IPv6 Packet Too Big: Why TCP Uploads Stall and How to Fix It
Small requests pass while a 64 KiB upload stalls at a 1280-byte link. Restore Packet Too Big feedback and see how a cached PMTU hides the fault.
IPv6
The 128-bit successor to IPv4 and the protocol every modern network is migrating toward. Articles tagged IPv6 cover address format and types, the simplified header, Neighbor Discovery, SLAAC autoconfiguration, and Cisco IOS XE configuration.
Latest posts
IPv6 Neighbor Discovery Troubleshooting: Stuck at Link-Local
An IPv6 host with only an FE80:: address is waiting on a Router Advertisement that never came. Captured on CML with IOS XE 17.18.2: the five NDP messages, the neighbor cache states, the A/M/O flags in a real RA, and why filtering ICMPv6 unplugs a segment.
IPv6 Address Examples: Every Type, Worked Through
A worked example of every IPv6 address type you will actually meet, broken down field by field. Learn to read the first hextet and instantly know if you are looking at link-local, global unicast, multicast, or unique-local.
IPv6 Transition Mechanisms: Which One, and When
Dual-stack, tunneling, and translation each solve a different IPv6 migration problem. This decision guide shows you which mechanism fits which situation and how the three fit together on real networks.
NAT64 Explained: Stateful, Stateless, and the DNS64 Half of the Story
How NAT64 lets an IPv6-only client reach an IPv4-only server: the full stateful configuration on Cisco IOS XE 17.18, the address synthesis math done by hand, the DNS64 half that makes it usable, and an honest platform finding from the lab.
IPv6 over IPv4 Tunnels: Manual, GRE, and 6to4 Compared
Configure a manual IPv6-over-IPv4 tunnel on Cisco IOS XE 17.18, prove it with a real ping across an IPv4-only core, and compare the three tunnel types you need to know: manual, GRE, and 6to4.
IS-IS for IPv6: Multi-Topology and the Single-Topology Trap
IS-IS runs a single topology by default, forcing IPv6 down the IPv4 shortest path even where IPv6 is broken. See how multi-topology IS-IS fixes the black hole, with real adjacency output on Cisco IOS XE 17.18.
EIGRP for IPv6: Classic and Named Mode on IOS XE
EIGRP for IPv6 is a separate protocol instance with no network statement and link-local next hops. You walk the classic interface-based config on IOS XE 17.18, see real adjacencies and routes, and clear up the shutdown gotcha.
IPv6 Services Closure: NPTv6, DHCPv6-PD, and General Prefix
DHCPv6 prefix delegation, general prefixes, and NPTv6: the IPv6 services engineers often skip, shown with real CML output of a delegated prefix flowing all the way to a downstream interface.
MPLS VPNv6 and 6VPE: IPv6 L3VPN over an IPv4 Core
6VPE carries IPv6 L3VPN across an IPv4 MPLS core using the same VRFs, RTs, and MP-BGP sessions you already run. This article builds it in CML and shows the two-label stack that makes it work.
IPv6 over BGP: MP-BGP for IPv6 and 6PE Explained
6PE carries IPv6 prefixes across an untouched IPv4 MPLS core using MP-BGP and labels. This post builds a working deployment on Cisco IOS XE and shows the labels, the IPv4-mapped next hops, and an end-to-end IPv6 traceroute.
IPv6 First-Hop Security Part 2: ND Inspection, Snooping, and Source Guard
ND Inspection, IPv6 Snooping, and IPv6 Source Guard stop address spoofing that RA Guard cannot see. This covers how the device-tracking binding table learns which IPv6 address belongs on which port and how the switch enforces it.