By default, IOS sends BGP messages to EBGP neighbors with an IP time-to-live (TTL) of 1. (This can be adjusted with
ebgp-multihop attached to the desired neighbor or peer group under BGP configuration.) Sending BGP messages with a TTL of one requires that the peer be directly connected, or the packets will expire in transit. Likewise, a BGP router will only accept incoming BGP messages with a TTL of 1 (or whatever value is specified by
ebgp-multihop), which can help mitigate spoofing attacks.
However, there is an inherent vulnerability to this approach: it is trivial for a remote attacker to adjust the TTL of sent packets so that they appear to originating from a directly-connected peer.
By spoofing legitimate-looking packets toward a BGP router at high volume, a denial of service (DoS) attack may be accomplished.
A very simple solution to this, as discussed in RFC 5082, is to invert the direction in which the TTL is counted. The maximum value of the 8-bit TTL field in an IP packet is 255; instead of accepting only packets with a TTL set to 1, we can accept only packets with a TTL of 255 to ensure the originator really is exactly one hop away. This is accomplished on IOS with the TTL security feature, by appending
ttl-security hops <count> to the BGP peer statement.
Only BGP messages with an IP TTL greater than or equal to 255 minus the specified hop count will be accepted. TTL security and EBGP multihop are mutually exclusive;
ebgp-multihop is no longer needed when TTL security is in use.