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.
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CCNP Routing Study Guide- P11 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 nego- tiate the highest common version numbers that each supports. If a BGP ver- sion 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 termi- nate 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 typically use the logical IP address assigned to the interface. This number is then used for every BGP session. Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com 264 Chapter 7 BGP’s Basic Components Optional Parameters Length This field is used to indicate the length of the Optional Parameters field in the OPEN message. If there are no optional parameters in the field, then this length is set to zero. Optional Parameters This field contains any optional parameters inserted into the OPEN message. Each optional parameter includes a one-octet parameter type, a one-octet parameter length, and a variable-length parameter value. UPDATE Message This type of message is the actual topology information sent between two BGP speakers. An UPDATE messages can contain a new route, routes to be withdrawn, or both. However, only one new route can be advertised by an UPDATE message. The UPDATE message adds additional fields to the BGP common header, as shown in Figure 7.9. FIGURE 7.9 The additional fields added to the BGP common header when using the UPDATE message type Withdrawn Withdrawn Total Path Total Path Network layer Common Routes Routes Attributes Attributes Reachability Header + Length (length Length (length Information (2 octets) varies) (2 octets) varies) (length varies) Let’s look at the additional fields added to the BGP common header when the UPDATE message type is used. Withdrawn Routes Length This field is used to indicate the length of the Withdrawn Routes field and specifies this information in the number of octets. The BGP specification itself officially calls this field the Unfeasible Routes Length field. Withdrawn Routes The Withdrawn Routes field can contain a list of IP prefixes for which the BGP speaker sending the UPDATE message wants to notify its BGP peer that a route path either no longer exists or cannot be accessed due to the addition of a policy. We’ll discuss this topic in more detail in Chapter 9. Each IP prefix Copyright ©2001 SYBEX , Inc., Alameda, CA www.sybex.com BPG Common Header 265 being withdrawn adds two fields to the Withdrawn Routes field. An IP Prefix Length, which is one octet, identifies the length of the second field, called the IP Prefix field. This field is of variable length and identifies the IP prefix for the route that needs to be withdrawn. If the prefix does not equal at least eight bits, then the rest of the field is padded with additional bits to make each integer a multiple of eight bits. Total Path Attributes Length This field is used to indicate how large the Total Path Attrib ...