthetically, but due to DRS and a large number of VMs it occurred frequently enough, that it was a serious problem for us.
Here’s what they figured out:
Simplify netlab Topologies with Link Groups
Last month I described how you can simplify your VLAN- or VRF lab topologies with VRF- and VLAN links, automatically setting vlan.access or vrf attribute on a set of links. Link groups allow you to do the same for any set of link attributes.
Imagine you have a small network with three PE-routers connected to a central P-router:
Worth Reading: Trapped by Technology Fallacies
Michele Chubirka published a must-read article on technology fallacies including this gem:
Technologists often assume that all problems can be beaten into submission with a technology hammer.
As I’ve been saying for ages (not that anyone would listen): all the technology in the world won’t save you unless you change the mentality and rearchitect broken processes.
Why Is Source Address Validation Still a Problem?
I mentioned IP source address validation (SAV) as one of the MANRS-recommended actions in the Internet Routing Security webinar but did not go into any details (as the webinar deals with routing security, not data-plane security)… but I stumbled upon a wonderful companion article published by RIPE Labs: Why Is Source Address Validation Still a Problem?.
The article goes through the basics of SAV, best practices, and (most interesting) using free testing tools to detect non-compliant networks. Definitely worth reading!
Video: Chassis Switch Architectures
Pete Lumbis concluded his ASICs for Networking Engineers presentation with a brief overview of types of switching ASICs and a wrap-up.
You can watch his entire 90-minute presentation (sliced into shorter videos) with Free ipSpace.net Subscription.
Find the Optimal Level of Automation Abstraction
Tom Ammon sent me his thoughts on choosing the right level of abstraction in your network automation solution as a response to my What Is Intent-Based Networking blog post, and allowed me to publish them on ipspace.net.
I totally agree with your what vs how example with OSPF. I work on a NOS team where if we wanted, we could say, instead of “run OSPF on these links”, do this: