| Figure 6-4 shows the EIGRP packet header. Notice that following the autonomous systems number are the Type/Length/Value (TLV) triplets. The TLV triplets carry route entries, as well as provide the fields for DUAL process management. Some common TLVs are the EIGRP parameter TLV, the IP internal route TLV, and the IP external route TLV. Figure 6-4. EIGRP Packet Header  The EIGRP packet parameters are described as follows : -
Version ‚ Specifies different versions of EIGRP. Version 2 of EIGRP was imple-mented beginning with Cisco IOS Software Releases 10.3(11), 11.0(8), and 11.1(3). EIGRP Version 2 is the most recent version that contains many enhancements to improve the stability and scalability of EIGRP. -
Opcode ‚ Specifies the types of EIGRP packet contained. Opcode 1 is the update packet, opcode 3 is the Query, opcode 4 is the reply, and opcode 5 is the EIGRP hello packet. -
Checksum ‚ Used as the regular IP checksum, calculated based on the entire EIGRP packet, excluding the IP header. -
Flags ‚ Involves only two flags now. The flag indicates either an init for new neighbor relationship or the conditional receive for EIGRP RTP. -
Sequence ‚ Specifies the sequence number used by the EIGRP RTP. -
Acknowledgment ‚ Used to acknowledge the receipt of an EIGRP reliable packet. -
Autonomous System Number ‚ Specifies the number for the identification of EIGRP network range. One of the most common EIGRP TLVs is the EIGRP parameter TLV, as shown in Figure 6-5, which contains the parameter needed to establish a neighbor relationship. The constant K values are included in this TLV, as well as the hold time. The K values between two routers must agree before they can establish a neighbor relationship. Figure 6-5. EIGRP Parameters TLV  Figure 6-6 and Figure 6-7 show two other common EIGRP TLVs ‚ the IP internal route TLV and the IP external route TLV, respectively. The EIGRP internal routes are routes originated from the same EIGRP autonomous system number as the receiving router. The EIGRP external routes are routes that are being redistributed into the EIGRP autonomous systems. Figure 6-6. EIGRP IP Internal Route TLV  Figure 6-7. EIGRP IP External Route TLV  The EIGRP IP internal route TLV contains this information: -
Next hop ‚ IP address of the next hop to which packets should be forwarded. -
Delay ‚ Delay parameter of the route metric. The delay value is the sum of all the delay parameters on the interface across the path to the destination network. -
Bandwidth ‚ Bandwidth parameter of the route metric. The bandwidth is obtained from the interface, and it is the lowest bandwidth on the interface across the path to the destination network. -
MTU ‚ The interface MTU parameter of the route metric. -
Hop count ‚ Number of hops to the destination network. -
Reliability ‚ The reliability of the interface, out of a possible range of 1 to 255. A reliability of 1 indicates that the reliability is 1/255, whereas a reliability of 255 indicates that the interface is 100 percent reliable. -
Load ‚ The load of the interface, out of a possible range of 1 to 255. A load value of 1 indicates that the interface has a very light load, while a load value of 255 indicates that the interface is highly saturated . -
Prefix length ‚ The subnet mask of the destination network. In EIGRP IP external route TLV, more information of the route is included: -
Originating router ‚ The router ID of the router that originates the external EIGRP routes. -
Originating autonomous system number ‚ The EIGRP autonomous system number of the routes before getting redistributed into this EIGRP autonomous number. -
External protocol metric ‚ The metric of the routes before getting redistributed into EIGRP. -
External protocol ID ‚ The type of routing protocol that originates the routes that were redistributed into EIGRP. The routing protocol type can be BGP, OSPF, RIP, IGRP, and so forth. |
