Ask what separates DMVPN Phase 1 from Phase 2 from Phase 3 and you will usually get a memorized answer: "Phase 1 is hub-and-spoke, Phase 2 adds spoke-to-spoke, Phase 3 adds redirects." True, and
Every interesting thing DMVPN does, it does with NHRP. Spokes appearing on the hub without configuration? NHRP registration. Spoke-to-spoke tunnels forming on demand? NHRP resolution. Phase 3's ability to summarize routes and still cut hub-free
DMVPN has a reputation for being complicated, and it is entirely undeserved. The technology is three well-understood pieces (multipoint GRE, NHRP, and a routing protocol) assembled so that each covers a gap the others leave. Once you can say
Multicast fails silently, and nearly every ticket collapses into three signatures: a silent receiver, an RPF failure, or a missing RP. Captured on CML with IOS XE 17.18.2: healthy PIM sparse mode state before and after the source goes active, how to read every field of show ip mroute, and each...
Standard PIM sparse mode scales beautifully for a handful of sources and thousands of receivers. Push it the other direction, thousands of sporadic senders, or stretch it across administrative boundaries, and two very different tools take over: bidirectional PIM collapses
Everything hard about PIM sparse mode traces back to one assumption: receivers do not know who the sources are. Drop that assumption and the rendezvous point, the register tunnel, the shared tree, and the SPT switchover all become unnecessary. That
Unicast routing looks forward: where is this packet going? Multicast routing looks backward: did this packet arrive from the right direction? That reversal is the reverse path forwarding check, and it is both the reason multicast never loops and the
PIM sparse mode is where multicast stops being theory and becomes state machines. It is the protocol that turns "a host joined a group" into an actual delivery tree across your network, and it does it with a
IGMP is the only part of multicast a host ever speaks. Before PIM builds a single tree, before an RP hears a single register, a receiver has to raise its hand on the local segment and say "I want
Every multicast design decision starts with an address. Which range you pick determines whether routers forward the traffic at all, whether you need an RP, whether it can leave your organization, and even how it collides at layer 2. This
Send one packet, deliver it to a hundred receivers, and put exactly one copy on each link along the way. That is the entire promise of IP multicast, and it is the delivery model behind market data feeds, IPTV, live
Redistribution fails three ways: routes that never arrive, routes that echo back and loop, and routes that arrive but take the long way. All three captured on CML with IOS XE 17.18.2, including the EIGRP external at AD 170 that loses to OSPF's 110 while the shorter direct path sits unused in the...