Please help me in building the logic of the scenario. Here is my topology
 >> (subinterface) ---- E1 Link ----(access port)>>>(subinterface)
Here is the relevant config.
encapsulation dot1Q 15 native
ip address 192.168.249.161 255.255.255.252
description *** ***
encapsulation dot1Q 15
ip address 192.168.249.162 255.255.255.252
Created a VLAN - 15 and made a port access port of VLAN 15 pointing towards 2821.
Trunking between 4948 and 7206 is enabled.Native Vlan between 4948 and 7206 is Vlan 13.
description *** TO ROUTER 2821 ***
switchport access vlan 15
switchport mode access
Now the things work ok with this configuration.I am confused with this line in the 2821 config . i.e; encapsulation dot1Q 15 native. If I don't use the word native in the end , I cant ping from 2821 to 7206 but still it shows 4948 in its show cdp. As soon as I put native word in the end of this command ping and everthing starts working fine.
One more thing if I plug the E 1(Rj -45) to my laptop (instead of router 2821, just for testing) the laptop can ping to the remote 7206.
Exlpanation required please.
For your first question, if the router is only on one VLAN, then you simply configure the switch as an access port in the VLAN, and configure the router physical interface without any tagging encapsulation.
I'm not sure what you ar trying to get at with the second question, but I think this is what you want: suppose the router has two subinterfaces, say with encapsulation dot1q 20 and 30. The switch passes a frame in VLAN 20 to the router still with its tag. The router knows it is for interface Fa0/0.20 because the tage corresponds to the encapsulation on Fa0/0.20. The router interface strips off the tag, and passes the frame to its routing process (or more strictly its forwarding processes). The routing table decides that the packet has to be forwarded to interface Fa0/0.30. Interface Fa0/0.30 adds a tag 30, and passes the frame to the switch.
Does that answer your question?