Information Technology Reference
In-Depth Information
EIGRP internal routes have an administrative distance of 90. EIGRP summary routes have an
administrative distance of 5, and EIGRP external routes (from redistribution) have an adminis-
trative distance of 170. Administrative distance is covered in Chapter 10.
EIGRP Components
The characteristics of EIGRP are as follows:
•
Protocol-dependent modules
•
Neighbor discovery and recovery
•
Reliable Transport Protocol (RTP)
•
DUAL
Protocol-Dependent Modules
EIGRP uses different modules that independently support IP, Internetwork Packet Exchange
(IPX), and AppleTalk routable protocols. These modules are the logical interface between
DUAL and routing protocols, such as IPX RIP, AppleTalk Routing Table Maintenance Protocol
(RTMP), and IGRP. The EIGRP module sends and receives packets but passes received
information to DUAL, which makes routing decisions. EIGRP automatically redistributes with
IGRP if both protocols are configured with the same autonomous system number. When
configured to support IPX, EIGRP communicates with the IPX RIP protocol and forwards the
route information to DUAL to select best paths. AppleTalk EIGRP automatically redistributes
routes with AppleTalk RTMP to support AppleTalk networks. IPX EIGRP is covered in Chapter
12, “Multiservice Networking, IPX Networking, and Security.” AppleTalk is no longer an
objective for the Cisco CCIE written test and is not covered in this topic.
Neighbor Discovery and Recovery
EIGRP discovers and maintains information about its neighbors. Hello packets are multicast
(224.0.0.10) every 5 seconds for most networks. The router builds a table with EIGRP neighbor
information. The holdtime to maintain a neighbor is 3 times the hello time: 15 seconds. If a
hello is not received in 15 seconds, the neighbor is removed from the table. Hellos are multicast
every 60 seconds on multipoint WAN interfaces (X.25, Frame Relay, ATM) with speeds less
than 1544 Mbps, inclusive. The neighbor holdtime is 180 second on these types of interfaces.
Example 7-24 shows an EIGRP neighbor database. The table lists the neighbor's IP address, the
interface to reach it, the neighbor's holdtime timer, and the uptime.
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