Link aggregation
Two switches joined by several cables can treat them as one bigger link. Watch spanning tree block the spare cables, then LACP bundle all four into one 4 Gbit/s link, a hash spread different flows over the cables while keeping each flow on one, and the bundle carry on when a cable is pulled out.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01Two switches are joined by four plain cables, with no LACP. What does spanning tree do?
Parallel cables form a loop. Spanning tree breaks loops by blocking all but one path, so three cables sit idle as spares.
02Four 1 Gbit/s cables are bundled with LACP. How does spanning tree see them?
A link aggregation group is one logical link. Spanning tree sees one port at each end, so there is no loop and no cable is blocked.
03One huge file copy is the only traffic on a bundle of four 1 Gbit/s cables. What is its top speed?
Every frame of one flow hashes to the same cable, so a single flow never uses more than one cable: 1 Gbit/s here. Bundles help many flows together.
04Why does a switch keep every frame of one flow on the same cable?
If frames of one flow were spread over cables they could overtake each other. Keeping a flow on one cable keeps its frames in order, which IEEE 802.1AX requires.
05Link 3 of a four-cable LAG is unplugged. What happens?
The switches drop the failed cable from the bundle and keep running at 3 Gbit/s. Only the flows that were on link 3 are hashed onto other cables.
Glossary
- Link aggregation
- Combining several physical links between two devices into one logical link, for more total bandwidth and for redundancy. Standardised in IEEE 802.1AX.
- LAG
- Link aggregation group: the bundle of member links that acts as one link. Also called a port channel, EtherChannel, bond or team.
- LACP
- Link Aggregation Control Protocol. Switches send LACP messages on every member link to agree which links belong in the bundle and to notice when one fails.
- Member link
- One of the physical cables inside a LAG. Usually all members run at the same speed in full duplex.
- Spanning tree (STP)
- A protocol switches run to stop loops: when there are several paths, it blocks all but one.
- Flow
- One conversation between two programs, such as one file copy; its frames share the same addresses and ports.
- Hash
- A number a switch computes by mixing header fields (MAC addresses, IP addresses, ports). The same fields always give the same hash, which picks the cable.
Notes and sources
- Link aggregation and LACP are defined in IEEE 802.1AX (first published as IEEE 802.3ad). Vendors call a LAG a port channel, EtherChannel, bond or team.
- The hash shown (XOR of the IP address and port bytes, 8 buckets) is an example. IEEE 802.1AX leaves the formula to each maker; it only requires that one conversation stays on one link.
- Each switch hashes the frames it sends, so the reply traffic of a flow may use a different cable from the request traffic.
- In practice switches require every member to run at the same speed, full duplex, and end on the same partner switch. (IEEE 802.1AX-2014 and later no longer forbid mixed speeds; the older 802.3ad and 802.1AX-2008 did.)
- IPs are from the private range 192.168.1.0/24 (RFC 1918), MACs from the RFC 7042 documentation block, source ports from the dynamic range 49152-65535 (RFC 6335).
- Frames cross a link in microseconds; the figure slows everything down so you can watch.
- IEEE 802.1AX: Link Aggregation
- IEEE 802.1Q: Bridges and Bridged Networks (spanning tree)
- IEEE 802.3: Ethernet (1000BASE-T, Slow Protocols)
- Linux kernel documentation: Linux Ethernet Bonding Driver HOWTO (802.3ad mode, transmit hash policies)
- RFC 7042: IANA considerations for IEEE 802 parameters (documentation MACs)
- RFC 1918: Address allocation for private internets
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