Bugs can be present at other places than an ASBR. Imagine the following very
ordinary network:
AS100 --- [ ASBR2 --- AR3 --- ASBR4 --- ] AS500
ASBR2 will have iBGP routing exchange with ASBR4 by some means. This may be an
ordinary iBGP session, via a route-reflector, or whatever.
If AR3 is buggy but it is in the forwarding path between ASBR2 and ASBR4, then
ASBR4 potentially will send packets to AR3, yet AR3 has treat-as-withdraw a
route. ASBR4 is not subject to the bug, receives and installs the iBGP-learnt
route from ASBR2, and happily announces it to AS500.
[SR] Ok. This however does not happen in bgp-free core (ASBR2 will tunnel
the packet to ASBR4 even if AR3 is in the forwarding path).
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