Does BGP use TCP or UDP?
TCP.
If TCP then we are in trouble.
I don't think so.
Almost everyone
has access to the Internet via BGP. The line IP address is usually made up
of a pair of addresses in the same subnet. You can IP spoof block all your
internal IP addresses but if you block the IP address of your BGP connection
to your BGP peer and BGP uses TCP, then the examples jbash gave out will
stop BGP updates as well.
This was my example:
interface ethernet 0
ip address 1.2.3.4 255.255.255.0
ip access-group 101 in
!
interface ethernet 1
ip address 5.6.7.8
ip access-group 101 in
!
access-list 101 deny tcp 1.2.3.4 0.0.0.0 1.2.3.4 0.0.0.0
access-list 101 deny tcp 5.6.7.8 0.0.0.0 5.6.7.8 0.0.0.0
access-list 101 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
That only blocks the router talking to itself, not talking to any other
host, whether on the same subnet or not. As far as I know, you don't have
to have a TCP connection to yourself to run BGP, just to your neighbors.
-- John B.