3550#show run

interface FastEthernet1/2
 switchport mode trunk
!
interface Vlan1
 ip address 172.16.1.1 255.255.255.0
!
interface Vlan10
 ip address 172.16.10.1 255.255.255.0
!
interface Vlan20
 ip address 172.16.20.1 255.255.255.0
!

2950#show run
Building configuration...

interface FastEthernet1/1
 switchport mode trunk
!
interface FastEthernet1/11
 switchport access vlan 10
!
interface FastEthernet1/12
 switchport access vlan 20
!
interface FastEthernet1/13
 switchport access vlan 10
!
interface Vlan1
 no ip address
!

pc1#show run

interface FastEthernet0/0
 ip address 172.16.10.2 255.255.255.0
 no sh
ip route 0.0.0.0 0.0.0.0 172.16.10.1

end

pc2#show run
!
interface FastEthernet0/0
 ip address 172.16.20.2 255.255.255.0
 no sh
ip route 0.0.0.0 0.0.0.0 172.16.20.1
!
end

PC3#show run

interface FastEthernet0/0
 ip address 172.16.10.3 255.255.255.0
 no sh
ip route 0.0.0.0 0.0.0.0 172.16.10.1
!