| This chapter covers the following key topics about Enhanced IGRP (EIGRP): -
Metrics -
EIGRP neighbor relationships -
The Diffusing Update Algorithm (DUAL) -
DUAL finite-state machine -
EIGRP reliable transport protocol -
EIGRP packet format -
EIGRP behavior -
EIGRP summarization -
EIGRP query process -
Default routes and EIGRP -
Unequal-cost load balancing in EIGRP As the size of network grows larger, you can see that the classical distance vector routing protocols such as IGRP and RIP won't scale to the needs of the network. Some of the biggest scalability problems of IGRP and RIP are as follows : -
Full periodic routing updates that consume bandwidth ‚ RIP sends out its entire routing table every 30 seconds; IGRP sends out its entire routing table every 90 seconds. This consumes significant bandwidth. -
RIP hop-count limitation of 15 hops ‚ This limitation makes RIP protocol a non-scalable routing protocol in today's networks because most medium- sized networks have more than 15 routers. -
No support of VLSM and discontiguous networks ‚ This also hinders the capability to scale large networks for RIP and IGRP. Because of this factor, router summarization is not supported. -
Slow convergence time ‚ Because RIP and IGRP send periodic routing updates, a network that is not available in one part of the network could take minutes for the other part of the network to discover that it's no longer available. -
Not 100 percent loop-free ‚ RIP and IGRP do not keep topology tables, so there is no mechanism for them to ensure a 100 percent loop-free routing table. Because of these shortcomings of IGRP and RIP, Cisco developed an enhanced version of IGRP that not only fixed all the problems of IGRP and RIP but also developed a routing protocol robust enough to scale to today's network growth. This enhanced version is called Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP is neither a classic distance vector routing protocol nor a link-state protocol ‚ it is a hybrid of these two classes of routing protocol. Like a distance vector protocol, EIGRP gets its update from its neighbors. Like a link-state protocol, it keeps a topology table of the advertised routes and uses the Diffusing Update Algorithm (DUAL) to select a loop-free path . The convergence time in a network is the time that it takes for all the routers in the network to agree on a network change. The shorter the convergence time is, the quicker a router can adapt to a network topology change. Unlike a traditional distance vector protocol, EIGRP has fast convergence time and does not send full periodic routing updates. Unlike a link-state protocol, EIGRP does not know what the entire network looks like; it depends only on its neighbor's advertisement. Because EIGRP has characteristics of both distance vector and link-state protocols, Cisco has classified EIGRP as an advanced distance vector routing protocol. Advantages of EIGRP include the following: -
100% loop-free ‚ EIGRP is guaranteed to have a 100 percent loop-free forwarding table if all the networks are contained within one autonomous system. -
Easy configuration ‚ Configuration of EIGRP is extremely easy and is the same as IGRP and RIP at the basic level. -
Fast convergence ‚ Convergence time for EIGRP is much faster than that for RIP and IGRP. -
Incremental update ‚ In an EIGRP network, no routing update is exchanged except for a network change. Also, only the change is updated, not the entire routing table. This saves CPU power and is more efficient. -
Use of multicast address ‚ IGRP and RIP use the broadcast address of 255.255.255.255 to send their packets. This means that every device on the same network segment receives the updates. EIGRP sends its packet over the multicast address of 224.0.0.10, which ensures that only the EIGRP-enabled devices receive the EIGRP packets. -
Better utilization of bandwidth ‚ EIGRP obtains the bandwidth parameter from the interface in which EIGRP packets will be sent out. It is a parameter in which its values are assigned to a particular interface. For example, by default, all serial interfaces have a bandwidth of 1544 kbps; however, this bandwidth parameter is configurable. EIGRP can use up to 50 percent of the interface bandwidth to carry EIGRP packets. This ensures that EIGRP packets will not starve the routed data packet during a major network convergence event. RIP and IGRP do not have this feature, so potentially large amounts of RIP or IGRP updates would prevent regular data packets from going through. -
Support for VLSM and discontiguous networks ‚ Unlike RIP and IGRP, EIGRP supports VLSM and discontiguous networks. This enables EIGRP to be implemented in the modern network and lends itself to better network scalability. |
