EIGRP uses five packet types:
- Hello: EIGRP uses hello packets in the discovery of neighbors. They are multicast to 224.0.0.10. By default, EIGRP sends hello packets every 5 seconds (60 seconds on WAN links with 1.544 Mbps speeds or less).
- Acknowledgment: An acknowledgment packet acknowledges the receipt of an update packet. It is a hello packet with no data. EIGRP sends acknowledgment packets to the unicast address of the sender of the update packet.
- Update: Update packets contain routing information for destinations. EIGRP unicasts update packets to newly discovered neighbors; otherwise, it multicasts update packets to 224.0.0.10 when a link or metric changes. Update packets are acknowledged to ensure reliable transmission.
- Query: EIGRP sends query packets to find feasible successors to a destination. Query packets are always multicast unless they are sent as a response; then they are unicast back to the originator.
- Reply: EIGRP sends reply packets to respond to query packets. Reply packets provide a feasible successor to the sender of the query. Reply packets are unicast to the sender of the query packet.
EIGRP Design
When designing a network with EIGRP, remember that it supports VLSM and network summarization. EIGRP allows for the summarization of routes in a hierarchical network. EIGRP is not limited to 16 hops as RIP is; therefore, the network diameter can exceed this limit. In fact, the EIGRP diameter can be 225 hops. The default diameter is 100. EIGRP can be used in site-to-site WANs and IPsec virtual private networks (VPNs). In the enterprise campus, EIGRP can be used in data centers, server distribution, building distribution, and the network core.
EIGRP does not broadcast its routing table periodically, so there is no large network overhead. You can use EIGRP for large networks; it is a potential routing protocol for the core of a large network. EIGRP further provides for route authentication.
As shown in Figure 3-8, when you use EIGRP, all segments can have different subnet masks.
Figure 3-8 EIGRP Design
EIGRP is suited for almost all enterprise environments, including LANs and WANs, and is simple to design. The only caveat is that it is a Cisco-proprietary routing protocol that cannot be used with routers from other vendors. The use of EIGRP is preferred over RIP in all environments.