Neighbor Table Contents
An EIGRP router multicasts hello packets to discover neighbors. It forms an adjacency with these neighbors so that it can exchange route updates. Only adjacent routers exchange routing information. Each router builds a neighbor table from the hello packets it receives from adjacent EIGRP routers running the same network layer protocol. EIGRP maintains a neighbor table for each configured network-layer protocol. You can display the IP neighbor table with the show ip eigrp neighbors command, as shown in Example 2-1.
Example 2-1: Sample Output for the show ip eigrp neighbors Command
R1#show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.1.102 Se0/0/1 10 00:07:22 10 2280 0 5
R1#
This neighbor table includes the following key elements:
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H (handle)— A number used internally by the Cisco IOS to track a neighbor. This column lists the order in which a peering session was established with the specified neighbor, starting with 0.
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Interface— The interface on this router through which it is receiving hello packets for the neighbor, and therefore through which the neighbor can be reached.
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Hold Time— The maximum time, in seconds, that the router waits to hear from the neighbor without receiving anything from a neighbor before considering the link unavailable. Originally, the expected packet was a hello packet, but in current Cisco IOS software releases, any EIGRP packets received after the first hello from that neighbor resets the timer.
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Uptime— The elapsed time, in hours, minutes, and seconds since the local router first heard from this neighbor.
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Smooth Round Trip Timer (SRTT)— The average number of milliseconds it takes for an EIGRP packet to be sent to this neighbor and for the local router to receive an acknowledgment of that packet. This timer is used to determine the retransmit interval, also known as the retransmit timeout (RTO).
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RTO— The amount of time, in milliseconds, that the router waits for an acknowledgment before retransmitting a reliable packet from the retransmission queue to a neighbor.
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Queue (Q) count— The number of EIGRP packets (update, query, and reply) waiting in the queue to be sent out. If this value is constantly higher than 0, a congestion problem might exist. A 0 indicates that no EIGRP packets are in the queue.
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Seq Num— The sequence number of the last update, query, or reply packet that was received from this neighbor.
EIGRP Reliability
EIGRP’s reliability mechanism ensures delivery of critical route information to neighboring routers. This information is required to allow EIGRP to maintain a loop-free topology. For efficiency, only certain EIGRP packets are transmitted reliably.
All packets carrying routing information (update, query, and reply) are sent reliably (because they are not sent periodically). A sequence number is assigned to each reliable packet and an explicit acknowledgment is required for that sequence number.
Recall that RTP is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. RTP supports an intermixed transmission of multicast and unicast packets.
RTP ensures that ongoing communication is maintained between neighboring routers. As such, a retransmission list is maintained for each neighbor. This list indicates packets not yet acknowledged by a neighbor within the RTO. It is used to track all the reliable packets that were sent but not acknowledged.
If the RTO expires before an ACK packet is received, the EIGRP process retransmits another copy of the reliable packet, up to a maximum of 16 times or until the hold time expires.
The use of reliable multicast packets is efficient. However, a potential delay exists on multiaccess media where multiple neighbors reside. The next reliable multicast packet cannot be transmitted until all peers have acknowledged the previous multicast. If one or more peers are slow to respond, this adversely affects all peers by delaying the next transmission. RTP is designed to handle such exceptions: Neighbors that are slow to respond to multicasts have the unacknowledged multicast packets retransmitted as unicasts. This allows the reliable multicast operation to proceed without delaying communication with other peers, helping to ensure that convergence time remains low in the presence of variable-speed links.
The multicast flow timer (seen in the show ip eigrp interfaces command output) determines how long to wait for an ACK packet before switching from multicast to unicast. The RTO determines how long to wait between the subsequent unicasts. The EIGRP process for each neighbor calculates both the multicast flow timer and RTO, based on the SRTT. The formulas for the SRTT, RTO, and multicast flow timer are Cisco-proprietary.
In a steady-state network where no routes are flapping, EIGRP waits the specified hold-time interval before it determines that an EIGRP neighbor adjacency is down. Therefore, by default, EIGRP waits up to 15 seconds on high-speed links and up to 180 seconds on slower WAN links. When EIGRP determines that a neighbor is down and the router cannot reestablish the adjacency, the routing table removes all networks that could be reached through that neighbor. The router attempts to find alternative routes to those networks so that convergence can occur.
The 180-second hold time on low-speed links can seem excessive, but it accommodates the slowest-speed links, which are generally connected to less-critical remote sites. In some networks with mission-critical or time-sensitive applications (such as IP telephony), even on high-speed links, 15 seconds is too long. The point to remember is that other conditions can override the hold time and allow the network to converge quickly. For example, if the network is unstable and routes are flapping elsewhere because a remote site is timing out on its adjacency, EIGRP hold timers begin counting down from 180 seconds. When the upstream site sends the remote site an update, and the remote site does not acknowledge the update, the upstream site attempts 16 times to retransmit the update. The retransmission occurs each time the RTO expires. After 16 retries, the router resets the neighbor relationship. This causes the network to converge faster than waiting for the hold time to expire.
DUAL
Diffusing Update Algorithm (DUAL) is the finite-state machine that selects which information is stored in the topology and routing tables. As such, DUAL embodies the decision process for all EIGRP route computations. It tracks all routes advertised by all neighbors, uses the metric to select an efficient and loop-free path to each destination, and inserts that choice in the routing table.
Advertised Distance and Feasible Distance
The AD is the EIGRP metric for an EIGRP neighbor router to reach a particular network. This is the metric between the next-hop neighbor router and the destination network.
The FD is the EIGRP metric for this router to reach a particular network. This is the sum of the AD for the particular network learned from an EIGRP neighbor, plus the EIGRP metric to reach that neighbor (the metric between this router and the next-hop router).
A router compares all FDs to reach a specific network in its topology table. The route with the lowest FD is offered to its IP routing table. This is the successor route. The FD for the chosen route becomes the EIGRP routing metric to reach that network in the routing table.
For example, in Figure 2-5, Routers A and B send their routing tables to Router C, whose tables are shown in the figure. Both Routers A and B have paths to network 10.1.1.0/24 (among many others that are not shown).
The routing table on Router A has an EIGRP metric of 1000 for 10.1.1.0/24. Therefore, Router A advertises 10.1.1.0/24 to Router C with a metric of 1000. Router C places the 10.1.1.0/24 network from Router A in its EIGRP topology table with an AD of 1000. Router B has network 10.1.1.0/24 with a metric of 1500 in its IP routing table. Therefore, Router B advertises 10.1.1.0/24 to Router C with an AD of 1500. Router C places the 10.1.1.0/24 network from Router B in the EIGRP topology table with an AD of 1500.
Router C in Figure 2-5 has two entries to reach 10.1.1.0/24 in its topology table. The EIGRP metric for Router C to reach either Router A or B is 1000. This cost (1000) is added to the respective AD from each router, and the results represent the FDs that Router C must travel to reach network 10.1.1.0/24. Router C chooses the least-cost FD (in this case 2000, via Router A) and installs it in its IP routing table as the best route to reach 10.1.1.0/24. The EIGRP metric in the routing table is the best FD from the EIGRP topology table. In this case Router C’s routing table shows the route to 10.1.1.0/24 is via Router A with a metric of 2000.
Successor and Feasible Successor
A successor, also called a current successor, is a neighboring router used for packet forwarding that has a least-cost path to a destination that is guaranteed not to be part of a routing loop.
The FD, not the AD, affects the selection of the best routes for incorporation in the routing table. The AD is used only to calculate the FD. A router is chosen as a successor because it has the lowest FD of all possible paths to that destination network. The successor is the next router in line to reach that destination—it is the next-hop router in the best path to reach that destination network.
An EIGRP router selects the best path to reach a given network and then installs the destination network, the metric to reach that network, the outbound interface to reach the next-hop router, and the IP address of the next-hop router into the IP routing table. If the EIGRP topology table has many entries that have an equal-cost FD to a given destination network, all successors (up to four by default) for that destination network are installed in the routing table.
All IP routing protocols can install only the next-hop router information in the routing table. Information about the subsequent routers in the path is not put in the routing table. Each router relies on the next-hop router to make a reliable decision to reach a specific destination network. The hop-by-hop path through a network goes from one router to the next. Each router makes a path selection to reach a given network and installs the best next-hop address along the path to reach that destination network. A router trusts a route’s successor (the best next-hop router) to send traffic toward that destination address.
The routing table is essentially a subset of the topology table. The topology table contains more detailed information about each route, any backup routes, and information used exclusively by DUAL.
An FS is a router providing a backup route. The route through the FS must be loop free. In other words, it must not loop back to the current successor. FSs are selected at the same time the successors are identified. These FS routes are kept in the topology table. The topology table can retain multiple FS routes for a destination.
An FS must be mathematically proven. To qualify as an FS, a next-hop router must have an AD less than the FD of the current successor route for the particular network. This is known as the feasibility condition. This requirement ensures that the FS cannot use a route through the local router (which would be a routing loop), because the AD through the FS is less than the best route through the local router. For example, as shown in Router C’s topology table in Figure 2-6 (for the network shown earlier in Figure 2-5), Router B is an FS, because the AD through Router B (1500) is less than the FD of the current successor, Router A (2000).
When a router loses a route, it looks at the topology table for an FS. If one is available, the route does not go into an active state. Instead, the best FS is promoted as the successor and is installed in the routing table. The FS can be used immediately, without any recalculation. If there are no FSs, a route goes into active state, and route computation occurs. Through this process, a new successor is determined (if there is one). The amount of time it takes to recalculate the route affects the convergence time.
Figure 2-7 illustrates another example. Router C’s initial topology table is shown at the top of the figure. Router B is the successor for network 10.1.1.0/24, and Router D is the FS.
In Figure 2-7, the link between Router B and Router C fails. Router C removes the route 10.1.1.0/24 through Router B from its routing table and searches the EIGRP topology table for an FS; Router D is an FS. Because Router D can still reach the network and does not send an update or query packet to inform Router C of the lost route, Router C immediately uses the path through Router D. Router C chose this path as an FS because the AD through Router D (1500) is less than the FD of the best route, through Router B (2000). This path is guaranteed to be loop free.
Figure 2-8 illustrates another scenario that shows how DUAL ensures a loop-free network. Router B sends the routing update about network 10.0.0.0/8, with an AD of 30. Router A receives the update, calculates the FD value (130), and sends an update to both of its neighbors, routers C and D. Routers A, C, and D are in a loop. The update sent to Router C is sent on to Router D, which then sends it to Router A. The AD of the route that Router D sends to Router A is 330. This AD value is higher than the FD (130) on Router A, calculated from the original update received from Router B. Because the FD of the route on Router A is smaller than the AD of the update coming from Router D, the route via Router D does not become an FS. Thus, DUAL ensures there will be no routing loop in the network.
DUAL Example
The mathematical formula to ensure that the FS is loop free requires that the AD of the backup route be less than the FD of the successor. When the AD of the second-best route is greater than or equal to the FD of the successor, an FS cannot be chosen. In this case, a discovery process that uses EIGRP queries and replies must be used to find any alternative paths to the lost networks.
The following example examines partial entries for network 10.1.1.0/24 in the topology tables for Routers C, D, and E in Figure 2-9, to give you a better understanding of EIGRP behavior. The partial topology tables shown in Figure 2-9 indicate the following:
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AD— The advertised distance is equal to the cost of the path to network 10.1.1.0/24 as advertised by neighboring routers. For example, consider Router E’s neighbors: Router D has an AD of 2 and Router C has an AD of 3, for 10.1.1.0/24.
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FD— The feasible distance is equal to the sum of the AD for a neighbor to reach 10.1.1.0/24, plus the metric to reach that neighbor. For example, again consider Router E: The FD of the route to 10.1.1.0/24 via Router D is Router D’s AD (2) plus the metric to reach Router D from Router E (1), for a total of 3. The FD of the route to 10.1.1.0/24 via Router C is Router C’s AD (3) plus the metric to reach Router C from Router E (1), for a total of 4.
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Successor— The successor is the forwarding path used to reach network 10.1.1.0/24. The cost of this path is equal to the FD. For example, Router E chooses the path to 10.1.1.0/24 with the lowest FD, which is via Router D. This is the route that Router E puts in its routing table.
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FS— The feasible successor is an alternative loop-free path to reach network 10.1.1.0/24. For example, in Router C, the path via Router D is an FS because the AD (2) is less than the FD (3) via the successor Router B. Routers D and E do not have any FSs because the AD of the alternate routes are not less than the FD of their current successors.
The network shown in Figure 2-9 is stable and converged.
| Note | As mentioned earlier, EIGRP implements split horizon. For example, Router E does not pass its route for network 10.1.1.0/24 to Router D, because Router E uses Router D as its next hop to network 10.1.1.0/24. |
In Figure 2-10, Routers B and D detect a link failure. In Router D, DUAL marks the path to network 10.1.1.0/24 through Router B as unusable after being notified of the link failure,
The following steps then occur, as shown in Figure 2-11:
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At Router D, there is no FS to network 10.1.1.0/24, because the AD via Router C (3) is greater than the FD via Router B (2). Therefore, DUAL does the following:
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Sets the metric to network 10.1.1.0/24 as unreachable (–1 is unreachable).
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Because an FS cannot be found in the topology table, the route changes from the passive state to the active state. In the active state, the router sends out queries to neighboring routers looking for a new successor.
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Sends a query to Routers C and E for an alternative path to network 10.1.1.0/24.
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Marks Routers C and E as having a query pending (q).
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At Router E, DUAL marks the path to network 10.1.1.0/24 through Router D as unusable.
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At Router C, DUAL marks the path to network 10.1.1.0/24 through Router D as unusable.
The following steps then occur, as shown in Figure 2-12:
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At Router D:
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DUAL receives a reply from Router C that indicates no change to the path to network 10.1.1.0/24.
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DUAL removes the query flag from Router C.
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DUAL stays active on network 10.1.1.0/24, awaiting a reply from Router E to its query (q).
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At Router E, there is no FS to network 10.1.1.0/24, because the AD from Router C (3) is not less than the original FD (also 3).
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DUAL generates a query to Router C.
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DUAL marks Router C as query pending (q).
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At Router C, DUAL marks the path to network 10.1.1.0/24 through Router E as unusable.
The following steps then occur, as shown in Figure 2-13:
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At Router D, DUAL stays active on network 10.1.1.0/24, awaiting a reply from Router E (q).
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At Router E:
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DUAL receives a reply from Router C indicating no change.
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It removes the query flag from Router C.
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It calculates a new FD and installs a new successor route in the topology table.
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It changes the route to network 10.1.1.0/24 from active to passive (converged).
The following steps then occur, as shown in Figure 2-14 at Router D:
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DUAL receives a reply from Router E.
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It removes the query flag from Router E.
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It calculates a new FD.
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It installs new successor routes in the topology table. Two routes (through Routers C and E) have the same FD, and both are marked as successors.
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It changes the route to network 10.1.1.0/24 from active to passive (converged).
The following steps then occur, as shown in Figure 2-15:
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At Router D, two successor routes are in the topology table for network 10.1.1.0/24. Both successor routes are listed in the routing table, and equal-cost load balancing is in effect.
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The network is stable and converged.
Figure 2-9, the original topology before the link failure, shows traffic from Router E for 10.1.1.0/24 passing through Routers D and B. In Figure 2-15, the new topology shows traffic from Routers D and E for 10.1.1.0/24 going through Routers C and B. Notice that throughout the entire convergence process, routes to network 10.1.1.0/24 become active only on Routers D and E. The route to network 10.1.1.0/24 on Router C remains passive because the link failure between Routers B and D does not affect the successor route from Router C to network 10.1.1.0/24.
| Note | When DUAL decides that a packet needs to be transmitted to a neighbor, the packets are not actually generated until the moment of transmission. Instead, the transmit queues contain small, fixed-size structures that indicate which parts of the topology table to include in the packet when it is finally transmitted. This means that the queues do not consume large amounts of memory. It also means that only the latest information is transmitted in each packet. If a route changes state several times, only the last state is transmitted in the packet, thus reducing link utilization. |
EIGRP Metric Calculation
DUAL selects routes based on the EIGRP composite metric. Five criteria are associated with the EIGRP composite metric, but EIGRP uses only two by default:
The following criteria, although available, are not commonly used, because they typically result in frequent recalculation of the topology table:
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Reliability— The worst reliability between the source and destination, based on keepalives.
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Loading— The worst load on a link between the source and destination based on the packet rate and the interface’s configured bandwidth.
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Maximum transmission unit (MTU)— The smallest MTU in the path. (MTU is included in the EIGRP update but is actually not used in the metric calculation.)
EIGRP calculates the metric by adding together weighted values of different variables of the path to the network in question. The default constant weight values are K1 = K3 = 1, and K2 = K4 = K5 = 0.
In EIGRP metric calculations, when K5 is 0 (the default), variables (bandwidth, bandwidth divided by load, and delay) are weighted with the constants K1, K2, and K3. The following is the formula used:
If these K values are equal to their defaults, the formula becomes
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metric = (1 * bandwidth) + [(0 * bandwidth) / (256 − load)] + (1 * delay)
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metric = bandwidth + [0] + delay
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metric = bandwidth + delay
If K5 is not equal to 0, the following additional operation is performed:
K values are carried in EIGRP hello packets. Mismatched K values can cause a neighbor to be reset (even though only K1 and K3 are used, by default, in metric compilation). These K values should be modified only after careful planning; changing these values can prevent your network from converging and is generally not recommended.
It is important to note that the format of the delay and bandwidth values is different from those displayed by the show interfaces command, as follows:
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The EIGRP delay value is the sum of the delays in the path, in tens of microseconds, multiplied by 256. The show interfaces command displays delay in microseconds.
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The EIGRP bandwidth is calculated using the minimum bandwidth link along the path, represented in kilobits per second (kbps). 107 is divided by this value, and then the result is multiplied by 256.
EIGRP uses the same metric formula as IGRP did, but EIGRP represents its metrics in a 32-bit format rather than the 24-bit representation used by IGRP. This representation allows a more granular decision to be made when determining the successor and feasible successor.
The EIGRP metric value ranges from 1 to 4,294,967,296. (The IGRP metric value ranged from 1 to 16,777,216.) EIGRP metrics are backward compatible with IGRP, as illustrated in Figure 2-16. When integrating IGRP routes into an EIGRP domain using redistribution, the router multiplies the IGRP metric by 256 to compute the EIGRP-equivalent metric. When sending EIGRP routes to an IGRP routing domain, the router divides each EIGRP metric by 256 to achieve the proper 24-bit metric.
Figure 2-17 illustrates an example network used to illustrate the metric calculation. In this figure, Router A has two paths to reach Router D (and therefore any networks behind Router D). The bandwidths (in kbps) and the delays (in tens of microseconds) of the various links are also shown in the figure.
The least bandwidth along the top path (A —> B —> C —> D) is 64 kbps. The EIGRP bandwidth calculation for this path is as follows:
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bandwidth = (107 / least bandwidth in kbps) * 256
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bandwidth = (10,000,000 / 64) * 256 = 156,250 * 256 = 40,000,000
The delay through the top path is as follows:
Therefore, the EIGRP metric calculation for the top path is as follows:
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metric = bandwidth + delay
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metric = 40,000,000 + 1,536,000
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metric = 41,536,000
The least bandwidth along the lower path (A —> X —> Y —> Z —> D) is 256 kbps. The EIGRP bandwidth calculation for this path is as follows:
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bandwidth = (107 / least bandwidth in kbps) * 256
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bandwidth = (10,000,000 / 256) * 256 = 10,000,000
The delay through the lower path is as follows:
Therefore, the EIGRP metric calculation for the lower path is as follows:
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metric = bandwidth + delay
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metric = 10,000,000 + 2,048,000
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metric = 12,048,000
Router A therefore chooses the lower path, with a metric of 12,048,000, over the top path, with a metric of 41,536,000. Router A installs the lower path with a next-hop router of X and a metric of 12,048,000 in the IP routing table.
The bottleneck along the top path, the 64-kbps link, can explain why the router takes the lower path. This slow link means that the rate of transfer to Router D would be at a maximum of 64 kbps. Along the lower path, the lowest speed is 256 kbps, making the throughput rate up to that speed. Therefore, the lower path represents a better choice, such as to move large files quickly.
This section concludes the discussion of EIGRP terminology and operation. The rest of the chapter explores planning, configuring, and verifying EIGRP implementation.
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