Information Technology Reference
In-Depth Information
Protocol-Dependent Modules
EIGRP uses different modules that independently support IP, Internetwork Packet Ex-
change (IPX), and AppleTalk routed protocols. These modules are the logical interface be-
tween 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 Ap-
pleTalk area not CCDA objectives and are not covered in this topic.
Neighbor Discovery and Recovery
EIGRP discovers and maintains information about its neighbors. It 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 3 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 (X.25,
Frame Relay, ATM) with speeds less than a T-1 (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.
Example 10-5 shows an EIGRP neighbor database. The table lists the neighbor's IP ad-
dress, the interface to reach it, the neighbor holdtime timer, and the uptime.
Example 10-5
EIGRP Neighbor Database
Router#
show ip eigrp neighbor
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(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 up-
dates and uses sequence numbers to ensure ordered delivery. It sends update packets us-
ing multicast address 224.0.0.10. It acknowledges updates using unicast hello packets with
no data.
DUAL
EIGRP implements DUAL to select paths and guarantee freedom from routing loops. J. J.
Garcia Luna-Aceves developed DUAL. It is mathematically proven to result in a loop-free
make convergence slower.