diff --git a/devtools/Makefile b/devtools/Makefile
index 81b620676..0badc4982 100644
--- a/devtools/Makefile
+++ b/devtools/Makefile
@@ -1,12 +1,12 @@
 DEVTOOLS_SRC := devtools/gen_print_wire.c devtools/gen_print_onion_wire.c devtools/print_wire.c
 DEVTOOLS_OBJS := $(DEVTOOLS_SRC:.c=.o)
-DEVTOOLS := devtools/bolt11-cli devtools/decodemsg devtools/onion devtools/dump-gossipstore devtools/gossipwith devtools/create-gossipstore devtools/mkcommit devtools/mkfunding devtools/mkclose devtools/mkgossip devtools/mkencoded devtools/checkchannels devtools/mkquery devtools/lightning-checkmessage
+DEVTOOLS := devtools/bolt11-cli devtools/decodemsg devtools/onion devtools/dump-gossipstore devtools/gossipwith devtools/create-gossipstore devtools/mkcommit devtools/mkfunding devtools/mkclose devtools/mkgossip devtools/mkencoded devtools/checkchannels devtools/mkquery devtools/lightning-checkmessage devtools/topology
 ifeq ($(EXPERIMENTAL_FEATURES),1)
 DEVTOOLS += devtools/blindedpath
 endif
 
 DEVTOOLS_TOOL_SRC := $(DEVTOOLS:=.c)
-DEVTOOLS_TOOL_OBJS := $(DEVTOOLS_TOOL_SRC:.c=.o)
+DEVTOOLS_TOOL_OBJS := $(DEVTOOLS_TOOL_SRC:.c=.o) devtools/clean_topo.o
 
 DEVTOOLS_BOLT_DEPS += $(BOLT_DEPS) tools/gen/print_impl_template tools/gen/print_header_template
 
@@ -93,9 +93,11 @@ devtools/mkquery: $(DEVTOOLS_COMMON_OBJS) $(CCAN_OBJS) $(BITCOIN_OBJS) wire/from
 
 devtools/lightning-checkmessage: $(DEVTOOLS_COMMON_OBJS) $(CCAN_OBJS) $(BITCOIN_OBJS) wire/fromwire.o wire/towire.o devtools/lightning-checkmessage.o
 
+devtools/topology: $(DEVTOOLS_COMMON_OBJS) $(CCAN_OBJS) $(BITCOIN_OBJS) wire/fromwire.o wire/towire.o wire/tlvstream.o common/gossmap.o common/random_select.o common/dijkstra.o common/route.o devtools/clean_topo.o devtools/topology.o
+
 # Make sure these depend on everything.
 ALL_PROGRAMS += $(DEVTOOLS)
-ALL_OBJS += $(DEVTOOLS_OBJS) $(DEVTOOLS_TOOL_OBJS)
+ALL_OBJS += $(DEVTOOLS_OBJS) $(DEVTOOLS_TOOL_OBJS) 
 
 check-source: $(DEVTOOLS_SRC:%=check-src-include-order/%) $(DEVTOOLS_TOOLS_SRC:%=check-src-include-order/%)
 
diff --git a/devtools/clean_topo.c b/devtools/clean_topo.c
new file mode 100644
index 000000000..07f05d432
--- /dev/null
+++ b/devtools/clean_topo.c
@@ -0,0 +1,53 @@
+#include <common/gossmap.h>
+#include <devtools/clean_topo.h>
+
+static void visit(struct gossmap *map,
+		  struct gossmap_node *n,
+		  bool *visited)
+{
+	visited[gossmap_node_idx(map, n)] = true;
+
+	for (size_t i = 0; i < n->num_chans; i++) {
+		int dir;
+		struct gossmap_chan *c = gossmap_nth_chan(map, n, i, &dir);
+		struct gossmap_node *peer;
+
+		peer = gossmap_nth_node(map, c, !dir);
+		if (!visited[gossmap_node_idx(map, peer)])
+			visit(map, peer, visited);
+	}
+}
+
+void clean_topo(struct gossmap *map, bool remove_singles)
+{
+	struct gossmap_node *n, *next;
+	bool *visited;
+
+	/* Remove channels which are not enabled in both dirs. */
+	for (struct gossmap_chan *c = gossmap_first_chan(map);
+	     c;
+	     c = gossmap_next_chan(map, c)) {
+		if (!c->half[0].enabled || !c->half[1].enabled) {
+			gossmap_remove_chan(map, c);
+		}
+	}
+
+	if (remove_singles) {
+		for (n = gossmap_first_node(map); n; n = next) {
+			next = gossmap_next_node(map, n);
+			if (n->num_chans == 1)
+				gossmap_remove_node(map, n);
+		}
+	}
+
+	/* Remove isolated nodes (we assume first isn't isolated!) */
+	visited = tal_arrz(NULL, bool, gossmap_max_node_idx(map));
+	visit(map, gossmap_first_node(map), visited);
+
+	for (n = gossmap_first_node(map); n; n = next) {
+		next = gossmap_next_node(map, n);
+		if (!visited[gossmap_node_idx(map, n)])
+			gossmap_remove_node(map, n);
+	}
+	tal_free(visited);
+}
diff --git a/devtools/clean_topo.h b/devtools/clean_topo.h
new file mode 100644
index 000000000..40349480c
--- /dev/null
+++ b/devtools/clean_topo.h
@@ -0,0 +1,13 @@
+#ifndef LIGHTNING_DEVTOOLS_CLEAN_TOPO_H
+#define LIGHTNING_DEVTOOLS_CLEAN_TOPO_H
+#include "config.h"
+
+struct gossmap;
+
+/* Cleans topology:
+ * 1. Removes channels not enabled in both dirs.
+ * 2. (if remove_singles) remove nodes with only one connection.
+ * 3. Remove isolated nodes (we assume first node is well-connected!).
+ */
+void clean_topo(struct gossmap *map, bool remove_singles);
+#endif /* LIGHTNING_DEVTOOLS_CLEAN_TOPO_H */
diff --git a/devtools/topology.c b/devtools/topology.c
new file mode 100644
index 000000000..b311b1812
--- /dev/null
+++ b/devtools/topology.c
@@ -0,0 +1,306 @@
+#include <assert.h>
+#include <ccan/err/err.h>
+#include <ccan/opt/opt.h>
+#include <ccan/time/time.h>
+#include <common/amount.h>
+#include <common/dijkstra.h>
+#include <common/gossmap.h>
+#include <common/node_id.h>
+#include <common/pseudorand.h>
+#include <common/random_select.h>
+#include <common/route.h>
+#include <common/type_to_string.h>
+#include <devtools/clean_topo.h>
+#include <inttypes.h>
+#include <stdio.h>
+
+/* We ignore capacity constraints */
+static bool channel_usable(const struct gossmap *map,
+			   const struct gossmap_chan *c,
+			   int dir,
+			   struct amount_msat amount,
+			   void *unused)
+{
+	if (!gossmap_chan_set(c, dir))
+		return false;
+	if (!c->half[dir].enabled)
+		return false;
+	return true;
+}
+
+/* Note: dijkstra() sets dir to the neighbor side; i.e. c->half[dir].node_idx is the
+ * neighbor. */
+static bool channel_usable_to_excl(const struct gossmap *map,
+				   const struct gossmap_chan *c,
+				   int dir,
+				   struct amount_msat amount,
+				   struct gossmap_node *excl)
+{
+	if (!channel_usable(map, c, dir, amount, NULL))
+		return false;
+
+	/* Don't go via excl. */
+	if (c->half[dir].nodeidx == gossmap_node_idx(map, excl))
+		return false;
+	return true;
+}
+
+/* What nodes can reach n without going through exclude? */
+static size_t count_possible_sources(const struct gossmap *map,
+				     struct gossmap_node *n,
+				     struct gossmap_node *exclude,
+				     bool is_last_node)
+{
+	const struct dijkstra *dij;
+	size_t distance_budget, num;
+
+	dij = dijkstra(tmpctx, map, n, AMOUNT_MSAT(0), 0,
+		       channel_usable_to_excl, route_path_shorter, exclude);
+
+	if (is_last_node)
+		distance_budget = ROUTING_MAX_HOPS - 1;
+	else
+		distance_budget = ROUTING_MAX_HOPS - 2;
+
+	assert(dijkstra_distance(dij, gossmap_node_idx(map, n)) == 0);
+	assert(dijkstra_distance(dij, gossmap_node_idx(map, exclude)) == UINT_MAX);
+
+	num = 0;
+	for (n = gossmap_first_node(map); n; n = gossmap_next_node(map, n)) {
+		if (dijkstra_distance(dij, gossmap_node_idx(map, n)) <= distance_budget)
+			num++;
+	}
+	return num;
+}
+
+/* Note: dijkstra() sets dir to the neighbor side; i.e. c->half[dir].node_idx is the
+ * neighbor. */
+static bool channel_usable_from_excl(const struct gossmap *map,
+				     const struct gossmap_chan *c,
+				     int dir,
+				     struct amount_msat amount,
+				     struct gossmap_node *excl)
+{
+	if (!channel_usable(map, c, !dir, amount, NULL))
+		return false;
+
+	/* Don't go via excl. */
+	if (c->half[dir].nodeidx == gossmap_node_idx(map, excl))
+		return false;
+	return true;
+}
+
+static size_t memcount(const void *mem, size_t len, char c)
+{
+	size_t count = 0;
+	for (size_t i = 0; i < len; i++) {
+		if (((char *)mem)[i] == c)
+			count++;
+	}
+	return count;
+}
+
+static void visit(const struct gossmap *map,
+		  struct gossmap_node *n,
+		  struct gossmap_node *exclude,
+		  bool *visited)
+{
+	if (n == exclude)
+		return;
+	if (visited[gossmap_node_idx(map, n)])
+		return;
+	visited[gossmap_node_idx(map, n)] = true;
+
+	for (size_t i = 0; i < n->num_chans; i++) {
+		int dir;
+		struct gossmap_chan *c;
+		c = gossmap_nth_chan(map, n, i, &dir);
+
+		if (!channel_usable(map, c, dir, AMOUNT_MSAT(0), NULL))
+			continue;
+		visit(map, gossmap_nth_node(map, c, !dir), exclude, visited);
+	}
+}
+
+/* What nodes can n reach without going through exclude? */
+static size_t count_possible_destinations(const struct gossmap *map,
+					  struct gossmap_node *start,
+					  struct gossmap_node *exclude,
+					  bool is_first_node)
+{
+	const struct dijkstra *dij;
+	size_t distance_budget, num;
+
+	dij = dijkstra(tmpctx, map, start, AMOUNT_MSAT(0), 0,
+		       channel_usable_from_excl, route_path_shorter, exclude);
+
+	if (is_first_node)
+		distance_budget = ROUTING_MAX_HOPS - 1;
+	else
+		distance_budget = ROUTING_MAX_HOPS - 2;
+
+	assert(dijkstra_distance(dij, gossmap_node_idx(map, start)) == 0);
+	assert(dijkstra_distance(dij, gossmap_node_idx(map, exclude)) == UINT_MAX);
+
+	num = 0;
+	for (struct gossmap_node *n = gossmap_first_node(map);
+	     n;
+	     n = gossmap_next_node(map, n)) {
+		if (dijkstra_distance(dij, gossmap_node_idx(map, n)) <= distance_budget)
+			num++;
+#if 0
+		else
+			printf("Can't reach %s (%u) if we exclude %s\n",
+			       type_to_string(tmpctx, struct node_id,
+					      gossmap_node_get_id(map, n)),
+			       dijkstra_distance(dij, gossmap_node_idx(map, n)),
+			       type_to_string(tmpctx, struct node_id,
+					      gossmap_node_get_id(map, exclude)));
+#endif
+	}
+
+	/* Now double-check with flood-fill. */
+	bool *visited = tal_arrz(tmpctx, bool, gossmap_max_node_idx(map));
+	visit(map, start, exclude, visited);
+	assert(memcount(visited, tal_bytelen(visited), true) == num);
+	return num;
+}
+
+static bool measure_least_cost(struct gossmap *map,
+			       struct gossmap_node *src,
+			       struct gossmap_node *dst)
+{
+	const struct dijkstra *dij;
+	u32 srcidx = gossmap_node_idx(map, src);
+	/* 10ksat, budget is 0.5% */
+	const struct amount_msat sent = AMOUNT_MSAT(10000000);
+	const struct amount_msat budget = amount_msat_div(sent, 200);
+	const u32 riskfactor = 0;
+	/* Max distance is 20 */
+	const u32 distance_budget = ROUTING_MAX_HOPS;
+	struct amount_msat maxcost, fee;
+	struct route **path;
+	struct timemono tstart, tstop;
+	struct node_id srcid;
+
+	gossmap_node_get_id(map, src, &srcid);
+	printf("# src %s (%u channels)\n",
+	       type_to_string(tmpctx, struct node_id, &srcid),
+	       src->num_chans);
+
+	tstart = time_mono();
+	dij = dijkstra(tmpctx, map, dst,
+		       sent, riskfactor, channel_usable,
+		       route_path_cheaper, NULL);
+	tstop = time_mono();
+
+	printf("# Time to find path: %"PRIu64" usec\n",
+	       time_to_usec(timemono_between(tstop, tstart)));
+
+	if (dijkstra_distance(dij, srcidx) > distance_budget) {
+		printf("failed (%s)\n",
+		       dijkstra_distance(dij, srcidx) == UINT_MAX ? "unreachable" : "too far");
+		return false;
+	}
+	if (!amount_msat_add(&maxcost, sent, budget))
+		abort();
+	if (amount_msat_greater(dijkstra_amount(dij, srcidx), maxcost)) {
+		printf("failed (too expensive)\n");
+		return false;
+	}
+
+	path = route_from_dijkstra(map, dij, src);
+	printf("# path length %zu\n", tal_count(path));
+	if (!amount_msat_sub(&fee, dijkstra_amount(dij, srcidx), sent))
+		abort();
+	printf("# path fee %s\n",
+	       type_to_string(tmpctx, struct amount_msat, &fee));
+
+	/* Count possible sources */
+	for (size_t i = 0; i < tal_count(path); i++) {
+		struct gossmap_node *prev, *cur;
+
+		/* N+1th node is at end of Nth hop */
+		prev = gossmap_nth_node(map, path[i]->c, path[i]->dir);
+		cur = gossmap_nth_node(map, path[i]->c, !path[i]->dir);
+
+		printf("source set size node %zu/%zu: %zu\n",
+		       i+1, tal_count(path),
+		       count_possible_sources(map, prev, cur, cur == dst));
+	}
+
+	/* Count possible destinations. */
+	for (size_t i = 0; i < tal_count(path); i++) {
+		struct gossmap_node *cur, *next;
+
+		/* N+1th node is at end of Nth hop */
+		cur = gossmap_nth_node(map, path[i]->c, path[i]->dir);
+		next = gossmap_nth_node(map, path[i]->c, !path[i]->dir);
+
+		printf("destination set size node %zu/%zu: %zu\n",
+		       i, tal_count(path),
+		       count_possible_destinations(map, next, cur, cur == src));
+	}
+	return true;
+}
+
+int main(int argc, char *argv[])
+{
+	struct timemono tstart, tstop;
+	struct gossmap_node *n, *dst;
+	struct gossmap *map;
+	struct node_id dstid;
+	bool no_singles = false;
+
+	setup_locale();
+	setup_tmpctx();
+
+	opt_register_noarg("--no-single-sources", opt_set_bool, &no_singles,
+			   "Eliminate single-channel nodes");
+	opt_register_noarg("-h|--help", opt_usage_and_exit,
+			   "<gossipstore> <srcid>|all <dstid>\n"
+			   "A topology test program.",
+			   "Get usage information");
+	opt_parse(&argc, argv, opt_log_stderr_exit);
+	if (argc != 4)
+		opt_usage_exit_fail("Expect 3 arguments");
+
+	tstart = time_mono();
+	map = gossmap_load(NULL, argv[1]);
+	if (!map)
+		err(1, "Loading gossip store %s", argv[1]);
+	tstop = time_mono();
+
+	printf("# Time to load: %"PRIu64" msec\n",
+	       time_to_msec(timemono_between(tstop, tstart)));
+
+	clean_topo(map, no_singles);
+	printf("# Reduced to %zu nodes and %zu channels\n",
+	       gossmap_num_nodes(map), gossmap_num_chans(map));
+
+	if (!node_id_from_hexstr(argv[3], strlen(argv[3]), &dstid))
+		errx(1, "Bad dstid");
+	dst = gossmap_find_node(map, &dstid);
+	if (!dst)
+		errx(1, "Unknown destination node '%s'", argv[3]);
+
+	if (streq(argv[2], "all")) {
+		for (n = gossmap_first_node(map);
+		     n;
+		     n = gossmap_next_node(map, n)) {
+			measure_least_cost(map, n, dst);
+			clean_tmpctx();
+		}
+	} else {
+		struct node_id srcid;
+		if (!node_id_from_hexstr(argv[2], strlen(argv[2]), &srcid))
+			errx(1, "Bad srcid");
+		n = gossmap_find_node(map, &srcid);
+		if (!n)
+			errx(1, "Unknown source node '%s'", argv[2]);
+		if (!measure_least_cost(map, n, dst))
+			exit(1);
+	}
+
+	tal_free(map);
+}