vlan problem help needed - ccna aspirant

songccna · Joined: 23 Dec 2004 Posts: 3

preparing for the ccna, and practiced with a simulator,in this
i have 11 pc's connected to switch 2950 and another 11 pcs connected to switch 1912 as shown in the topology.
here,
in swicth 2950,

ip address= 10.0.10.50
default-gateway=10.0.10.100

fa0/1,2,3,4=vlan 1 (one)

fa0/5,6,7,8=vlan 2(two)

fa0/9,10,11,12=vlan 3(three); fa0/12 used for trunking, though i did not assign it to vlan 3, it took it as vlan 3(?)

pc1-11 are assigned ip addresses 10.0.10.1 thro 10.0.10.11

in switch 1912,

ip address= 10.0.20.50
default-gateway=10.0.20.100

e0/1-5=vlan 1(one)
e0/6-8=vlan 2(two)
e0/9-11=vlan 3(three)
and fa0/26 used for trunking

pc12-22 are assigned ip addresses 10.0.20.1 thro 10.0.20.11

ping

pc1 to pc 11 =OK

pc1 to pc17,21 =not pinging (sure as these 2 are in different vlans)

the problem is when i tried to
ping from

pc21 to pc1 =pings (though it is different vlan), why it is happening?
from all pcs connected to 1912 a ping to other pcs on 2950 whether they are in same vlan or different , the ping succeeds

similarly when i tried within the same switch 2950,
from pc1 to pc 6, it pings

it should not succeed right? as its in differnt vlan

i tried to debug,
with
show int f0/12 switchport, it displayed trunking encapsulation as dotq and while in 1912 as isl,
i thought this may be the cause.

but how to change the encapsulation?

plz someone clarify this

Joshua Delcore · Joined: 26 Apr 2005 Posts: 7

I am also going through my CCNA so I am no expert but If I remember right those two switches use different trucking protocols so they cannot talk to each other even within the same VLAN (maybe unless its the default VLAN 1). Also, you need a router to have each VLAN talk to each other....each VLAN is a separate Broadcast network which needs a routing protocol to reach them......Again I am not even a CCNA yet but I think this may answer your question if I am right.
Joshua Delcore