address (as per your scenario). You look up the destination in the routing table, and don't find it. So we look in RFC792 on page 5: If, according to the information in the gateway's routing tables, the network specified in the internet destination field of a datagram is unreachable, e.g., the distance to the network is infinity, the gateway may send a destination unreachable message to the internet source host of the datagram. In addition, in some networks, the gateway may be able to determine if the internet destination host is unreachable. Gateways in these networks may send destination unreachable messages to the source host when the destination host is unreachable. -------------> who does? the source is reachable.......via BGP.....its a valid internet address... And you send that to the bogus source address *HOW*? ------> how what??...it still isnt a problem for the actual traffic, the "source" network may exist on a BGP router as being advertised from another AS ......but not on the edge router from where it uplinks ......as was being discussed here Also, note the following: Another case is when a datagram must be fragmented to be forwarded by a gateway yet the Don't Fragment flag is on. In this case the gateway must discard the datagram and may return a destination unreachable message. Getting Path MTU Discovery to work is tough enough without some bozo network engineer assuming that assymetric paths with unroutable endpoint addresses will actually work. Yeah, sure - the destination *MIGHT* have a route back, but if *you* don't have a route back, things will break in subtle ways. ---------------> suggest u read the thread... we were : 1. discussing a ip spoofed attacks 2. the network/ip may exist on a BGP running router as being advertised from antoher AS/ differnet ISP.. its still present on the internet, .... but its a BGP route, not an IGP route...although that network uplinks from ur network...whats the problem? where does all this cause a problem? all ur edges will 0.0.0.0/0 to some bgp running router and the packet will get there.. ......there are enuf asymmetric networks, i can assure of of that... for one, you could simply try running a traceroute to some tracert sites from ur PC and a reverse trace from those servers to you.... ull find lots...