The reported distance value is the feasible distance for the advertising router. R3 advertises the R4 advertises the For a route to be considered a backup route, the RD received for that route must be less than the FD calculated locally. This logic guarantees a loop-free path. A route with that satisfies the feasibility condition is maintained as a backup route.
The feasibility condition ensures that the backup route is loop free. EIGRP contains a topology table, which makes it different from a true distance vector routing protocol. Each entry in the table contains the following:. The command show ip eigrp topology [ all-links ] provides the topology table. By default, only the successor and feasible successor routes are displayed, but the optional all-links keyword shows the paths that did not pass the feasibility condition.
Figure shows the topology table for R1 from Figure This section focuses on the Examine the network The successor upstream router advertises the successor route with an RD of The second path entry has a metric of and has an RD of Because is less than , the second entry passes the feasibility condition and classifies the second entry as the feasible successor for the prefix.
The During a topology change, routes go into an active A state when computing a new path. EIGRP neighbors exchange the entire routing table when forming an adjacency, and they advertise incremental updates only as topology changes occur within a network. The neighbor adjacency table is vital for tracking neighbor status and the updates sent to each neighbor. EIGRP uses its own IP protocol number 88 and uses multicast packets where possible; it uses unicast packets when necessary.
Communication between routers is done with multicast using the group address EIGRP uses multicast packets to reduce bandwidth consumed on a link one packet to reach multiple devices. While broadcast packets are used in the same general way, all nodes on a network segment process broadcast packets, whereas with multicast, only nodes listening for the particular multicast group process the multicast packets.
The sequence value zero does not require a response from the receiving EIGRP router; all other values require an ACK packet that includes the original sequence number. Ensuring that packets are received makes the transport method reliable.
All update, query, and reply packets are deemed reliable, and hello and ACK packets do not require acknowledgment and could be unreliable. If the originating router does not receive an ACK packet from the neighbor before the retransmit timeout expires, it notifies the non-acknowledging router to stop processing its multicast packets. The originating router sends all traffic by unicast until the neighbor is fully synchronized. Upon complete synchronization, the originating router notifies the destination router to start processing multicast packets again.
All unicast packets require acknowledgment. EIGRP retries up to 16 times for each packet that requires confirmation, and it resets the neighbor relationship when the neighbor reaches the retry limit of Other protocols that require reliable connection-oriented communication, such as TCP, cannot use multicast addressing.
The following parameters must match for the two routers to become neighbors:. I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands.
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Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Routing loops happen when information about the loss of a route does not reach all routers in the network because an update packet gets dropped or corrupted.
These routers that have not received the information about the loss of the route inject bad routing information back into the network by telling their neighbors about the route they know. EIGRP uses reliable transmission for all updates between neighbors. Neighbors acknowledge the receipt of updates, and if an acknowledgment is not received, EIGRP retransmits the update. RIP and IGRP employ a battery of techniques to reduce the likelihood of routing loops: split horizon, hold-down timers, and poison reverse.
These techniques do not guarantee that loops will not occur and, in any case, result in long convergence times. DUAL is able to achieve such low convergence times by maintaining a table of loop-free paths to every destination, in addition to the least-cost path. DUAL is described in more detail later in this chapter. A protocol-dependent module encapsulates DUAL messages and handles interactions with the routing table.
In summary, DUAL requires:. A method for the discovery of new neighbors and their loss see the next section, Section 4. Reliable transmission of update packets between neighbors see the later section Section 4. A router discovers a neighbor when it receives its first hello packet on a directly connected network. The router requests DUAL to send a full route update to the new neighbor. In response, the neighbor sends its full route update.
Thus, a new neighbor relationship is established in the following steps:. When a router A receives a hello packet from a new neighbor B , A sends its topology table to router B in unicast updates with the initialization bit turned on.
When router B receives a packet with the initialization bit on, it sends its topology table to router A. The interval between hello packets from any EIGRP-speaking router on a network is five seconds by default on most media types. Each hello packet advertises hold-time -- the length of time the neighbor should consider the sender up.
The default hold-time is 15 seconds. If no hellos are received for the duration of the hold-time, DUAL is informed that the neighbor is down. Thus, in addition to detecting a new neighbor, hello packets are also used to detect the loss of a neighbor. Lengthening the hello-interval will also lengthen the route convergence time.
If the hello-interval is changed, the hold-time should also be modified. A rule of thumb is to keep the hold-time at three times the hello-interval. Note that the hello-interval and hold-time need not be the same for all routers on a network.
The default hello-interval is 60 seconds with a hold-time of seconds on multipoint interfaces such as ATM, Frame Relay, and X. Hello packets are multicast; no acknowledgments are expected. The first column -- labeled H -- is the order in which the neighbors were learned. The hold-time for The hold-time for a neighbor should not exceed 15 seconds or fall below 10 seconds if the hold-time fell below 10 s, that would indicate the loss of one or more hello packets.
After a neighbor relationship has been established between A and B the only EIGRP overhead is the exchange of hello packets, unless there is a topological change in the network. The EIGRP transport mechanism uses a mix of multicast and unicast packets, using reliable delivery when necessary. All transmissions use IP with the protocol type field set to The IP multicast address used is DUAL requires guaranteed and sequenced delivery for some transmissions. This is achieved using acknowledgments and sequence numbers.
So, for example, update packets containing routing table data are delivered reliably with sequence numbers to all neighbors using multicast. Acknowledgment packets -- with the correct sequence number -- are expected from every neighbor. If the correct acknowledgment number is not received from a neighbor, the update is retransmitted as a unicast.
The sequence number seq num in the last packet from the neighbor is recorded to ensure that packets are received in sequence. The number of packets in the queue that might need retransmission is shown as a queue count QCnt , and the smoothed round trip time SRTT is used to estimate how long to wait before retransmitting to the neighbor.
The retransmission timeout RTO is the time the router will wait for an acknowledgment before retransmitting the packet in the queue. Some transmissions do not require reliable delivery. For example, hello packets are multicast to all neighbors on an Ethernet segment, whereas acknowledgments are unicast. Neither hellos nor acknowledgments are sent reliably. Queries are multicast or unicast using reliable delivery, whereas replies are always reliably unicast. Query and reply packets are discussed in more detail in the next section.
This table is referred to as the topology table. Unlike traditional DV protocols that save only the best least-cost path for every destination, DUAL saves all paths in the topology table.
The least-cost path s is copied from the topology table to the routing table. In the event of a failure, the topology table allows for very quick convergence if another loop-free path is available. If a loop-free path is not found in the topology table, a route recomputation must occur, during which DUAL queries its neighbors, who, in turn, may query their neighbors, and so on An update from a router R contains the cost to reach the destination network N from R.
This cost is referred to as the reported distance RD. In other words, the RD for Ames to reach We are not done yet since there is something else that R2 will save in its topology table. We know the advertised distance is 5 since this is what R3 told us. We also know the metric of the link between R2 and R3 since this is directly connected.
R2 now knows the metric for the total path to the destination, this total path is called the feasible distance and it will be saved in the topology table. The advertised distance, your neighbor tells you how far it is for him to reach the destination and the feasible distance which is your total distance to get to the destination. R2 is sending its feasible distance towards R1 which is R1 will save this information in the topology table as the advertised distance.
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