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Typically, networks are configured with multiple paths to a remote network. When these paths are
equal or nearly equal, it makes sense to utilize all the available paths. Unlike Layer 2 forwarding,
Layer 3 forwarding has the capability to load-balance between multiple paths. That is, the router
can send frames out multiple interfaces to reduce the amount of traffic sent to a single network
connection. The key to this feature is that the network paths must be of equal cost (or nearly equal
for some protocols like EIGRP). EIGRP uses a metric to compute the costs to a given network.
EIGRP Metric
The EIGRP metric can be based on several criteria, but EIGRP uses only two of these criteria by
default:
■ Bandwidth: The smallest bandwidth between source and destination
■ Delay: The cumulative interface delay in microseconds along the path
IP-EIGRP: Ext 172.69.40.0 255.255.255.0 metric 2272256 - 1657856 614400
IP-EIGRP: 192.135.245.0 255.255.255.0, - do advertise out Ethernet0/1
IP-EIGRP: Ext 192.135.245.0 255.255.255.0 metric 40622080 - 40000000 622080
IP-EIGRP: 192.135.244.0 255.255.255.0, - do advertise out Ethernet0/1
Table 5-6 debug ip eigrp Output
Field Description
IP-EIGRP Indicates that this is an IP EIGRP packet.
Ext Indicates that the following address is an external destination rather than an
internal destination, which would be labeled as Int.
do not advertise out Indicates interfaces out which EIGRP will not advertise the given route. This
configuration prevents routing loops (split horizon).
M Displays the computed metric, which includes the sent metric (SM) and the cost
between this router and the neighbor. The first number is the composite metric.
The next two numbers are the inverse bandwidth and the delay, respectively.
SM Displays the metric as reported by the neighbor.
Example 5-5 Analyzing Sent/Received EIGRP Packets (Continued)
182 Chapter 5: Implementing EIGRP
The following criteria can be used but are not recommended because they typically result in
frequent recalculation of the topology table:
■ Reliability: This value represents the worst reliability between the source and destination,
based on keepalives.
■ Load: This value represents the worst load on a link between the source and destination,
computed based on the packet rate and the configured bandwidth of the interface.
Load Balancing Across Equal Paths
Equal-cost load balancing is the capability of a router to distribute traffic over all its network ports
that are the same metric from the destination address. Load balancing increases the use of network
segments and increases effective network bandwidth.
For IP, Cisco IOS Software applies load balancing across up to four equal-cost paths by default.
With the maximum-paths maximum-path router configuration command, up to 16 equal-cost
routes can be kept in the routing table. If you set the maximum-path to 1, you disable load
balancing. When a packet is process switched, load balancing over equal-cost paths occurs on a
per-packet basis. When packets are fast switched, load balancing over equal-cost paths occurs on
a per-destination basis.
Configuring Load Balancing Across Unequal-Cost Paths
EIGRP can also balance traffic across multiple routes that have different metrics, which is called
unequal-cost load balancing. The degree to which EIGRP performs load balancing is controlled
with the variance command.
The multiplier parameter for the variance command is a value from 1 to 128, used for load
balancing. The default is 1, which indicates that only equal-cost load balancing is being
performed. The multiplier defines the range of metric values that are accepted for load balancing
by the EIGRP process.
NOTE Although the maximum transmission unit (MTU) is exchanged in EIGRP packets
between neighbor routers, MTU is not factored into the EIGRP metric calculation.=
NOTE If you test load balancing, do not ping to or from routers that use fast-switching
interfaces because these router-generated packets are process switched rather than fast switched
and might produce confusing results.
NOTE By default, traffic is distributed proportionately among the links with unequal costs,
with respect to the metric.
Implementing EIGRP 183
Example: Variance
In Figure 5-6, a variance of 2 is configured, and the range of the metric values, which are the
feasible distances for Router E to get to network 172.16.0.0, is 20 to 45. This range of values
determines the feasibility of a potential route.
Figure 5-6 Variance Example
A route is feasible if the next router in the path is closer to the destination than to the current router
and if the metric of the alternate path is within the variance. Load balancing can use only feasible
paths, and the routing table includes only these paths. The two feasibility conditions are as follows:
■ The local best metric, which is the current feasible distance, must be greater than the best
metric (the advertised distance) that is learned from the next router. In other words, the next
router in the path must be closer to the destination than to the current router; this criterion
prevents routing loops.
■ The metric of the alternate path must be less than the variance multiplied by the local best
metric (the current feasible distance).
If both of these conditions are met, the route is determined to be feasible and can be added to the
routing table.
In Figure 5-6, three paths to network 172.16.0.0 exist with the following metrics:
■ Path 1: 30 (through B)
■ Path 2: 20 (through C)
■ Path 3: 45 (through D)
B
D
E C
Network
172.16.0.0
FD = Feasible Distance
AD = Advertised Distance
20
10
20
10
10
25
(config) #router eigrp 200
(config–router) #variance 2
Network
172.16.0.0
Neighbor
B
C
D
FD
30
20
45
AD
10
10
25
184 Chapter 5: Implementing EIGRP
By default, the router places only path 2 (through C) in the routing table because it is the least-cost
path. To load-balance over paths 1 and 2, use a variance of 2 because 20 * 2 = 40, which is greater
than the metric through path 1.
In Figure 5-6, Router E uses Router C as the successor because it has the lowest feasible distance
(20). With the variance 2 command applied to Router E, the path through Router B meets the
criteria for load balancing. In this case, the feasible distance through Router B is less than twice
the feasible distance for the successor (Router C).
Router D is not considered for load balancing with this variance because the feasible distance
through Router D is greater than twice the feasible distance for the successor (Router C). In the
example, however, Router D would never be a feasible successor no matter what the variance is.
This decision is because the advertised distance of Router D is 25, which is greater than the Router
E feasible distance of 20; therefore, to avoid a potential routing loop, Router D is not considered
a feasible successor.
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