Load Balancing
Command Meaning
show ip eigrp
topology
Shows the topology table used with EIGRP
variance Sets a variance used for unequal-cost load balancing
Take a look at a routing table with EIGRP routing configured on the network:
R1#sh ip route
10.0.0.0/24 is subnetted, 12 subnets
D 10.1.11.0 [90/2684416] via 10.1.3.1, 00:50:37, Serial0/0/1
[90/2684416] via 10.1.2.1, 00:50:37, Serial0/0/0
D 10.1.10.0 [90/2707456] via 10.1.3.1, 01:04:40, Serial0/0/1
[90/2707456] via 10.1.2.1, 01:04:40, Serial0/0/0
D 10.1.9.0 [90/2707456] via 10.1.3.1, 01:24:09, Serial0/0/1
[90/2707456] via 10.1.2.1, 01:24:09, Serial0/0/0
D 10.1.8.0 [90/2707456] via 10.1.3.1, 01:24:09, Serial0/0/1
[90/2707456] via 10.1.2.1, 01:24:09, Serial0/0/0
D 10.1.12.0 [90/2684416] via 10.1.3.1, 00:10:10, Serial0/0/1
[90/2684416] via 10.1.2.1, 00:10:10, Serial0/0/0
C 10.1.3.0 is directly connected, Serial0/0/1
C 10.1.2.0 is directly connected, Serial0/0/0
D 10.1.1.0 [90/2172416] via 10.1.3.1, 01:24:11, Serial0/0/1
[90/2172416] via 10.1.2.1, 01:24:11, Serial0/0/0
C 10.1.7.0 is directly connected, FastEthernet0/1
C 10.1.6.0 is directly connected, FastEthernet0/0
D 10.1.5.0 [90/2681856] via 10.1.3.1, 01:24:11, Serial0/0/1
[90/2681856] via 10.1.2.1, 01:24:11, Serial0/0/0
D 10.1.4.0 [90/2681856] via 10.1.3.1, 01:24:11, Serial0/0/1
[90/2681856] via 10.1.2.1, 01:24:11, Serial0/0/0
You can see that here you have two links to every route in the internetwork, and again,
EIGRP will load balance across the s0/0/0 and s0/0/1 links by default because they’re the same
metric.
But how about bundling links? Well, EIGRP can allow you to do this too—with just a
slightly different configuration! I’ll show you how this works by configuring the links between
the Corp and R1 router with the same subnet, meaning both links will have all interfaces
within the same subnet. Check out my configuration:
Corp#config t
Corp(config)#int s0/0/1
Corp(config-if)#ip address 10.1.2.4 255.255.255.0
R1#config t
R1(config)#int s0/0/1
R1(config-if)#ip address 10.1.2.3 255.255.255.0
R1(config-if)#do show run | begin interface
interface Serial0/0/0
description 1st Connection to Corp Router
ip address 10.1.2.2 255.255.255.0
!
interface Serial0/0/1
description 2nd connection to Corp Router
ip address 10.1.2.3 255.255.255.0
Now both links have all four interfaces in the same subnet:
R1(config-if)#do show ip route
10.0.0.0/24 is subnetted, 12 subnets
D 10.1.11.0 [90/2684416] via 10.1.2.4, 00:04:44, Serial0/0/1
[90/2684416] via 10.1.2.1, 00:04:44, Serial0/0/0
D 10.1.10.0 [90/2707456] via 10.1.2.4, 00:04:44, Serial0/0/1
[90/2707456] via 10.1.2.1, 00:04:44, Serial0/0/0
D 10.1.9.0 [90/2707456] via 10.1.2.4, 00:04:44, Serial0/0/1
[90/2707456] via 10.1.2.1, 00:04:44, Serial0/0/0
D 10.1.8.0 [90/2707456] via 10.1.2.4, 00:04:44, Serial0/0/1
[90/2707456] via 10.1.2.1, 00:04:44, Serial0/0/0
D 10.1.12.0 [90/2684416] via 10.1.2.4, 00:04:44, Serial0/0/1
[90/2684416] via 10.1.2.1, 00:04:44, Serial0/0/0
D 10.1.3.0 [90/3193856] via 10.1.2.4, 00:04:44, Serial0/0/1
[90/3193856] via 10.1.2.1, 00:04:44, Serial0/0/0
C 10.1.2.0 is directly connected, Serial0/0/0
is directly connected, Serial0/0/1
D 10.1.1.0 [90/2172416] via 10.1.2.4, 00:03:56, Serial0/0/1
[90/2172416] via 10.1.2.1, 00:03:56, Serial0/0/0
C 10.1.7.0 is directly connected, FastEthernet0/1
C 10.1.6.0 is directly connected, FastEthernet0/0
D 10.1.5.0 [90/2681856] via 10.1.2.4, 00:04:46, Serial0/0/1
[90/2681856] via 10.1.2.1, 00:04:46, Serial0/0/0
D 10.1.4.0 [90/2681856] via 10.1.2.4, 00:04:46, Serial0/0/1
[90/2681856] via 10.1.2.1, 00:04:46, Serial0/0/0
Now I’ll show what’s in the Corp topology table by using the show ip eigrp topology
command. I’ll cover this command more specifically in the “Verifying EIGRP” section.
Corp# sh ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(10.1.5.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.11.0/24, 1 successors, FD is 2172416
via 10.1.5.2 (2172416/28160), Serial0/2/0
P 10.1.10.0/24, 1 successors, FD is 2172416
via 10.1.5.2 (2195456/281600), Serial0/2/0
P 10.1.9.0/24, 1 successors, FD is 2195456
via 10.1.4.2 (2195456/281600), Serial0/1/0
P 10.1.8.0/24, 1 successors, FD is 2195456
via 10.1.4.2 (2195456/72960), Serial0/1/0
P 10.1.12.0/24, 1 successors, FD is 2172416
via 10.1.5.2 (2172416/28160), Serial0/2/0
P 10.1.3.0/24, 1 successors, FD is 76839936
via Connected, Serial0/0/1
via 10.1.2.2 (9849856/7719936), Serial0/0/0, serno 89
P 10.1.2.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0/0
via 10.1.2.2 (2681856/551936), Serial0/0/0
P 10.1.1.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/1
P 10.1.7.0/24, 1 successors, FD is 793600
via 10.1.2.2 (2195456/281600), Serial0/0/0
via 10.1.3.2 (77081600/281600), Serial0/0/1
P 10.1.6.0/24, 1 successors, FD is 793600
via 10.1.2.2 (2195456/281600), Serial0/0/0
via 10.1.3.2 (77081600/281600), Serial0/0/1
P 10.1.5.0/24, 1 successors, FD is 2169856
via Connected, Serial0/2/0
P 10.1.4.0/24, 1 successors, FD is 2169856
via Connected, Serial0/1/0
nexthop
neighbor through which packets will travel to their destinations. Plus, each entry also has
two numbers in parentheses. The first indicates the feasible distance, and the second one indicates
the advertised distance to a remote network.
Now here’s where things get interesting—notice that under the 10.1.7.0 and 10.1.6.0
outputs there are two links to each network and the feasible distance and advertised distance
are different. What this means is that you have one successor to the networks and one feasible
successor—a backup route!
For the route to be a feasible successor, its advertised distance must be less
than the feasible distance of the successor route. That’s so very cool! You
need to remember that even though both routes to network 10.1.6.0 and
10.1.7.0 are in the topology table, only the successor route (the one with the
lowest metrics) will be copied and placed into the routing table.
EIGRP will load balance across both links automatically when they are of equal variance
(equal cost), but EIGRP can also load balance across unequal cost links if you use the
variance command. Changing a variance value enables EIGRP to install multiple, loop-free
routes with unequal cost in a local routing table.
For example, if the variance is set to 2, an unequal cost path with a metric less than two
times the successor metric will be installed in the local routing table. Remember, an unequal
cost path will be used only if it is a feasible successor.