EIGRP can load balance up to 4 equal-cost by default but it can also be configured to load balance up to 16 equal or non equal cost links. Using the previous Basic EIGRP lab, I see how this is done on R1.
R1#show ip route eigrp
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D 10.1.1.0/24 [90/27769856] via 10.1.2.1, 00:02:59, Serial0/0/0
[90/27769856] via 10.1.3.1, 00:02:59, Serial0/0/1
D 10.1.4.0/30 [90/2681856] via 10.1.2.1, 00:02:59, Serial0/0/0
[90/2681856] via 10.1.3.1, 00:02:59, Serial0/0/1
D 10.1.5.0/30 [90/2172416] via 10.1.2.1, 00:02:59, Serial0/0/0
[90/2172416] via 10.1.3.1, 00:02:59, Serial0/0/1
172.16.0.0/24 is subnetted, 1 subnets
D 172.16.10.0 [90/2174976] via 10.1.2.1, 00:02:59, Serial0/0/0
[90/2174976] via 10.1.3.1, 00:02:59, Serial0/0/1
D 192.168.30.0/24 [90/2684416] via 10.1.2.1, 00:02:59, Serial0/0/0
[90/2684416] via 10.1.3.1, 00:02:59, Serial0/0/1
D 192.168.40.0/24 [90/2684416] via 10.1.2.1, 00:02:59, Serial0/0/0
[90/2684416] via 10.1.3.1, 00:02:59, Serial0/0/1
Each remote network has two available paths, serial 0/0/0 and serial 0/0/1 , these paths have the same cost 90. Because these routes have the same cost to the remote network, EIGRP will automatically load balance between the two paths creating a pipe of 3MB instead of a single serial pipe of 1.5MB. To verify the load balancing I used traceroute, ping wouldn't work because EIGRP doesn't load balance a single flow of data because of Cisco Express Forwarding but would if I had multiple data flows to the remote network.
R1#traceroute 10.1.4.1
Type escape sequence to abort.
Tracing the route to 10.1.4.1
1 10.1.3.1 7 msec 6 msec 3 msec
R1#traceroute 10.1.4.1
Type escape sequence to abort.
Tracing the route to 10.1.4.1
1 10.1.2.1 5 msec 5 msec 5 msec
Next I will configure serial 0/0/1 between R1 and Corp link with a different cost by changing the bandwidth. EIGRP using bandwidth and delay to determine cost of a route.
R1(config)#interface serial 0/0/1
R1(config-if)#bandwidth 256
%DUAL-5-NBRCHANGE: IP-EIGRP 51: Neighbor 10.1.3.1 (Serial0/0/1) is down: interface down
R1(config-if)#
%DUAL-5-NBRCHANGE: IP-EIGRP 51: Neighbor 10.1.3.1 (Serial0/0/1) is up: new adjacency
R1(config-if)#delay 300000
%DUAL-5-NBRCHANGE: IP-EIGRP 51: Neighbor 10.1.3.1 (Serial0/0/1) is down: interface down
R1(config-if)#
%DUAL-5-NBRCHANGE: IP-EIGRP 51: Neighbor 10.1.3.1 (Serial0/0/1) is up: new adjacency
EIGRP notices a routing change and creates a new adjacency between the routers. Taking a look at the routing table I notice that there is only one route to the remote networks instead of two load balanced ones.
R1#show ip route eigrp
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
D 10.1.1.0/24 [90/27769856] via 10.1.2.1, 00:45:15, Serial0/0/0
D 10.1.4.0/30 [90/2681856] via 10.1.2.1, 00:45:15, Serial0/0/0
D 10.1.5.0/30 [90/2172416] via 10.1.2.1, 00:45:15, Serial0/0/0
172.16.0.0/24 is subnetted, 1 subnets
D 172.16.10.0 [90/2174976] via 10.1.2.1, 00:45:15, Serial0/0/0
D 192.168.30.0/24 [90/2684416] via 10.1.2.1, 00:45:15, Serial0/0/0
D 192.168.40.0/24 [90/2684416] via 10.1.2.1, 00:45:15, Serial0/0/0
R1#show ip eigrp topology 10.1.4.0 255.255.255.252
IP-EIGRP (AS 51): Topology entry for 10.1.4.0/30
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2681856
Routing Descriptor Blocks:
10.1.2.1 (Serial0/0/0), from 10.1.2.1, Send flag is 0x0
Composite metric is (2681856/2169856), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 40000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
10.1.3.1 (Serial0/0/1), from 10.1.3.1, Send flag is 0x0
Composite metric is (87311872/2169856), Route is Internal
Vector metric:
Minimum bandwidth is 256 Kbit
Total delay is 3020000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
With the show topology command I can see the feasible distance and the feasible successor
Before bandiwth change
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS 51
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status
P 192.168.20.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/1
P 192.168.10.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/0
P 10.1.2.0/30, 1 successors, FD is 2169856
via Connected, Serial0/0/0
P 10.1.1.0/24, 2 successors, FD is 27769856
via 10.1.2.1 (27769856/25625600), Serial0/0/0
via 10.1.3.1 (27769856/25625600), Serial0/0/1
P 10.1.5.0/30, 2 successors, FD is 2172416
via 10.1.2.1 (2172416/28160), Serial0/0/0
via 10.1.3.1 (2172416/28160), Serial0/0/1
P 10.1.3.0/30, 1 successors, FD is 2169856
via Connected, Serial0/0/1
P 10.1.4.0/30, 2 successors, FD is 2681856
via 10.1.2.1 (2681856/2169856), Serial0/0/0
via 10.1.3.1 (2681856/2169856), Serial0/0/1
After bandwidth change
R1#show ip eigrp topology
IP-EIGRP Topology Table for AS 51
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status
P 192.168.20.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/1
P 192.168.10.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0/0
P 10.1.2.0/30, 1 successors, FD is 2169856
via Connected, Serial0/0/0
P 10.1.1.0/24, 1 successors, FD is 27769856
via 10.1.2.1 (27769856/25625600), Serial0/0/0
via 10.1.3.1 (112399872/25625600), Serial0/0/1
P 10.1.5.0/30, 1 successors, FD is 2172416
via 10.1.2.1 (2172416/28160), Serial0/0/0
via 10.1.3.1 (86802432/28160), Serial0/0/1
P 10.1.3.0/30, 1 successors, FD is 86799872
via Connected, Serial0/0/1
P 10.1.4.0/30, 1 successors, FD is 2681856
via 10.1.2.1 (2681856/2169856), Serial0/0/0
via 10.1.3.1 (87311872/2169856), Serial0/0/1
P 172.16.10.0/24, 1 successors, FD is 2174976
via 10.1.2.1 (2174976/30720), Serial0/0/0
via 10.1.3.1 (86804992/30720), Serial0/0/1
P 192.168.30.0/24, 1 successors, FD is 2684416
via 10.1.2.1 (2684416/2172416), Serial0/0/0
via 10.1.3.1 (87314432/2172416), Serial0/0/1
P 192.168.40.0/24, 1 successors, FD is 2684416
via 10.1.2.1 (2684416/2172416), Serial0/0/0
via 10.1.3.1 (87314432/2172416), Serial0/0/1
The second number after the FD is the advertised distance (AD)
(27769856/25625600)
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