RTLPlayground/doc/lacp.md

LACP (Link Aggregation Control Protocol, IEEE 802.3ad)

A static link aggregation group works only if both ends agree about it in advance. Nothing checks that they do: cable a member to the wrong neighbour and the group keeps hashing frames onto a link that goes somewhere else. LACP is the protocol that makes the two ends agree, and keeps agreeing — it discovers which links reach the same partner, aggregates only those, and drops a link out of the group when the partner stops answering.

The switch runs LACP in software on the 8051. Frames are the Slow Protocols group: destination 01:80:C2:00:00:02, ethertype 0x8809, subtype 1. The implementation covers the parts that decide whether an aggregate forms: the Receive machine and the Selection and Mux machines of 802.3ad clause 43, recordPDU and update_NTT, and the mis-cabling protection that refuses to aggregate links whose partners differ. It does not implement the Churn Detection machines or the Marker protocol, and the Mux machine is the coupled variant, so collecting and distributing are enabled together rather than in sequence.

Up to LACP_NUM_LAGS (4) groups can be placed under LACP; the rest stay static.

Getting the frames to the CPU

Reserved multicast addresses are handled by the RMA block before the L2 lookup happens. Each address has its own register and the action field sits in bits [5:4]: 0 forward, 1 trap to CPU, 2 drop, 3 forward excluding the CPU. The registers are packed and consecutive by entry rather than by address byte, so the step of 4 counts entries, not bytes of the address: ...:00 sits at 0x4ecc and ...:02 at 0x4ed4, but ...:08 sits at 0x4ee0.

#define RTL837X_RMA0_CONF	0x4ecc	/* 01:80:C2:00:00:00, entry 0 */
#define RTL837X_RMA2_CONF	0x4ed4	/* 01:80:C2:00:00:02, Slow Protocols */
#define RTL837X_RMA_ACT_MASK	0x30
#define RTL837X_RMA_ACT_TRAP	0x10
#define RTL837X_RMA_ACT_DROP	0x20

The engine puts this address on trap while it runs. A trap takes the frame out of the forwarding path and hands it to the port named in EXT_CPU_CTRL (0x6724), whose reset value names no port at all: the destination is meant to be an external CPU on a physical port, which these boards do not have. That is why the trap first looked like a dead end - the receive counters stayed frozen while the partner kept sending, and the ingress discard counter grew instead. trap_init() points the register at the CPU port once at startup, and the same frames then arrive.

FORWARD reaches the CPU too, because the CPU port is in the forwarding domain, but a forwarded frame is subject to the L2 lookup and on its own floods to every other port in the VLAN. That leaks a neighbour's LACPDUs to hosts that must not see them and, with two bonds on one switch, poisons both; a reserved link-local group must not be forwarded at all. Containing it takes a static L2 multicast entry with a CPU-only member mask, and because lookups are IVL on this chip, one entry per distinct pvid over all front panel ports - a VID 0 entry is never matched, and a port outside the LACP groups needs its pvid covered as well, or a LACPDU arriving there misses the lookup and is flooded through its VLAN.

The trap needs none of that and reaches further, because it acts on the address before the VLAN lookup and before the ingress filters. An untagged LACPDU is delivered on a port that admits tagged frames only, where a forwarded one is dropped by the acceptable-frame-type filter; and it is delivered on a port STP holds in blocking, so a group converges without waiting out the forward delay. Both were measured against forward on the same ports of an SWTGW218AS.

A trapped frame arrives without the VLAN tag the hardware inserts into a forwarded one, so the LACPDU body sits four bytes earlier. TRAP_RX_BODY() gives the offset of the body for either form, and lacp_in() reads the PDU from there.

Neighbouring addresses in the same block still differ in other respects - ...:00 is exempt from the ingress STP-state check and ...:02 is not, which the trap makes moot only because it acts before that check - so measure before carrying a conclusion about one of them across to another.

When LACP is switched off the action goes to DROP, which acts before the L2 lookup, so there is nothing to undo.

What has to match across the members

802.3ad requires that every link in an aggregate be full duplex and run at the same speed. Beyond the standard, anything that changes how a frame is treated has to be identical across members, because the hash picks the member per frame and a difference would make the behaviour depend on that choice:

• must match: speed and duplex, VLAN membership and pvid, ingress filtering, port isolation
• may differ: EEE, LED configuration, counters, cable diagnostics

None of this is enforced yet — port_lag_members_set() writes a member mask and a hash setting and validates nothing. See #377 for the discussion of whether a mismatch should be rejected or applied to the whole group.

Protocol timing

#define LACP_FAST_PERIODIC	0x0032	/* fast TX 1 s; partner expires at 3 s   */
#define LACP_SLOW_PERIODIC	0x05dc	/* slow TX 30 s; partner expires at 90 s */
#define LACP_SHORT_TIMEOUT	0x00c8	/* silent partner dropped after 4 s   (802.3ad: 3 s)  */

One unit is one step of the LACP timers, taken every four system ticks, so 50 per second whatever the pace of the main loop; 50 units came out as 1.00 s on the wire in all 116 intervals of a two minute capture.

The two transmit periods are worth a word, because they are the one thing here that is not ours to choose freely. Our own timeouts only decide how patient we are with a partner that has gone quiet, and being slow there costs nobody anything. The transmit period is the opposite: it decides when the partner gives up on us. 802.3ad pairs a 1 s fast period with a 3 s short timeout, and a 30 s slow period with a 90 s long timeout, and the ratio is the whole point.

Getting that wrong does not break the aggregate, which is what makes it easy to miss. An earlier version of this code transmitted every 2 to 3 s while the partner was asking for the fast rate. The links still aggregated and still carried traffic, but every time a gap crossed 3 s the partner's receive machine timed us out and recovered: 13 of its 93 LACPDUs came back carrying EXPIRED and reporting our SYNC as cleared. Since a received actor state is recorded verbatim as the partner state and sent back out, the flap was visible from both ends, and the bond's churn machines never settled. With the periods above, the same capture shows no expiry at all and both churn states read none.

Our own timeouts sit one period above the standard's 3 s and 90 s, which absorbs a single lost LACPDU plus the tick jitter. A partner whose LACP has stopped while its link stayed up is out of the trunk after that; anything longer keeps hashing traffic onto a member that no longer agrees to carry it.

LACP API

void lacp_init(void) __banked;	/* boot init: clear per-LAG state */
void lacp_off(void) __banked;
void lacp_lag_set(uint8_t lag, uint16_t ports) __banked;
void lacp_in(void) __banked;
void lacp_timers(void) __banked;
void lacp_show(void) __banked;

lacp_init() has to run before the startup configuration replays, because xdata is not zeroed and the port-to-LAG map uses 0xff as "no LAG"; a stray zero would make a port look like it belongs to LAG 0.

LACP configuration on the serial console

A group is put under LACP by adding lacp to the lag command:

lag 1 lacp 7 8      # ports 7 and 8 become LACP candidates of group 1
lag 1 lacp off      # group 1 goes back to static (so does "lag 1 lacp")
lag 1 7 8           # plain static aggregation, no protocol

A group put under LACP gives up the static trunk it had: the hardware trunk is cleared and rebuilt from the ports that converge. A port named in another LACP group moves to the new one. The protocol starts with the first group put under LACP and stops when the last one is taken out; there is no separate switch for it. Two commands act on the whole engine:

lacp off            # every group back to static, protocol stopped
lacp show

lacp show prints the engine state, then one line per group with its candidate ports, the elected aggregator and the trunk members actually programmed into the hardware, then one line per port with the actor and partner state bytes and the receive-machine state.

LACP configuration via the Web Interface

Each group on the LAG page has a Static/LACP mode next to its member ports; for an LACP group it shows the elected aggregator and the active members. Below the groups a table has one row per LACP port with its group, actor and partner state, receive state, LACPDU count and partner system. The page reads /lacp.json every three seconds while it is open. The member checkboxes of a static group come from the hardware registers in /lag.json.

A test with a Linux 802.3ad bond

Two ports of the switch to two interfaces of one host:

ip link add bond0 type bond mode 802.3ad
ip link set enp1s0 down && ip link set enp1s0 master bond0
ip link set enp2s0 down && ip link set enp2s0 master bond0
ip link set bond0 up

On the switch, with those two ports as the group:

lag 1 lacp 7 8

lacp show should then give both ports an actor state of 0x3f, which is ACTIVITY, TIMEOUT, AGGREGATION, SYNC, COLLECTING and DISTRIBUTING together, a partner state with SYNC in it, and the partner's system MAC. lag show should list both ports as trunk members, since that reads the hardware rather than the protocol's opinion of itself.

The host's view is the one worth trusting, in /proc/net/bonding/bond0. Both slaves want the same Aggregator ID; two different numbers mean the bond has not aggregated anything and each link is sitting in its own group, which looks healthy at a glance and is not. Both churn states should read none. monitoring that never settles is the signature of our transmit period being too slow for the rate the partner asked for.

Three things are worth checking past convergence, because each of them has been broken here at some point:

• tcpdump -i enp1s0 ether proto 0x8809 on each slave should show the switch's LACPDUs and that slave's own, and never the other slave's. A sibling leak is what the trap prevents: the PDU never enters the forwarding path at all.
• No illegal loopback in the host's log.
• In a hex dump of a frame from the switch, the three reserved bytes after the actor state should be zero. They used to carry whatever the transmit buffer held from the previous frame.

Pulling one slave down and putting it back is the cheapest failure test. The group narrows to the surviving member and comes back within tens of seconds, and traffic across the transition loses a few percent at the edges rather than half of everything, which is what it would lose if the ASIC kept hashing onto the dead member.

Known limits

• No Churn Detection machines and no Marker protocol.
• Coupled Mux: collecting and distributing are enabled together.
• One aggregator per group.
• Selection compares the partner's system only, not its key: two aggregates that one partner system offers on the same group land in one trunk here.
• Member compatibility is not validated; see #377.