before it stops and waits for an acknowledgment. For example, if 1000-byte packets are being sent, BGP would stop and wait for an acknowledgment only when 65 packets had not been acknowledged, when using the maximum window size. TCP is designed to use a sliding window in which the receiver acknowledges at the halfway point of the sending window. This method allows any TCP application, such as BGP, to continue to stream packets without having to stop and wait as OSPF or EIGRP would require.

Content 6.1 BGP Concepts and Terminology 6.1.10 BGP Databases A router running BGP keeps several tables to store BGP information that it receives from and sends to other routers. These tables include a neighbor table, a BGP table (also called a forwarding database or topology database), and an IP routing table. For BGP to establish an adjacency, you must configure it explicitly for each neighbor. BGP uses TCP as its transport protocol (port 179). It forms a TCP connection with each of the configured neighbors and keeps track of the state of these relationships by periodically sending a BGP TCP keepalive message. Note
BGP sends TCP keepalives every 60 seconds by default. Routers that run a BGP routing process are often referred to as BGP speakers. Two BGP speakers that form a TCP connection between one another for the purpose of exchanging routing information are referred to as neighbors or peers. After establishing an adjacency, the neighbors exchange the BGP routes that are in their IP routing table. Each router collects these routes from each neighbor that successfully establishes an adjacency and then places them in its BGP forwarding database. All routes that have been learned from each neighbor are placed into the BGP forwarding database. The best routes for each network are selected from the BGP forwarding database using the BGP route selection process and then offered to the IP routing table. Each router compares the offered BGP routes to any other possible paths to those networks, and the best route, based on administrative distance, is installed in the IP routing table. EBGP routes (BGP routes learned from an external autonomous system) have an administrative distance of 20. IBGP routes (BGP routes learned from within the autonomous system) have an administrative distance of 200.

Content 6.1 BGP Concepts and Terminology 6.1.11 BGP Message Types The four BGP message types are open, keepalive, update, and notification. After a TCP connection is established, the first message sent by each side is an open message. If the open message is acceptable, the side that receives the message sends a keepalive message confirming the open message. After the receiving side confirms the open message and establishes the BGP connection, the BGP peers can exchange any update, keepalive, and notification messages. BGP peers initially exchange their full BGP routing tables. Incremental updates are sent only after topology changes in the network. BGP peers send keepalive messages to ensure that the connection between the BGP peers still exists. They send notification packets in response to errors or special conditions. Here are more details about the different types of BGP messages:

Open message: An open message includes the following information:
- Version number: The highest common version that both routers support is used. All BGP implementations today use BGP4.
- AS number: The AS number of the local router. The peer router verifies this information. If it is not the AS number that is expected, the BGP session is torn down.
- Hold time: Maximum number of seconds that can elapse between the successive keepalive and update messages from the sender. On receipt of an open message, the router calculates the value of the hold timer by using whichever is smaller: its configured hold time or the hold time that was received in the open message.
- BGP router ID: 32-bit field indicating the BGP ID of the sender. The BGP ID is an IP address that is assigned to that router, and it is determined at startup. The BGP router ID is chosen in the same way that the OSPF router ID is chosen: it is the highest active IP address on the router, unless a loopback interface with an IP address exists. In this case, the router ID is the highest loopback IP address. The router ID can also be statically configured.
- Optional parameters: These parameters are Type, Length, and Value (TLV)-encoded. An example of an optional parameter is session authentication.
Keepalive message: BGP keepalive messages are exchanged between BGP peers often enough to keep the hold timer from expiring. If the negotiated hold-time interval is 0, periodic keepalive messages are not sent. A keepalive message consists of only a message header.
Update message: A BGP update message has information on one path only; multiple paths require multiple update messages. All the attributes in the update message refer to that path, and the networks are those that can be reached through it. An update message can include the following fields:
- Withdrawn routes: This list displays IP address prefixes for routes that are withdrawn from service, if any.
- Path attributes: These attributes include the AS path, origin, local preference, and so on (as described later in this module). Each path attribute includes the attribute TLV. The attribute type consists of the attribute flags, followed by the attribute type code.
- Network-layer reachability information: This field contains a list of IP address prefixes that are reachable by this path.
Notification message: A BGP notification message is sent when an error condition is detected. The BGP connection is closed immediately after this is sent. Notification messages include an error code, an error subcode, and data related to the error. Figure displays the field for error codes that can be used to troubleshoot BGP connections.

Interactive Media Activity Drag and Drop: BGP Message Types Upon completion of this activity, the student will be able to identify the properties of BGP messages types.

Content 6.2 EBGP and IBGP 6.2.1 BGP Neighbor Relationships No one router can handle every connection with all the routers that run BGP. Tens of thousands of routers run BGP and are connected to the Internet, representing more than 21,000 autonomous systems. A BGP router forms a direct neighbor relationship with a limited number of other BGP routers. Through these BGP neighbors, a BGP router learns of the paths through the Internet to reach any advertised network. Any router that runs BGP is known as a BGP speaker. The term “BGP peer” has a specific meaning: a BGP speaker that is configured to form a neighbor relationship with another BGP speaker for the purpose of directly exchanging BGP routing information with each other. A BGP speaker has a limited number of BGP neighbors with which it peers and forms a TCP-based relationship. BGP peers are also known as BGP neighbors and can be either internal or external to the autonomous system. A BGP peer must be configured with the BGP neighbor command. The administrator instructs the BGP speaker to establish a relationship with the address listed in the neighbor command and to exchange the BGP routing updates with that neighbor.

Content 6.2 EBGP and IBGP 6.2.2 Establishing a Connection Between External BGP Neighbors Recall that when BGP is running between routers in different autonomous systems, it is called EBGP. Generally, routers running EBGP are directly connected to each other. For two routers to exchange BGP routing updates, the TCP-reliable transport layer on each side must successfully pass the TCP three-way handshake before the BGP session can be established. Therefore, the IP address used in the BGP