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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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► Full mesh of BGP sessions within an AS: IBGP speakers assume a full mesh<br />

of BGP sessions have been established between peers in the same AS. In<br />

Figure 5-22 on page 219, all three BGP peers in AS 1 are interconnected with<br />

BGP sessions.<br />

When a BGP speaker receives a route update from an IBGP peer, the receiving<br />

speaker uses EBGP to propagate the update to external peers. Because the<br />

receiving speaker assumes a full mesh of IBGP sessions have been established,<br />

it does not propagate the update to other IBGP peers.<br />

For example, assume that there was no IBGP session between R1 <strong>and</strong> R3 in<br />

Figure_82. R1 receives the update about 10.0.0.0/8 from AS 3. R1 forwards the<br />

update to its BGP peers, namely R2. R2 receives the IBGP update <strong>and</strong> forwards<br />

it to its EBGP peers, namely R6. No update is sent to R3. If R3 needs to receive<br />

this information, R1 <strong>and</strong> R3 must be configured to be BGP peers.<br />

5.9.3 Protocol description<br />

BGP establishes a reliable <strong>TCP</strong> connection between peers. Sessions are<br />

established using <strong>TCP</strong> port 179. BGP assumes the transport connection will<br />

manage fragmentation, retransmission, acknowledgement, <strong>and</strong> sequencing.<br />

When two speakers initially form a BGP session, they exchange their entire<br />

routing table. This routing information contains the complete AS path used to<br />

reach each destination. The information avoids the routing loops <strong>and</strong><br />

counting-to-infinity behavior observed in R<strong>IP</strong> networks. After the entire table has<br />

been exchanged, changes to the table are communicated as incremental<br />

updates.<br />

BGP packet types<br />

All BGP packets contain a st<strong>and</strong>ard header. The header specifies the BGP<br />

packet type. The valid BGP packet types include:<br />

► OPEN2 : This message type establishes a BGP session between two peer<br />

nodes.<br />

► UPDATE: This message type transfers routing information between GP<br />

peers.<br />

► NOTIFICATION: This message is sent when an error condition is detected.<br />

► KEEPALIVE: This message determines if peers are reachable.<br />

2 RFC 1771 uses uppercase to name BGP messages. The same convention is used in this section.<br />

220 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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