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.
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 addresses look intimidating until you have seen enough real ones to recognize the patterns. The first hextet usually tells you everything: what kind of address it is and what it is for. This post is a worked reference - one
There is no single "IPv6 migration button." Instead there are three transition mechanisms, and the skill is knowing which one to reach for in a given situation. Dual-stack, tunneling, and translation each solve a different shape of
Dual-stack and tunnels both assume IPv6 exists on both ends of a conversation. NAT64 solves the harder case: an IPv6-only client that must reach a server which only speaks IPv4. It does this by translating between the two
Sometimes you have IPv6 islands that need to talk to each other, but the network in between is IPv4-only and you cannot change it (a provider core, a legacy WAN, a merged network you do not fully control). The
IS-IS is the quietest way to break IPv6 in your network. It will happily form an adjacency, show every neighbor as up, and route IPv4 perfectly, while dropping your IPv6 traffic into a black hole on any link where
EIGRP did not just bolt IPv6 support onto the existing process. On Cisco IOS XE, EIGRP for IPv6 is a separate protocol instance with its own topology table, its own neighbors, and a configuration model that trips up engineers who
IPv6 does several things differently from IPv4, and a few of them are genuinely better - once you understand them. Prefix delegation hands a whole subnet to a downstream router automatically. A general prefix lets you renumber an entire site by
You have an IPv4 MPLS L3VPN running. Your customers each have their own VRF, their own routing table, their own isolated slice of the core. It works, it scales, it makes money. Then a customer asks for IPv6, and you
Here is a problem that a great many service providers had, and some still have: a fully working, fully tuned, IPv4-only MPLS core, and customers who want IPv6. Upgrading every P router in the core to dual-stack is
RA Guard and DHCPv6 Guard stop the loud attacks: rogue routers and rogue DHCP servers. But there is a quieter layer of IPv6 abuse that operates below them, at the level of individual addresses. A host that spoofs its neighbor&
In IPv4, a rogue DHCP server is a nuisance you can contain with DHCP snooping. In IPv6, the equivalent threat is worse and easier to trigger, because IPv6 hosts configure themselves from Router Advertisements, and any device on the segment