There are two types of BGP Sessions, the configuration is similar expect eBGPs a unique AS in the
Internal BGP (iBGP)
Sessions that established within the same AS are classified as internal BGP sessions. The administrative distance assigned upon placement in the routers table is 200. The TTL on packets from iBGP routers are set to 255.
iBGP can be implemented where transit connectivity is required through between two AS networks. By implementing a full mesh iBGP network, problems such as scalability can be prevented. If an internal routing protocol is used such as EIGRP or OSPF, problems may occur due to high amount of routes required to be injected into the routing protocols databases.
iBGP will allow preservation of path attributes too, and custom routing for values other than metric can be used to determine where traffic should go.
External BGP (eBGP)
Sessions that established outside of the AS of the originating router are classified as External BGP (eBGP) sessions. When assigned into the routing table, prefixes are marked with an administrative distance of 20.
The time to live on eBGP packets are set to one by default. This means that packets drop in transit if they need to reach another router via a hop.
Each advertising hop updates the next-hop address to the IP address sourcing the eBGP connection, and the advertisers ASN is added to the AS_Path attribute. This helps prevent any network loops in BGP, as if the AS_Patch is detected, it will discard the packet.