Configuring EIGRP Split Horizon

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Real World Application & Core Knowledge

In the previous lab you configured static neighbors on a frame relay hub-and-spoke between R1 and R2, R1 and R3, R1 and R4. After verification you should have noticed that routes from R2 did not get propagated to R4 through the hub. This is by default the normal operation of EIGRP and this is caused by a loop prevention mechanism called split-horizon.

The split horizon rule simply states that routes will not be advertised back out an interface in which they were received on. After all; if a router sends route updates to a neighbor why would that router need to have the neighboring router re-advertise those routes back to the originating router? The simple answer is that its not needed.

However in some scenarios EIGRP IP split horizon is required to be disabled to ensure intended operation; for example a hub and spoke topology where the physical interface has multiple IP’s mapped to specific PVC’s out a single physical interface. In this case the normal behavior is that routes learned via an interface will not be re-advertised back out that interface so in that case, with the previous lab R2’s routes being advertised to R1’s Serial0/0 interface would not be re-advertised back out R1’s Serial0/0 interface to R3 and R4.

In a case like this you’d need to disable EIGRP split-horizon using the no ip split-horizon eigrp as# command in interface configuration mode. This disables split-horizon on a per interface basis for the specified EIGRP autonomous system.

This lab will continue to build upon the topology used in Lab 8-2 and other labs that are found in Section 8.

Familiarize yourself with the following new command(s);

Command	Description
no ip split-horizon eigrp as#	This command is executed in interface configuration mode to disable ip split-horizon for the specified EIGRP autonomous system.

Lab Prerequisites

If you just completed Lab 8-2 you may start where you left off, if not you can load the Free CCNA Workbook GNS3 topology; start and establish a console session with R1, R2, R3, R4 and R5 then load their initial configurations included below by copying the config from the textbox and pasting it into the routers console.

!##################################################
!#  Free CCNA Workbook Lab 8-3 R1 Initial Config  #
!##################################################
!
enable
configure terminal
!
hostname R1
no ip domain-lookup
!
interface Loopback0
 description ### SIMULATED NETWORK ###
 ip address 10.80.10.1 255.255.255.0
!
interface Serial0/0
 description ### PHYSICAL FRAME RELAY INTERFACE ###
 ip address 10.80.234.1 255.255.255.248
 encapsulation frame-relay
 serial restart-delay 0
 no frame-relay inverse-arp
 frame map ip 10.80.234.2 122 broadcast
 frame map ip 10.80.234.3 123 broadcast
 frame map ip 10.80.234.4 124
 no shut
 exit
!
router eigrp 10
 network 10.80.234.1 0.0.0.0
 network 10.80.10.1 0.0.0.0
 no auto-summary
 neighbor 10.80.234.4 Serial0/0
 neighbor 10.80.234.3 Serial0/0
 neighbor 10.80.234.2 Serial0/0
 exit
!
line con 0
 logging sync
 no exec-timeout
!
end

!##################################################
!#  Free CCNA Workbook Lab 8-3 R2 Initial Config  #
!##################################################
!
enable
configure terminal
!
hostname R2
no ip domain-lookup
!
interface Loopback0
 description ### SIMULATED NETWORK ###
 ip address 10.80.20.1 255.255.255.0
!
interface Serial0/0
 description ### PHYSICAL FRAME RELAY INTERFACE ###
 no ip address
 encapsulation frame-relay
 serial restart-delay 0
 no frame-relay inverse-arp
 no shut
!
interface Serial0/0.221 point-to-point
 description ### FRAME RELAY LINK TO R1 ###
 ip address 10.80.234.2 255.255.255.248
 frame-relay interface-dlci 221
!
interface Serial0/2
 ### POINT-TO-POINT LINK TO R3 ###
 ip add 10.80.23.1 255.255.255.252
 encapsulation ppp
 no shut
 exit
!
router eigrp 10
 network 10.80.23.1 0.0.0.0
 network 10.80.234.2 0.0.0.0
 network 10.80.20.1 0.0.0.0
 no auto-summary
 neighbor 10.80.234.1 Serial0/0.221
 exit
!
line con 0
 logging sync
 no exec-timeout
!
end

!##################################################
!#  Free CCNA Workbook Lab 8-3 R3 Initial Config  #
!##################################################
!
enable
configure terminal
!
hostname R3
no ip domain-lookup
!
interface Loopback0
 description ### SIMULATED NETWORK ###
 ip address 10.80.30.1 255.255.255.0
!
interface Serial0/0
 description ### PHYSICAL FRAME RELAY INTERFACE ###
 no ip address
 encapsulation frame-relay
 serial restart-delay 0
 no frame-relay inverse-arp
 no shut
!
interface Serial0/0.321 point-to-point
 description ### FRAME RELAY LINK TO R1 ###
 ip address 10.80.234.3 255.255.255.248
 frame-relay interface-dlci 321
!
interface Serial0/1
 description ### POINT-TO-POINT LINK TO R2 ###
 ip address 10.80.23.2 255.255.255.252
 encapsulation ppp
 no shut
 exit
!
router eigrp 10
 network 10.80.23.2 0.0.0.0
 network 10.80.234.3 0.0.0.0
 network 10.80.30.1 0.0.0.0
 no auto-summary
 neighbor 10.80.234.1 Serial0/0.321
 exit
!
line con 0
 logging sync
 no exec-timeout
!
end

!##################################################
!#  Free CCNA Workbook Lab 8-3 R4 Initial Config  #
!##################################################
!
enable
configure terminal
!
hostname R4
no ip domain-lookup
!
interface Loopback0
 description ### SIMULATED NETWORK ###
 ip address 10.80.40.1 255.255.255.0
!
interface Serial0/0
 description ### PHYSICAL FRAME RELAY INTERFACE ###
 ip address 10.80.234.4 255.255.255.248
 encapsulation frame-relay
 serial restart-delay 0
 no frame-relay inverse-arp
 frame map ip 10.80.234.1 421
 no shut
!
interface Serial0/1
 description ### POINT-TO-POINT LINK TO R5 ###
 ip address 10.80.45.1 255.255.255.252
 encapsulation ppp
 no shut
 exit
!
router eigrp 10
 network 10.80.45.1 0.0.0.0
 network 10.80.234.4 0.0.0.0
 network 10.80.40.1 0.0.0.0
 no auto-summary
 neighbor 10.80.234.1 Serial0/0
 exit
!
line con 0
 logging sync
 no exec-timeout
!
end

!##################################################
!#  Free CCNA Workbook Lab 8-3 R5 Initial Config  #
!##################################################
!
enable
configure terminal
!
hostname R5
no ip domain-lookup
!
interface Loopback0
 description ### SIMULATED NETWORK ###
 ip address 10.80.50.1 255.255.255.0
!
interface Serial0/1
 description ### POINT-TO-POINT LINK TO R4 ###
 ip address 10.80.45.2 255.255.255.252
 encapsulation ppp
 no shut
 exit
!
router eigrp 10
 network 10.80.45.2 0.0.0.0
 network 10.80.50.1 0.0.0.0
 no auto-summary
 exit
!
line con 0
 logging sync
 no exec-timeout
!
end

Lab Objectives

Disable IP Split-Horizon for EIGRP Autonomous System 10 on R1’s Frame-Relay Hub interface.
Verify that routes from R2 and R3 are now being propagated through the hub to R4 and R5. Ping R2’s Lo0 interface from R5’s Lo0 interface to ensure IP reachability.

Lab Instruction

Objective 1. – Disable IP Split-Horizon for EIGRP Autonomous System 10 on R1′s Frame-Relay Hub interface.

R1>enable
R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#interface Serial0/0
R1(config-if)#no ip split-horizon eigrp 10
R1(config-if)#end
R1#
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor 10.80.234.4 (Serial0/0)
is resync: split horizon changed
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor 10.80.234.3 (Serial0/0)
is resync: split horizon changed
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor 10.80.234.2 (Serial0/0)
is resync: split horizon changed
%SYS-5-CONFIG_I: Configured from console by console
R1#

Objective 2. – Verify that routes from R2 and R3 are now being propagated through the hub to R4 and R5. Ping R2′s Lo0 interface from R5′s Lo0 interface to ensure IP reachability.

R5#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     10.80.0.0/8 is variably subnetted, 11 subnets, 4 masks
C       10.80.50.0/24 is directly connected, Loopback0
D       10.80.40.0/24 [90/2297856] via 10.80.45.1, 01:13:18, Serial0/1
D       10.80.23.1/32 [90/3705856] via 10.80.45.1, 00:01:44, Serial0/1
D       10.80.23.0/30 [90/3705856] via 10.80.45.1, 00:01:44, Serial0/1
D       10.80.23.2/32 [90/3705856] via 10.80.45.1, 00:01:44, Serial0/1
D       10.80.30.0/24 [90/3321856] via 10.80.45.1, 00:01:44, Serial0/1
C       10.80.45.1/32 is directly connected, Serial0/1
C       10.80.45.0/30 is directly connected, Serial0/1
D       10.80.20.0/24 [90/3321856] via 10.80.45.1, 00:01:45, Serial0/1
D       10.80.10.0/24 [90/2809856] via 10.80.45.1, 01:07:51, Serial0/1
D       10.80.234.0/29 [90/2681856] via 10.80.45.1, 01:13:18, Serial0/1
R5#

As shown above you can see that R5 now has routes to R2’s Lo0 interface and the next hop to that destination is R4 so with that in mind, R4 also knows how to get there otherwise it would not advertise that specific route.

To verify that R5 has ip reachability to R2’s Lo0 interface you can pink R2’s Lo0 interface from R5’s Lo0 interface as shown below;

R5#ping 10.80.20.1 source lo0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.80.20.1, timeout is 2 seconds:
Packet sent with a source address of 10.80.50.1 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 164/274/380 ms
R5#

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