BGP2. Black Hole
Take above as an example , There are three AS running BGP protocol .
In the AS 65102 . Router B has neighbor relationship with Router E , But they doesn’t connect with physical cable . and Router B has no any neighbor relationship with Router D,C
Now : while AS 65101 ,router A advertise its own routes and transit to Router B ,then Router B transits to E, then to F
But while a packet with the destination IP address of Router A goes back from Router F to Router A , Router F hand it to Router E ,then Router E checks its route table and hand to Router B .
Since Router E has no physical cable connect to Router B , So he deliver this packet to Router C or D .
But Router C or D has not any routes about this destination IP in routing table . So , he will just drop it .
This is an interesting conclusion that Routes might could pass through between different AS ,But it doesn’t mean to Data packet .
But why routes updates could pass through between different AS, but Data packet couldn’t ?
We still use above topology and give them some IP address :
1.Routes updates packet
Router A could hand its routes to B without any doubts ,because they have physical connections .
When Router B hand to its neighbor Router E . here is the form of the routes updates packet :
source IP address :2.2.2.2 source port: random routes updates
Destination IP : 3.3.3.3 Destination Port:179
As we know , BGP routes updates transits through TCP protocol which a point-to-point connection . Since there is OSPF route protocol running internal AS 65102 .Router b
and Router E is reachable . So the route updates packet could be passed successfully
2.Data packet
Router F could hand its routes to E without any doubts ,because they have physical connections .
when Router E hand to its neighbor Router B, E will hand it to IGP neighbor first ,as B and E has no physical connection .
When the Data packet comes to Router C or D ,it should be this :
source ip address: origin source Port :random
Destination IP : 1.1.1.1 destination port:unknow
it is obvious that the Source ip address and destination ip address never changes while the packet passes through the whole network . On router C or D , there is no any routes about
1.1.1.1 network ,as they doesn’t run BGP protocol . So the packet will be dropped .
Solution :
- Full mesh connection ——- link a physical cable between Router B and E
- all routers in as 65102 run BGP protocol
- MPLS/VPN
