tailieunhanh - CCNP Routing Study Guide- P11

CCNP Routing Study Guide- P11:T his book is intended to help you continue on your exciting new path toward obtaining your CCNP and CCIE certification. Before reading this book, it is important to have at least read the Sybex CCNA: Cisco Certified Network Associate Study Guide, Second Edition. You can take the CCNP tests in any order, but you should have passed the CCNA exam before pursuing your CCNP. | BPG Common Header 263 Version The Version field indicates the BGP version being used by the router sending the OPEN message. This field allows the BGP speakers to informally negotiate the highest common version numbers that each supports. If a BGP version speaker receives a packet indicating a version number of 4 and the BGP speaker receiving the packet is running a lower version of BGP then it will send an error message stating that it does not understand 4 and will terminate the TCP session. The BGPv4 speaker must then reopen the TCP session using the parameters used in the lower version of BGP. My Autonomous System This field indicates the autonomous system number ASN membership of the BGP router sending this OPEN message. Every AS must be identified by its own unique ASN which we discussed earlier in the chapter. Hold Time This field indicates the amount of time that the sender of the OPEN message wants to use for its hold-down timer. This hold time indicates the maximum amount of time that each endpoint will wait for another to send a message before considering the connection terminated. This means that if an UPDATE or a KEEPALIVE message is not sent in the indicated amount of time the session is considered closed. A value of zero indicates that the sender does not want to exchange KEEPALIVE messages. This mode is not recommended since one side will not know if the other has lost communication. The hold time value is the minimum value set locally on the router or the advertised hold time value. If the hold time value is not zero then the hold time must be at least three seconds. A neighboring endpoint can reject an OPEN message if the hold time value is unacceptable. BGP Identifier This field contains a value that identifies the BGP speaker. This is a random value chosen by the BGP router when sending an OPEN message. The value must be unique to all the other BGP speakers communicating with one another. Although the number can be random BGP speakers will .