lab/migration/PPPOE-HA.md

PPPoE high availability

Proven in labsim. Not applied to production.

What it does

One consumer ISP account, two routers. The 10 gig lease is bound to a cloned MAC (f0:9f:c2:12:9b:4f, the retired USG's) and the Vodafone line to a single credential, so neither may be live on both boxes. The WAN follows VRRP mastership — but the two halves use different control planes, and that is the whole design:

	plane	why
bond0.53 (10 gig)	VyOS config (disable)	only config can move a MAC
pppoe0 (Vodafone)	systemd unit gate	see below

Why PPPoE cannot live on the config plane

interfaces_pppoe.py treats disable and delete identically: both unlink /etc/ppp/peers/pppoe0, call PPPoEIf.remove() (withdrawing the FRR default route) and stop the unit. That path is pppd's own options file (ExecStart=/usr/sbin/pppd call %I), so the resting state destroyed exactly what the promotion path needed. ppp@pppoe0 then restart-looped against the missing file — 47 restarts observed, zero sessions at the access concentrator — and never tripped systemd's limiter, because RestartSec=5s against the default 10s/5-burst window is only two restarts per interval.

It also made op-mode connect interface pppoe0 unusable (it refuses without the peers file), and put every failover behind a priority-322 commit where one unrelated invalid node fails the whole thing.

The gate

pppoe0 is configured identically and enabled on both routers, so the peers file always exists. Dialling is gated by /etc/systemd/system/ppp@pppoe0.service.d/10-vrrp-wan-gate.conf:

ConditionPathExists=/run/vrrp-wan/may-dial
ConditionPathExists=/etc/ppp/peers/pppoe0
StartLimitIntervalSec=600
StartLimitBurst=6

/run is tmpfs, so the gate is shut at boot and neither box can dial before VRRP has decided. This is load-bearing, not a nicety: with the node enabled, interfaces_pppoe.py restarts ppp on every commit touching the pppoe subtree when the daemon is not running — so the backup actively tries to dial whenever anything commits (pulumi up, a hand commit, the boot-time config load). The gate is the only thing making that a no-op, which is why vrrp-wan-reconcile refuses to bless a box whose drop-in is missing: /etc is per-image, so a VyOS upgrade silently removes the protection, and failing closed turns that into "PPPoE never dials" rather than "both routers dial".

may-dial is a lease, not a flag. ConditionPathExists is evaluated at start only — it can prevent a dial, never revoke one. vrrp-wan-reconcile renews it every 30s; vrrp-wan-guard runs every 5s and only ever revokes, on either "I do not hold the VIP" or "the lease is stale".

Measured in labsim

Clean failover (force-fault)	pppoe0 moves in 20-26s, reproducible
10 gig down → PPPoE	route falls to pppoe0; LAN back online in 5s
Stale lease	guard hangs up within ~5s
Missing peers file	NRestarts=0 — no loop
Flap holdoff vs live session	session survives a forged 900s holdoff
Invariant	AC never showed two simdsl sessions

Two failure modes found by running it, not by reading it

The flap damper tore down a healthy WAN. ppp_dial() checked the hold-off and returned before renewing may-dial. That file is a lease the guard expires after LEASE_TTL, so tripping the damper stopped the renewal and the guard hung up pppoe0 on the master ~80s later:

DIAL FLAP: >=6 attempts in 600s -- holding off 900s
GUARD: lease stale (81s > 75s) -- hanging up pppoe0

A damper meant to suppress repeated dials was destroying an established session instead. An active session now renews the lease and returns before every other check; everything below only decides whether to start a new session. T12 is the regression test.

A missing peers file is silent. /etc/ppp/peers/pppoe0 is both pppd's options file and the gate's second condition, and it is only written by a commit that touches the pppoe subtree. Without it systemd logs skipped because of an unmet condition check exactly once and then nothing — a router that cannot dial at all looks identical to a healthy backup. ppp_dial() now says so on every tick, and distinguishes the two causes: configured-but-not-rendered (re-commit the subtree) versus no pppoe0 in the config at all.

The second cause is the one to watch in production: a commit that was never saved reverts on reboot and takes pppoe0 with it. That is exactly how the sim secondary lost its WAN and spent hours looking like an ISP problem. After any hand commit to the pppoe subtree, save — or the next reboot produces a standby that can never take over.

Deploying (not yet done)

1. sudo /config/vyos-known-good save on both.
2. migration/vrrp-wan-install --vip 192.168.1.1 --host vyos@10.0.1.253 then the same for .252. Then --check on both. No config change yet — verify nothing dials.
3. Confirm /config/wan-secrets is present and identical on both.
4. vyos002 first (the non-master), on its own commit — interfaces pppoe is priority 322 and one bad node fails everything: delete interfaces pppoe pppoe0 disable, commit-confirm 10.
5. Verify vyos002 did not dial — check on the wire, not from state: sudo tcpdump -i bond0.51 -nn pppoed should show no PADI. Then confirm; save.
6. vyos001: nothing to change; it already has pppoe0 enabled.
7. Confirm vif 53 disable is in both config.boots.
8. Add the drop-in re-install to the VyOS image-upgrade runbook.

Rollback, from either box: set interfaces pppoe pppoe0 disable on both and rm /run/vrrp-wan/may-dial. That restores today's behaviour exactly.

What the sim cannot prove

• Vodafone's session-control. The sim's accel-ppp defaults to replace, so a new auth kills the old session immediately. A real BRAS may deny and hold the session for its own dead-peer timer. The matrix runs all three modes to bracket the risk; it cannot tell you which one you will meet, and account rate-limiting or lockout on repeated dials has no sim analogue at all. The flap damper (6 dials / 600s → 15 min hold-off) exists for that.
• Whether Vodafone honours our LCP Terminate / PADT on a graceful stop.
• The cloned-MAC lease — whether the 10 gig ISP re-issues 87.192.101.48 to f0:9f:c2:12:9b:4f arriving on a different switch port. That risk belongs to bond0.53, not PPPoE, and is the largest untested item in the failover.
• Real dial time and MTU/MSS under load. PPPoE was proven on the USG; VyOS dialling Vodafone has never been done.
• Timing under load. The sim routers are idle 2-vCPU VMs; commit latency on the VP2440s under kea + BGP + conntrack will be worse, and commit latency is the dominant term in the bond0.53 half of a failover.

A model hazard to fix before applying

The imported baseline records the running state, not the safe one: vyos001 has no vif 53 disable (it is master), vyos002 does. So an apply performed while vyos002 held the VIP would enable vyos001's WAN as well, putting the cloned MAC on both boxes. An override must assert vif 53 disable on both routers, with the reconciler re-enabling whichever holds the VIP. Note the consequence: an apply then briefly disables the current master's 10 gig until the reconciler restores it (≤30s).