How do I make the discarding ports appear in different ports or in different bridges?

[chen-xiao-chen]

Hello, first of all thank you for sharing your excellent work.
When I use it in multiple instance situation,I found that the discarding port is the same in different mstid.For example,I added two bridges named as bra and brb,then connected two bridges using veth pair.The topological diagram is below:
(bra)veth01------------------------------------veth02(brb)
(bra)veth03------------------------------------veth04(brb)
my commands list is below:
#brctl addbr bra
#brctl addbr brb
#ip link add veth01 type veth peer name veth02
#ip link add veth03 type veth peer name veth04
#ifconfig bra up
#ifconfig brb up
#ip link set veth01 up
#ip link set veth02 up
#ip link set veth03 up
#ip link set veth04 up
#brctl addif bra veth01
#brctl addif bra veth03
#brctl addif brb veth02
#brctl addif brb veth04
#vconfig add veth01 10
#vconfig add veth01 20
#vconfig add veth02 10
#vconfig add veth02 20
#vconfig add veth03 10
#vconfig add veth03 20
#vconfig add veth04 10
#vconfig add veth04 20
#ip link set veth01.10 up
#ip link set veth01.20 up
#ip link set veth02.10 up
#ip link set veth02.20 up
#ip link set veth03.10 up
#ip link set veth03.20 up
#ip link set veth04.10 up
#ip link set veth04.20 up
#mstpctl addbridge bra
#mstpctl addbridge brb
#mstpctl setforcevers bra mstp
#mstpctl setforcevers brb mstp
#mstpctl createtree bra 10
#mstpctl createtree bra 20
#mstpctl createtree brb 10
#mstpctl createtree brb 20
#mstpctl setvid2fid bra 10:10
#mstpctl setvid2fid bra 20:20
#mstpctl setvid2fid brb 10:10
#mstpctl setvid2fid brb 20:20
#mstpctl setfid2mstid bra 10:10
#mstpctl setfid2mstid bra 20:20
#mstpctl setfid2mstid brb 10:10
#mstpctl setfid2mstid brb 20:20
#brctl stp bra on
#brctl stp brb on
#mstpctl settreeprio bra 10 0
#mstpctl settreeprio bra 20 15
#mstpctl settreeprio brb 10 15
#mstpctl settreeprio brb 20 0
#mstpctl showvid2fid bra
bra list of known MSTIS:
0 10 20
#mstpctl showvid2fid brb
brb list of known MSTIS:
0 10 20
#mstpctl showvid2fid bra
bra VID-to-FID allocation table:
FID 0:1-9,11-19,21-4094
FID 10: 10
FID 20: 20
#mstpctl showvid2fid brb
brb VID-to-FID allocation table:
FID 0:1-9,11-19,21-4094
FID 10: 10
FID 20: 20
In the instance 10,the discarding port is veth04,and in the instance 20,the discarding port is also veth04.
Is this possible or correct?
Maybe my configuration is not correct?
How do I configure if I want to make veth01 or veth03 become discarding port in the distance 20?
Thanks.