IP split-horizon
November 3rd, 2008 in RIP | tags: ,

When the ip split-horizon rule is enabled it will not send updates out the interface it received it on. When this rule is disabled it will send updates out the interface it was received on.

By default ip split-horizon is enabled on ethernet and frame-relay subinterfaces, this means that it will not send updates out the interfaces it receives it on. (ip split-horizon)

By default ip split-horizon is disabled on frame-relay physical interfaces, this means that it will send updates out the interfaces where it receives it on. (no ip split-horizon)

See below an example a simple RIPv2 network

IP split-horizon

R1 :


interface Serial1/0
 ip address 10.10.10.1 255.255.255.0
 encapsulation frame-relay
 ip split-horizon
 serial restart-delay 0
 frame-relay map ip 10.10.10.2 102 broadcast
 frame-relay map ip 10.10.10.3 103 broadcast
 no frame-relay inverse-arp
!
router rip
 version 2
 network 1.0.0.0
 network 10.0.0.0
 no auto-summary

R2 :


R2 :
interface Serial1/0
 ip address 10.10.10.2 255.255.255.0
 encapsulation frame-relay
 serial restart-delay 0
 frame-relay map ip 10.10.10.1 201 broadcast
 frame-relay map ip 10.10.10.3 201 broadcast
 no frame-relay inverse-arp
!
router rip
 version 2
 network 2.0.0.0
 network 10.0.0.0
 no auto-summary

R3 :


interface Serial1/0
 ip address 10.10.10.3 255.255.255.0
 encapsulation frame-relay
 serial restart-delay 0
 frame-relay map ip 10.10.10.1 301 broadcast
 frame-relay map ip 10.10.10.2 301 broadcast
 no frame-relay inverse-arp
!
router rip
 version 2
 network 3.0.0.0
 network 10.0.0.0
 no auto-summary

When configured like above the “show ip route” output looks like :

R1 :


R1#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
     2.0.0.0/32 is subnetted, 1 subnets
R       2.2.2.2 [120/1] via 10.10.10.2, 00:00:04, Serial1/0
     3.0.0.0/32 is subnetted, 1 subnets
R       3.3.3.3 [120/1] via 10.10.10.3, 00:00:18, Serial1/0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

R2 :


R2#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
R       1.1.1.1 [120/1] via 10.10.10.1, 00:00:17, Serial1/0
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
     3.0.0.0/32 is subnetted, 1 subnets
R       3.3.3.3 [120/2] via 10.10.10.3, 00:00:17, Serial1/0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

R3 :


R3#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
R       1.1.1.1 [120/1] via 10.10.10.1, 00:00:08, Serial1/0
     2.0.0.0/32 is subnetted, 1 subnets
R       2.2.2.2 [120/2] via 10.10.10.2, 00:00:08, Serial1/0
     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

You can see that router R2 can see router’s R3 loopback address. This is because, as we stated before, by default ip split-horizon is disabled on frame-relay physical interfaces.

Now let’s change router’s R1 configuration. We are going to move the configuration on router’s R1 physical interface to a frame-relay multipoint sub-interface.

R1 :


interface Serial1/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
!
interface Serial1/0.1 multipoint
 ip address 10.10.10.1 255.255.255.0
 frame-relay map ip 10.10.10.2 102 broadcast
 frame-relay map ip 10.10.10.3 103 broadcast
 no frame-relay inverse-arp

R2 :


R2#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
R       1.1.1.1 [120/1] via 10.10.10.1, 00:00:02, Serial1/0
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

R3 :


R3#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
R       1.1.1.1 [120/1] via 10.10.10.1, 00:00:29, Serial1/0
     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

As you can see above, routers R2 and R3 cannot see each other’s loopback interface, this is because ip split-horizon is enabled by default on frame-relay subinterfaces. So router R1 cannot send the updates it receives from router R2 to router R3 and vice versa. After disabling ip split-horizon on router R1’s subinterface :

R1 :


interface Serial1/0.1 multipoint
 ip address 10.10.10.1 255.255.255.0
 no ip split-horizon
 frame-relay map ip 10.10.10.2 102 broadcast
 frame-relay map ip 10.10.10.3 103 broadcast
 no frame-relay inverse-arp
end

R2 :


R2#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
R       1.1.1.1 [120/1] via 10.10.10.1, 00:00:03, Serial1/0
     2.0.0.0/32 is subnetted, 1 subnets
C       2.2.2.2 is directly connected, Loopback0
     3.0.0.0/32 is subnetted, 1 subnets
R       3.3.3.3 [120/2] via 10.10.10.3, 00:00:03, Serial1/0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

R3 :


R3#sh 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

     1.0.0.0/32 is subnetted, 1 subnets
R       1.1.1.1 [120/1] via 10.10.10.1, 00:00:19, Serial1/0
     2.0.0.0/32 is subnetted, 1 subnets
R       2.2.2.2 [120/2] via 10.10.10.2, 00:00:19, Serial1/0
     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     20.0.0.0/24 is subnetted, 1 subnets
C       20.20.20.0 is directly connected, FastEthernet2/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, Serial1/0

As you can see above after disabling ip split-horizon router R1 sends updates out the interface it receives updates on. You can see the status of ip split-horizon on an interface by issueing the “sh ip int -interface-”

1 Comment

KonstantinMillerJuly 7th, 2009 at 12:31 am

Hi. I like the way you write. Will you post some more articles?

Leave a comment

Your comment