For CCNA exam success, you had better know what split horizon is, how to turn it off, and when to turn it off. Knowing when to turn split horizon off is also important in production networks, because it can cause a hub-and-spoke network to have incomplete routing tables on the spokes.
Laptop Battery Split horizon exists for a very good reason - routing loop prevention. The rule of split horizon states that a router cannot send an advertisement for a route out the same interface that it came in on. Split horizon is on by default on all interfaces running RIP, IGRP, and EIGRP.
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Thinkpad In this CCNA tutorial, R1 will serve, as the hub and R2 and R3 will be the spokes. We'll first configure EIGRP over the 172.16.123.0 /24 network, the network connecting the three routers.
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Microsoft R1#conf t
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Laptop Computers R1(config)#router eigrp 100
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Laptop Computer R1(config-router)#no auto-summary
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Desktop Computer R1(config-router)#network 172.12.123.0 0.0.0.255
Notebooks R2#conf t
Lenovo R2(config)#router eigrp 100
Hard Drive R2(config-router)#no auto-summary
Travelstar R2(config-router)#network 172.12.123.0 0.0.0.255
Gateway R3#conf t
Laptop Parts R3(config)#router eigrp 100
Software R3(config-router)#no auto-summary
Hard Drives R3(config-router)#network 172.12.123.0 0.0.0.255
Electronics Running show ip eigrp neighbor on R1 shows that adjacencies to R2 and R3 are up.
Canon R1#show ip eigrp neighbor
Desktop Pc IP-EIGRP neighbors for process 100
Desktop Computers H Address Interface Hold Uptime SRTT RTO Q Seq Type
Think Pad (sec) (ms) Cnt Num
Repair 1 172.12.123.3 Se0/0 11 00:02:45 1 5000 0 1
Data Recovery 0 172.12.123.2 Se0/0 161 00:03:01 1 5000 0 1
Cisco Each router will now advertise its loopback address via EIGRP.
Keyboard R1#conf t
Monitor R1(config)#router eigrp 100
Desktop R1(config-router)#network 1.1.1.0 0.0.0.255
Infosys R2#conf t
Refurbished Laptops R2(config)#router eigrp 100
Wipro R2(config-router)#network 2.2.2.0 0.0.0.255
Lap Top R3#conf t
Refurbished R3(config)#router eigrp 100
Memory R3(config-router)#network 3.3.3.0 0.0.0.255
Intel Running show ip eigrp route on each router shows that R1 has a route for both R2's and R3's loopback. R2 and R3 will only see R1's loopback address, and not each other's. Why?
As400 R1#show ip route eigrp
Averatec 2.0.0.0/24 is subnetted, 1 subnets
Hardware D 2.2.2.0 [90/2297856] via 172.12.123.2, 00:03:19, Serial0/0
Dual Xeon 3.0.0.0/24 is subnetted, 1 subnets
Storage D 3.3.3.0 [90/2297856] via 172.12.123.3, 00:03:04, Serial0/0
Seagate R2#show ip route eigrp
Computer Sales 1.0.0.0/24 is subnetted, 1 subnets
Computer Hardware D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:03:40, Serial0/0.123
Printers R3#show ip route eigrp
Technology 1.0.0.0/24 is subnetted, 1 subnets
Mainframe D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:05:17, Serial0/0.31
Samsung EIGRP uses Split Horizon by default to prevent routing loops. In this lab, though, it prevents full network reachability. R2 and R3 both form neighbor relationships with R1's Serial physical interface. R2 advertises its loopback address to R1's Serial interface, as does R3. Split Horizon does not allow a route to be advertised back out the same interface it was received on. This prevents R1 from advertising R2's loopback to R3, or R3's loopback to R2.
Computer Repair Split Horizon must be disabled to allow full network reachability in this lab. To do so, run no ip split-horizon eigrp 100 on R1's Serial interface. When Split Horizon is disabled, that will cause the neighbor
relationships to fail, and then reestablish. Run show ip route eigrp 100 on both R2 and R3. The appropriate route to the remote loopback address will now appear.
Used Computers R1#conf t
Network R1(config)#int serial0
Digital Cameras R1(config-if)#no ip split-horizon eigrp 100
Desktops 10:02:23: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0/0) down: split horizon changed
Cognos 10:02:23: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0/0) down: split horizon changed
Hosting 10:02:27: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0/0) ip: new adjacency
Netfinity 10:02:54: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0/0) ip: new adjacency
Internet R2#show ip route eigrp
Cheap Computer 1.0.0.0/24 is subnetted, 1 subnets
Digital Camera D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:00:06, Serial0/0.123
Printer 3.0.0.0/24 is subnetted, 1 subnets
Xseries D 3.3.3.0 [90/2809856] via 172.12.123.1, 00:00:06, Serial0/0.123
Maxtor R3#show ip route eigrp
Data Storage 1.0.0.0/24 is subnetted, 1 subnets
Hitachi D 1.1.1.0 [90/2297856] via 172.12.123.1, 00:00:12, Serial0/0.31
2.0.0.0/24 is subnetted, 1 subnets
Rational D 2.2.2.0 [90/2809856] via 172.12.123.1, 00:00:12, Serial0/0.31
Websphere Disabling split horizon should be done with care, but knowing when and where to do so shows that you truly understand how this technology works - and that's a big step on the way to earning your CCNA!
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