diff --git a/.env.example b/.env.example
index 3b56c52..f7c34fa 100644
--- a/.env.example
+++ b/.env.example
@@ -55,7 +55,10 @@ WORKER=you@192.0.2.2
 # Only for private Hugging Face mirrors; the pinned checkpoints are public.
 #HF_TOKEN=
 
-# Detected from the RoCE device; set only if detection picks the wrong port.
+# The fabric port is detected on each Spark: start.sh lists the RoCE ports whose
+# link is up and that have an IPv4 address, picks the pair on the same subnet (the
+# cable between the two Sparks), and prints its choice. Set these only if it picks
+# the wrong port, e.g. rocep1s0f1 / enp1s0f1np1 for a cable on the second port.
 #FABRIC_HCA=rocep1s0f0
 #FABRIC_INTERFACE=enp1s0f0np0
 # The fabric also uses the RDMA device behind the ConnectX-7's other PCIe domain
diff --git a/docs/INSTALL-MANUAL.md b/docs/INSTALL-MANUAL.md
index 8169507..369cbb0 100644
--- a/docs/INSTALL-MANUAL.md
+++ b/docs/INSTALL-MANUAL.md
@@ -74,7 +74,9 @@ matches that converter; otherwise it moves any older overlay aside.
 Start rank **1** (the worker) first, then rank **0** (the leader). The
 `--leader-address` is rank 0's IPv4 address on the direct cable. Each rank
 finds its own fabric interface from the RoCE device (`--fabric-hca`, default
-`rocep1s0f0`). Pass `--fabric-interface` to override it.
+`rocep1s0f0`, the first port). If the cable is on the second port, pass
+`--fabric-hca rocep1s0f1` on that Spark (`./start.sh` detects this itself).
+Pass `--fabric-interface` to override the interface.
 
 ```sh
 # on rank 1:
diff --git a/start.sh b/start.sh
index 743fe71..3843a77 100755
--- a/start.sh
+++ b/start.sh
@@ -14,7 +14,6 @@ if [[ -f .env ]]; then set -a; . ./.env; set +a; fi
 : "${WORKER:?set WORKER in .env to the ssh destination of the other Spark (see .env.example)}"
 MODEL_ROOT=${MODEL_ROOT:-$HOME/models/sparkglm}
 PROFILE=${PROFILE:-4x1m}
-FABRIC_HCA=${FABRIC_HCA:-rocep1s0f0}
 IMAGE=${IMAGE:-$(install/build.sh --tag)}
 CHECKPOINT=nvidia/GLM-5.3-Flash-NVFP4@423acf37583782c51c142d145aef733d72943d93
 DRAFTER=incoai/GLM-5.3-Flash-DFlash2@7d74cdd881ed7e32c31175984a67823127b66cfe
@@ -27,6 +26,47 @@ on_worker() { ssh -o BatchMode=yes "$WORKER" "$@"; }
 script_on_worker() { local script=$1; shift; on_worker "bash -s -- $(printf '%q ' "$@")" < "$script"; }
 has_image() { docker image inspect "$IMAGE" >/dev/null 2>&1; }
 
+# Each Spark's RoCE ports whose link is up and that have an IPv4 address, one
+# "<hca> <interface> <address/prefix>" line each. Only PCIe domain 0 (rocep*):
+# the engine adds the device behind the other domain of the same port itself.
+fabric_ports() {
+  local dev net cidr
+  for dev in /sys/class/infiniband/rocep*; do
+    grep -qs ACTIVE "$dev/ports/1/state" || continue
+    for net in "$dev"/device/net/*; do
+      cidr=$(ip -4 -o addr show dev "${net##*/}" 2>/dev/null | awk '{print $4; exit}')
+      [[ -n $cidr ]] && echo "${dev##*/} ${net##*/} $cidr"
+    done
+  done
+}
+
+# Pick the fabric port on each Spark: the pair of active ports on the same IPv4
+# subnet is the cable between them. Sets hca0/iface0/ip0 (here) and hca1/ip1
+# (the worker). FABRIC_HCA in .env overrides it for both.
+pick_fabric() {
+  local here there choice
+  here=$(fabric_ports)
+  there=$(on_worker "bash -s" <<< "$(declare -f fabric_ports); fabric_ports")
+  [[ -n $here ]] || { echo "no RoCE port with its link up and an IPv4 address on this Spark; check the cable and the port's address, or set FABRIC_HCA in .env" >&2; exit 2; }
+  [[ -n $there ]] || { echo "no RoCE port with its link up and an IPv4 address on $WORKER; check the cable and the port's address, or set FABRIC_HCA in .env" >&2; exit 2; }
+  choice=$(python3 - "$here" "$there" <<'PY'
+import ipaddress, sys
+parse = lambda text: [line.split() for line in text.splitlines() if len(line.split()) == 3]
+here, there = parse(sys.argv[1]), parse(sys.argv[2])
+net = lambda cidr: ipaddress.ip_interface(cidr).network
+match = [(a, b) for a in here for b in there if net(a[2]) == net(b[2])]
+a, b = match[0] if match else (here[0], there[0])
+print(a[0], a[1], a[2].split('/')[0], b[0], b[1], b[2].split('/')[0], 'subnet' if match else 'first')
+PY
+)
+  read -r hca0 iface0 ip0 hca1 iface1 ip1 how <<< "$choice"
+  if [[ $how == subnet ]]; then
+    say "fabric: $hca0 ($iface0, $ip0) here, $hca1 ($iface1, $ip1) on $WORKER; set FABRIC_HCA in .env to choose another port"
+  else
+    say "fabric: no active port pair shares a subnet; using $hca0 ($iface0, $ip0) here and $hca1 ($iface1, $ip1) on $WORKER. If they are not cabled to each other, set FABRIC_HCA in .env"
+  fi
+}
+
 build() {
   say "image $IMAGE"
   if ! has_image; then
@@ -89,13 +129,18 @@ state() { docker inspect -f '{{.State.Status}}' "${NAME}0" 2>/dev/null || echo a
 worker_state() { on_worker docker inspect -f '{{.State.Status}}' "${NAME}1" 2>/dev/null || echo absent; }
 
 serve() {
-  local iface address common
-  iface=${FABRIC_INTERFACE:-$(ls "/sys/class/infiniband/$FABRIC_HCA/device/net" 2>/dev/null | head -1)}
+  local iface address common hca0 iface0 ip0 hca1 iface1 ip1 how
+  if [[ -n ${FABRIC_HCA:-} ]]; then
+    hca0=$FABRIC_HCA hca1=$FABRIC_HCA
+  else
+    pick_fabric
+  fi
+  iface=${FABRIC_INTERFACE:-$(ls "/sys/class/infiniband/$hca0/device/net" 2>/dev/null | head -1)}
   address=${LEADER_ADDRESS:-}
   [[ -n $address ]] || address=$(ip -4 -o addr show dev "$iface" 2>/dev/null | awk '{sub("/.*", "", $4); print $4; exit}')
   [[ -n $address ]] || { echo "no IPv4 address on the fabric interface '$iface'; set LEADER_ADDRESS" >&2; exit 2; }
   common=(--leader-address "$address" --model-root "$MODEL_ROOT" --image "$IMAGE"
-          --profile "$PROFILE" --fabric-hca "$FABRIC_HCA" ${FABRIC_INTERFACE:+--fabric-interface "$FABRIC_INTERFACE"}
+          --profile "$PROFILE" ${FABRIC_INTERFACE:+--fabric-interface "$FABRIC_INTERFACE"}
           ${GPU_MEMORY_UTILIZATION:+--gpu-memory-utilization "$GPU_MEMORY_UTILIZATION"}
           ${PREFIX_CACHE_DIR:+--prefix-cache-dir "$PREFIX_CACHE_DIR"} ${PREFIX_CACHE_GB:+--prefix-cache-gb "$PREFIX_CACHE_GB"})
   case ${DISPLAY_CARVEOUT:-0} in
@@ -115,8 +160,8 @@ serve() {
   esac
   say "start rank 1 on $WORKER, then rank 0 here (leader $address, profile $PROFILE)"
   stop
-  script_on_worker install/start-node.sh --rank 1 "${common[@]}"
-  install/start-node.sh --rank 0 "${common[@]}"
+  script_on_worker install/start-node.sh --rank 1 --fabric-hca "$hca1" "${common[@]}"
+  install/start-node.sh --rank 0 --fabric-hca "$hca0" "${common[@]}"
   say "loading (about 2.5 minutes)"
   for ((i = 0; i < 90; i++)); do
     if curl -sf "$API/health" >/dev/null; then