Four different message types are used in BGP-4:
• Open messages—Used to establish connections with peers.
• Keepalives—Sent periodically between peers to maintain connections and verify paths held by the router sending the keepalive. These packets are sent unreliably. If the periodic timer is set to a value of 0, this equates to infinity, and no keepalives are sent.
• Update messages—Contain paths to destination networks and the path attributes. Routes that are no longer available or withdrawn routes are included in updates. There is one path per update, requiring many updates for many paths. The information contained in the update includes the path attributes such as origin, Autonomous System path, neighbor, and inter-Autonomous System metric.
• Notification—Used to inform the receiving router of errors.
BGP-4 comes in two flavors: internal and external BGP-4. The difference depends on the function of the routing protocol. The router will determine if the peer BGP-4 router is going to be an external BGP-4 peer or an internal BGP-4 peer by checking the autonomous system number in the open message that was sent.
Internal BGP-4 is used within an autonomous system. It conveys information to all BGP-4 routers within the domain and ensures that they have a consistent understanding of the networks available. Internal BGP-4 is used within an ISP or a large organization to coordinate the knowledge of that autonomous system. The routers are not required to be physical neighbors on the same medium, and they often sit on the edges of the network. Internal BGP-4 is used to convey BGP-4 information about other autonomous systems across a transit autonomous system. Another routing protocol, an interior routing protocol such as OSPF, is used to route the BGP-4 packets to their remote locations. To achieve this, internal BGP requires the destination BGP neighbor's IP address to be contained within the normal routing table kept by another routing protocol (static routing, OSPF, EIGRP, and so on). The integration of these different routing protocols can be challenging.
External BGP-4 complies with the common perception of an external routing protocol; it sends routing information between differing autonomous systems. Therefore, the border router between different autonomous systems is the external BGP router. Figure 8-4 shows the application of internal and external BGP-4.
Figure 8-3 Establishing a BGP-4 Peer State
Router # reset BGP
1. Reset -connect/retry counter
2. Initiate TCP connection ->
3. Listens & waits
Open sent (awaiting reply)
Connect (top connection awaiting open sent)
(Is the router in same autonomous system internal or external BGP?)
Errors Active (had a connection, trying to reestablish it)
Heard you but had a problem
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