EIGRP is characterized by four components:
- Protocol-dependent modules
- Neighbor discovery and recovery
- Reliable Transport Protocol (RTP)
- Diffusing Update Algorithm (DUAL)
You should know the role of the EIGRP components, which are described in the following sections.
Protocol-Dependent Modules
EIGRP uses different modules that independently support IP—and even the older Internetwork Packet Exchange (IPX) and AppleTalk routing protocols. These modules are the logical interface between DUAL and routing protocols such as IPX RIP and AppleTalk Routing Table Maintenance Protocol (RTMP). The EIGRP module sends and receives packets but passes received information to DUAL, which makes routing decisions.
When configured to support IPX, EIGRP communicates with the IPX RIP and forwards the route information to DUAL to select the best paths. AppleTalk EIGRP automatically redistributes routes with AppleTalk RTMP to support AppleTalk networks. IPX and AppleTalk are not covered on the ENSLD 300-420 exam and are therefore not covered in this book.
Neighbor Discovery and Recovery
EIGRP discovers and maintains information about its neighbors. EIGRP neighbors exchange the entire routing table when forming an adjacency. Incremental updates are advertised only when topology changes occur. EIGRP multicasts hello packets (224.0.0.10) every 5 seconds on most interfaces. The router builds a table with EIGRP neighbor information. The holdtime to maintain a neighbor is three times the hello time: 15 seconds. If the router does not receive a hello in 15 seconds, it removes the neighbor from the table. EIGRP multicasts hellos every 60 seconds on multipoint WAN interfaces (that is, X.25, Frame Relay, ATM) with speeds less than a T1 (1.544 Mbps), inclusive. The neighbor holdtime is 180 seconds on these types of interfaces. To summarize, hello/holdtime timers are 5/15 seconds for high-speed links and 60/180 seconds for low-speed links.
If MD5 authentication is configured, then the key ID value and the key string password have to match to establish a neighbor relationship.
Example 3-5 shows an EIGRP neighbor database. The table in this example lists the neighbor’s IP address, the interface to reach it, the neighbor holdtime timer, and the uptime.
Example 3-5 EIGRP Neighbor Database
Router#
show ip eigrp neighbor
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq Type
c (sec) (ms) Cnt Num
1 172.17.1.1 Se0 11 00:11:27 16 200 0 2
0 172.17.2.1 Et0 12 00:16:11 22 200 0 3
RTP
EIGRP uses RTP to manage EIGRP packets. RTP ensures the reliable delivery of route updates and uses sequence numbers to ensure ordered delivery. It sends update packets using multicast address 224.0.0.10. It acknowledges updates using unicast hello packets with no data.