If I set WAN connection on PPPoE in my NAT Router, connection would be established but I observed that when I increase network load (by sending data with speed of above 80Mbps) softirq takes around 90-95% CPU usage. So that other processes are not able to execute in correct manner.

Ethernet cards are having 100Mbps capability.

The same case is not observed in case of static connection. Every thing works fine If I set WAN connection on static IP address.

To establish PPPoE connection I have used PPPoE Client. For that I have used pppd daemon to start PPPoE connection. Configuration of pppd is given below.

plugin rp-pppoe.so eth0
unit 50
noipdefault
hide-password
persist
holdoff 5
maxfail 0
usepeerdns
noauth
lcp-echo-interval 30
lcp-echo-failure 3
mru 1492
mtu 1492
nobsdcomp
nodeflate
nopcomp
novj
novjccomp
noaccomp -am
user vlan10
password 1234


With this configuration connection will be established.

Kernel version : 3.4.56
All necessary kernel module is loaded for PPPoE connection.

Any kind of help will be highly appreciated.

Thanks.

Moreover I come to know that NAPI is enable, but seems It is not applicable when I switch my WAN connection on PPPoE.

PPPoE is known to use processing power. I had pppoe on a 100Mbit internet connection - with pppoe my router (WRT54GL) capped the transfer rate at ~30Mbit. Plugging the cable directly in the computer i had the full 100Mbit (tried with torrents, constant 11 megabytes).
I had a dual core Athlon II @3GHz then and i had about ~50 or more CPU usage (this means 50% of one core) if i remember correctly.
So, avoid pppoe if its not needed.