What attack vectors does the BGP TTL security knob protect against?
BGP TTL security makes your router only accept packets that have a very high TTL number, typically 254. Since TTL is decremented every time a packet is routed, configuring this feature on both neighbors ensures that only a directly connected neighbor router could have sent you this packet.
When you configure for example a value of 5 in the tt-security feature, the router deduct 5 from 255, so it accepts only an IP TTL greater than or equal to 255 minus the value configured.
So, it doesn't really means it only accept a router directly connected. It avoids attacks from host manipulating the TTL in the TCP session.