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Configuration guides, write-ups of things that broke, questions with answers on them, and labs you can open in the browser — written by people using ZNetLab, reviewed before they appear, and readable without an account.
Articles
A configuration guide, a deep dive, or a write-up of something that broke.
MTU black holes: why ping works and SSH hangs
Small packets get through and large ones vanish. The mechanism that is meant to fix this is ICMP, which is exactly what people block.
@admin · 2026-10-09 · 2 min read
Spanning tree: the root, the roles, and why a loop melts a LAN
A layer-2 loop has no TTL to stop it. STP exists for that one reason, and it solves it by electing a root and then blocking exactly the right ports.
@admin · 2026-10-09 · 2 min read
Redistributing between EIGRP and OSPF without creating a loop
Two protocols with incompatible metrics, joined at two points, is the classic way to build a routing loop that passes every ping you think to run.
@admin · 2026-10-09 · 2 min read
Access lists: the order, the implicit deny, and where to put one
An ACL is read top to bottom and stops at the first match. Two consequences follow, and they account for most of the ACLs that do not do what their author meant.
@admin · 2026-10-09 · 2 min read
NAT overload: one address, a table, and how the reply finds its way home
PAT is not address translation with extra steps — it is a table keyed on the port, and knowing that explains every "works outbound, fails inbound" ticket.
@admin · 2026-10-09 · 2 min read
A static route, read field by field
Five fields, each of which decides something. The one people get wrong is the third, and the symptom is a route that is in the table and does not work.
@admin · 2026-10-09 · 2 min read
OSPF adjacency: the states, and where it really stops
Down, Init, 2-Way, ExStart, Exchange, Loading, Full. Which state it is stuck in tells you which thing to check, and the mapping is nearly one-to-one.
@admin · 2026-10-09 · 2 min read
Trunks, native VLANs, and the mismatch that drops traffic quietly
A trunk carries a tag per frame — except for one VLAN, which it does not. That exception is where the outages come from.
@admin · 2026-10-09 · 2 min read
What a switch actually does with a frame
Learn, forward, flood, filter. Four rules, applied per VLAN, explain every switching question on the exam and most of the ones in the ticket queue.
@admin · 2026-10-09 · 2 min read
Subnetting without the magic-number trick
The mask is a ruler, not a riddle. Four questions answer every subnetting problem you will be asked, and none of them needs a table.
@admin · 2026-10-09 · 2 min read
Blog posts
Shorter and more personal — an opinion, a lesson learned, a war story.
Reading a packet capture without drowning in it
Filter first, read second. A capture you open unfiltered is a capture you will close again.
@admin · 2026-10-09 · 1 min read
"It is a DNS problem" usually is not
Four things wear the DNS costume. Two commands tell them apart in under a minute.
@admin · 2026-10-09 · 1 min read
The lab taught me more than the exam did
An exam asks whether you recognise the answer. A lab asks whether you can produce it, which is a different skill.
@admin · 2026-10-09 · 1 min read
The three commands I run before I believe anything
Layer one, then addressing, then the path. In that order, and before any theory.
@admin · 2026-10-09 · 1 min read
I stopped memorising subnet tables
The table is a crutch that works until the question is phrased differently.
@admin · 2026-10-09 · 1 min read
Questions
Something you are stuck on. Include the topology and the actual output.
NAT is configured but show ip nat translations is empty
The ACL matches, the overload statement is there, and no translation is ever created.
@admin · 2026-10-09 · 1 answer
Trunk shows as trunking but VLAN 20 will not pass
VLAN 10 crosses the trunk fine. VLAN 20 does not, and both are configured.
@admin · 2026-10-09 · 1 answer
EtherChannel will not bundle — ports stay independent
Three links configured into a channel group, and the port-channel never comes up.
@admin · 2026-10-09 · 1 answer
OSPF neighbours stuck in EXSTART — both sides, no errors in the log
Two routers over a GRE tunnel sit at EXSTART and never progress.
@admin · 2026-10-09 · 1 answer
Ping works, SSH hangs — only over the site-to-site VPN
Small traffic crosses the tunnel fine, large traffic disappears without an error.
@admin · 2026-10-09 · 1 answer
Labs
A topology with tasks that grade themselves. Readers run it in the browser.
Lab: Junos and IOS in one OSPF area
The protocol is a standard; the two command lines are not. Same adjacency, two ways of asking about it.
@admin · 2026-10-09 · 1 min read
Lab: the QoS policy that will not attach
Classify, mark, queue — and then the one error message that stops the whole thing.
@admin · 2026-10-09 · 1 min read
Lab: inside, outside, and a DMZ
Three zones and the policies between them. A zone pair is directional, which is the whole lesson.
@admin · 2026-10-09 · 1 min read
Lab: DNS and HTTP across a routed network
A name, a server, and three different faults that look identical from the client.
@admin · 2026-10-09 · 1 min read
Lab: three cables, one logical link
Bundling links so spanning tree sees one. Easy to configure and easy to get silently wrong.
@admin · 2026-10-09 · 1 min read
Lab: IPv6 across a routed link, without a DHCP server
Two /64s, one router, and hosts that address themselves. The parts of IPv6 that are genuinely different.
@admin · 2026-10-09 · 1 min read
Lab: two routing domains, joined at two points
Redistribute at one point and nothing can loop. Redistribute at two — which is what redundancy means — and this is what you have to do about it.
@admin · 2026-10-09 · 1 min read
Lab: a provider core — OSPF underneath, iBGP on top
The two-layer design every service provider uses, small enough to hold in your head.
@admin · 2026-10-09 · 1 min read
Lab: a branch edge — NAT overload and an inbound ACL
Thirty hosts, one public address, and a filter on the way back in. The commonest router in the world.
@admin · 2026-10-09 · 1 min read
Lab: a redundant triangle that does not melt
Three switches in a ring. With spanning tree, resilience; without it, a storm you can watch in the counters.
@admin · 2026-10-09 · 1 min read
Lab: VLANs that stay apart until the router joins them
Two VLANs on one switch, a trunk, and sub-interfaces. The classic campus shape, in twenty minutes.
@admin · 2026-10-09 · 1 min read
Lab: two LANs, and the second route nobody adds
Two routers, two LANs, and the lesson that routing is one-directional. Fifteen minutes.
@admin · 2026-10-09 · 1 min read
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