Sending packet using libpcap speed limitation

Question

I wrote a program that captures Ethernet packets from some NIC (i.e eth0) at high speed (about 1Gbps) and forward traffic using pcap_sendpacket() to another NIC (i.e eth1).

While the forwarder Ethernet interface is not connected to another computer NIC, forwarding speed is desired, but when I connect it, sending speed gets down awfully (from 900 Mbps to 100 Mbps).

Can anybody explain the reason?

_{OS: Kubuntu linux 10.10
Language: C++
Library: Libpcap}

Answer 1

Because the destination NIC is a 100 Bbps interface :D :p

Answer 2

Or the auto-negotiation for the network device has been turned off and is trying 100Mbps firstly.