BAC: Add -cycle option so that both linear and triangle topologies could be tested

linear:

	B--A--C

triangle:

	B-----A
	 \   /
	  \ /
	   C

namespace_demo/lib.sh

@@ -6,6 +6,7 @@ PID=$$
 xunshare() {
     ref=$1
     shift
+    echo "create $ref"
     rm -rf "$ref"
     mkdir "$ref"
     touch $ref/mnt $ref/net
@@ -29,6 +30,7 @@ xlink() {
     B=$2
     a="${A,,}" # A -> a
     b="${B,,}" # B -> b
+    echo "link $A-$B"
 
     xnsenter $A -- ip link add $a-$b type veth peer br$A$B.$a
     xnsenter $A -- ip link set br$A$B.$a netns $PID

namespace_demo/topo-B-A-C.sh

-#!/bin/bash -ex
+#!/bin/bash -e
 # topo B-A-C creates the following network topology:
 #
 #
 #     ns:B      ns:A      ns:C
 #
-#       B  --- br0 ---  A  --- br1 ---  C
+#       B  -----  A  -----  C
 #        b-a   a-b a-c   c-a
 #
 #
+# if run with -cycle, it also creates B-C link:
+#
+#
+#       B  ...............  C
+#       |                   |
+#       '-------------------'
+#
 # should be run under unshare -mrun .
 # use xnsenter {A,B,C} to enter into namespaces of A/B/C.
 #
@@ -15,6 +22,8 @@
 
 . lib.sh
 
+cycle=n
+[ "$1" = "-cycle" ] && cycle=y
 
 # lo on everything
 xunshare A -- ip link set lo up
@@ -24,13 +33,19 @@ xunshare C -- ip link set lo up
 # links
 xlink B A   # B-A
 xlink A C   # A-C
+[ $cycle = y ] && xlink B C
 
+set -x
 
 # addresses
 xnsenter B -- ip addr add 10.0.0.1/24 dev b-a
 xnsenter A -- ip addr add 10.0.0.2/24 dev a-b
 xnsenter C -- ip addr add 20.0.0.1/24 dev c-a
 xnsenter A -- ip addr add 20.0.0.2/24 dev a-c
+if [ $cycle = y ]; then
+    xnsenter B -- ip addr add 30.0.0.1/24 dev b-c
+    xnsenter C -- ip addr add 30.0.0.2/24 dev c-b
+fi
 
 # multicast routing
 xnsenter B -- ip route add 224.0.0.0/4 dev b-a
@@ -39,6 +54,11 @@ xnsenter C -- ip route add 224.0.0.0/4 dev c-a
 xnsenter A -- ip route add    224.0.0.0/4 dev a-b   # NOTE on A 224.0.0.0/4 is routed
 xnsenter A -- ip route append 224.0.0.0/4 dev a-c   # to _both_ a-b and a-c
 
+if [ $cycle = y ]; then
+    xnsenter B -- ip route append 224.0.0.0/4 dev b-c
+    xnsenter C -- ip route append 224.0.0.0/4 dev c-b
+fi
+
 
 # mount private /var/run so that smcrouted can be started
 xnsenter A -- mount -t tmpfs none /var/run