Root bridge election, root ports and designated ports

The bridge ID

STP uses the old word "bridge" for a switch. Each switch has an 8-byte bridge ID (BID): a 2-byte priority part followed by the 6-byte MAC address of the switch. When two BIDs are compared, the priority is compared first, and the MAC address only breaks a tie. Lower is better.

With Cisco's per-VLAN STP, the 2-byte priority part is split in two: a 4-bit priority and a 12-bit extended system ID, which is simply the VLAN number. So the priority you see is the configured priority plus the VLAN number. With the default priority 32768, a switch shows 32769 in VLAN 1 and 32778 in VLAN 10. Because only 4 bits are left for the real priority, you can set it only in steps of 4096: 0, 4096, 8192 ... 61440.

Step 1: elect the root bridge

At first, every switch claims to be the root and sends BPDUs saying so. When a switch hears a BPDU with a lower (better) root BID than its own, it stops claiming and passes on the better one. Soon all switches agree: the switch with the lowest BID is the root bridge. If nobody changed the priority, every switch has 32768 and the switch with the lowest MAC address wins, which is often the oldest, slowest switch in the building. That is why you choose the root yourself (next topic).

All ports of the root bridge are designated ports and forward.

Step 2: every other switch picks one root port

The root port is the port with the best path to the root bridge. "Best" means the lowest root path cost: the sum of the port costs of every link on the way to the root. Cost depends on link speed.

If two ports have the same root path cost, the tie-breakers are, in order: the lowest neighbour (sender) bridge ID, then the lowest neighbour port priority (default 128), then the lowest neighbour port number. Note that these compare the values sent by the neighbour in its BPDU, not your own port numbers.

Step 3: one designated port per link

On every link (segment), exactly one port is the designated port: the one on the switch with the lowest root path cost for that link. It forwards traffic onto that link. If both switches have the same cost, the switch with the lower bridge ID wins.

Step 4: block the rest

Any port that is neither a root port nor a designated port is blocked (in RSTP it is called an alternate port). This is what breaks the loop.

Worked example: three switches in a triangle

SW1, SW2 and SW3 all have priority 32769 (VLAN 1). Their MACs are SW1 0011.1111.1111, SW2 0022.2222.2222, SW3 0033.3333.3333. All links are 1 Gbps (cost 4). Cabling: SW1 Gi0/1 to SW2 Gi0/1, SW1 Gi0/2 to SW3 Gi0/1, SW2 Gi0/2 to SW3 Gi0/2.

1. Root bridge: all priorities are equal, so the lowest MAC wins: SW1.
2. SW1 Gi0/1 and Gi0/2 are designated, because every port of the root bridge is designated.
3. SW2 root port: direct via Gi0/1 costs 4; via SW3 (Gi0/2) costs 4 + 4 = 8. Root port is Gi0/1.
4. SW3 root port: direct via Gi0/1 costs 4. Root port is Gi0/1.
5. Link SW2 to SW3: both switches have root cost 4, a tie. The lower bridge ID is SW2, so SW2 Gi0/2 is designated.
6. SW3 Gi0/2 is neither root nor designated: it is blocked. The loop is broken.

Now change one thing: the SW1 to SW2 link is only 100 Mbps (cost 19). SW2 compares 19 (direct) with 4 + 4 = 8 (through SW3), so its root port becomes Gi0/2. On the SW2 to SW3 link, SW3 now has the lower root cost (4 against 8), so SW3 Gi0/2 is designated. The SW1 to SW2 link has SW1 Gi0/1 as designated, and SW2 Gi0/1 is blocked. The slow link is the one that ends up unused, which is exactly what you want.

Common mistakes and how to fix them

• Thinking the highest priority wins. In STP, lower is always better: lower BID, lower cost.
• Adding the costs of the wrong ports. Root path cost adds the cost of each link on the way to the root, as seen by the receiving ports.
• Using your own port number as a tie-breaker. The tie-breaker is the neighbour's (sender's) port priority and number.
• Setting a priority like 10000. Only multiples of 4096 are accepted.