Where is the edge of the Internet? Re: no ip forged-source-address

alok alok.dube at apara.com
Thu Nov 7 19:59:50 UTC 2002

On Fri, Nov 08, 2002 at 01:01:33AM +0530, alok wrote:
> there was a comment from chris saying..."never possible to knw what
> an bgp customer uplinks via you" which is very true.. ..so i assume u mean
> non-bgp customers? loose or strict, rpf will not work for aasymterically
> connected bgp neighbouring AS....

How does loose not work in this scenario?

If it's not in the global tables -at all-, it's not reachable, and
might as well be discarded.

------> the scenario is this... a BGP customer uplinks network a.b.c.d via
me, but advertises it via some place else (some other network he peers with)
and some other bgp peer/router to bring that traffic back into his AS...

this can also happen mainly due to BGP metrics blah blah....

now, essentially a.b.c.d can be anything...and he need not tell me what he
uplinks from me, all he tells me are the networks he downlinks via me so as
to tell me what routemaps to put with acls for bgp advertisements from

infact people tend to use this very often (also a way of providing link
failure etc by multihoming) ..and they have the choice to uplink anything
from anywhere and downlink it from another location...they certainly dont
need to tell you what they uplink..as far as i know...

now the point is that if you use loose rfp here.... what are u filtering on?
you dont even know what he is uplinking to you...

i assume the subject is still DDoS attacks...using spoofed ips...

now when u dont know what he is uplinking from ur networks, how do u even
know what to block?

if u say "loose" simply means check if the entry for the network is there in
the routing table..then the entire internet is there in the routing
table...(thanks to bgp)....so it certainly work on bgp based "edges"

the other point u made about not reachable...well not reachaable from where?
from a ospf running node which uses ? a lot of ones own networks etc
may not be reachable from there i guess...as they are covered in default

for a bgp running router...all valid internet addresses are "reachable" ,
for an ospf router....all is reachable either via, and if u remove
default any, it doesnt even know what the customer networks are.....so a lot
"isnt" reachable....

i think as was rightly defined...the edge is the place where the end
user/host gets onto the net...

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