Objectives
The simulator ticks each one as soon as the network meets it.
PE: start BGP AS 65000 and peer with the CE at 10.0.0.2 (AS 65100).
CE: start BGP AS 65100, peer with 10.0.0.1, and advertise 192.168.10.0/24 and 10.99.0.0/16.
PE: advertise 203.0.113.0/24 to the customer.
PE: accept only 192.168.10.0/24 from the customer, using prefix-list CUST-IN and route-map CUST-IN applied inbound.
Run show ip bgp on PE: 192.168.10.0/24 is there and 10.99.0.0/16 is not.
PC1 pings 203.0.113.1.
Before you start
eBGP peers are normally directly connected. The network command advertises a prefix only if the exact prefix is already in the routing table. A route-map ends with an implicit deny, so one permit entry that matches the prefix list is enough to filter everything else.
Concept
eBGP: exchanging routes between autonomous systems
BGP connects networks run by different organisations (autonomous systems). Two routers in different ASes form an eBGP session over TCP port 179 and advertise only the prefixes they choose, each tagged with the AS path it has crossed.
Concept
Filtering what a customer can advertise
A provider must accept only the prefixes a customer is allowed to announce. A prefix list matched by an inbound route map drops anything else — a mistake on the customer side then cannot leak into the provider network.
