hello
one simple method will be to use only one vtp domain between your DS1 - DS2 and access switches
also you can configure vlan 10 and 20 on both DS routers and use HRSP for election process
for example:
DS1#
!
int vlan 10
ip add 172.18.10.10 255.255.255.0
standby 10 ip 172.18.10.1
!
on DS2
!
int vlan 10
ip add 172.18.10.20 255.255.255.0
standby 10 ip 172.18.10.1
!
doing this, your ip devices in network 172.18.10.0/24 will have default gateway 172.16.18.1
one solution to STP topology will be to have links between DS1 and DS2 root for vlan 10 and 20
futuremore we can do alteration of STP cost in L2 network, doing this you can try to block stp vlan 10 and 20 between access-switch10 and access-switch20
best regards
vanguardro
