The Illustrated Network- P44 pptx

is the 2-byte Attribute Type. There is a structure to the Attribute Type fi eld, as shown in Figure 15.7. There are four fl ag bits, four unused bits, and then an 8-bit Attribute Type code. There are other attribute codes in use with BGP, but these are not discussed in this chapter. One of the most important of these other attributes is the Extended Commu- nity attribute used in VPNs. The Update message ends with a variable-length NLRI fi eld. Each NLRI (route) is a Length/Prefi x pair. The length indicates the number of bits that is signifi cant in the following prefi x. There is no length fi eld for this list that ends the Update message. The number of NLRIs present is derived from the known length of all of the other fi elds. So, instead of saying “here’s a route and these are its attributes…” for every NLRI advertised the Update message basically says “here’s a group of path attributes and here are the routes that these apply to…” This cuts down on the number of messages that needs to be sent across the network. In this way, each Update message forms a unit of its own and has no further fragmentation concerns. The Notifi cation Message Error messages in BGP have an 8-bit Error Code, an 8-bit Subcode, and a variable-length Data fi eld determined by the Error Code and Subcode. The format of the BGP Notifi ca- tion message is shown in Figure 15.8. 8 bits 8 bits OTPEUUUU Attribute Type Code Flag bits: O: Optional bit 0 5 Optional 1 5 Well known T: Transitive bit 0 5 Transitive 1 5 Nontransitive P: Partial bit 0 5 Optional transitive attribute is partial 1 5 Optional transitive attribute is complete E: Extended length bit 0 5 Attribute length is 1 byte 1 5 Attribute length is 2 bytes U: Unused FIGURE 15.7 The BGP Attribute Type format. This is how NRLIs are grouped. CHAPTER 15 Border Gateway Protocol 399 A full discussion of BGP Notifi cation codes and subcodes is beyond the scope of this chapter. The major Error Codes are Message Header Error (1), Open Message Error (2), Update Message Error (3), Hold Timer Expired (4), Finite State Machine Error (5), used when the BGP implementation gets hopelessly confused about what it should be doing next, and Cease (6), used to end the session. 32 bits Data Error SubcodeError Code 1 byte 1 byte 1 byte 1 byte Error codes: 1: Message header error 2: Open message error 3: Update message error 4: Hold timer expired 5: Finite State Machine error 6: Cease FIGURE 15.8 The BGP Notifi cation message format. BGP benefi ts from using TCP as a transport protocol. 400 PART III Routing and Routing Protocols QUESTIONS FOR READERS Figure 15.9 shows some of the concepts discussed in this chapter and can be used to help you answer the following questions. “I don’t know 10.0.75.1! It’s not in this AS!” Router 192.168.14.1 “Oh! I know how to reach 192.168.14.1” IBGP without NHS IBGP with NHS EBGP (No IGP) Router in AS 65127 “I can reach 10.10.12/24. Use 10.0.75.1 as a next hop.” 10.0.75.2 10.0.75.1 FIGURE 15.9 How Next Hop Self allows internal routers to forward packets for BGP routes. Border router 192.168.14.1 substitutes its own address for the “real” next hop. 1. BGP distributes “reachability” information and not routes. Why doesn’t BGP distribute route information? 2. What does it mean to say that the BGP is a “path-vector” protocol? 3. What is “next hop self” and why is it important in BGP? 4. Which two major BGP router confi gurations are employed to deal with BGP scaling? 5. What are the ten major BGP attributes? 401 . portion of the Ethereal capture of the packet stream, detailing the UDP con- tent inside the IP packet. FIGURE 16.2 Multicast packet capture, showing the MAC address format used and the port in the. other attribute codes in use with BGP, but these are not discussed in this chapter. One of the most important of these other attributes is the Extended Commu- nity attribute used in VPNs. The. Length/Prefi x pair. The length indicates the number of bits that is signifi cant in the following prefi x. There is no length fi eld for this list that ends the Update message. The number of NLRIs