In eigrp it is on by default on frame relay multipoint interface which it is not in
ripv2.
You can disable using
(where x is the AS number)
no ip split-horizon eigrp x
It is on in eigrp not really as loop prevention as eigrp has the dual mechanism but
more of an efficence of routing updates.
The split horizon rule prohibits a router from advertising a route through an interface that the router itself uses to reach the destination. In order to disable the split horizon behavior, use the no ip split-horizon eigrp as-number interface command. Some important points to remember about EIGRP split horizon are:
* Split horizon behavior is turned on by default.
* When you change the EIGRP split horizon setting on an interface, it resets all adjacencies with EIGRP neighbors reachable over that interface.
* Split horizon should only be disabled on a hub site in a hub-and-spoke network.
* Disabling split horizon on the spokes radically increases EIGRP memory consumption on the hub router, as well as the amount of traffic generated on the spoke routers.
* The EIGRP split horizon behavior is not controlled or influenced by the ip split-horizon command.
LAB
----
I will do a lab on this even though it is very similar to the rip split horizon we
can see some interesting behaviour by dual. So R1 is the hub in the frame relay it
using main point multipoint interface both r5 and r2 have circuits to r1 but not to
each other also r5 has a direct serial connection to r4 which is not in the frame
relay network
r4
---
r4(config)#no router rip
r4(config)#router eigrp 1
r4(config-router)#network 44.44.44.44 0.0.0.0
r4(config-router)#no auto
r4(config-router)#network 10.17.17.0 0.0.0.255
r4(config-router)#exit
Serial0/1/0 10.17.17.4 YES manual down down
Loopback0 44.44.44.44 YES manual up up
r4(config)#
r5
---
r5#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES TFTP up down
FastEthernet0/1 unassigned YES TFTP administratively down down
Serial0/0/0 unassigned YES NVRAM up up
Serial0/0/0.1 10.229.254.5 YES manual up up
Serial0/1/0 10.17.17.5 YES manual up up
r5(config)#router eigrp 1
r5(config-router)#network 10.17.17.0 0.0.0.255
r5(config-router)#network 10.229.254.0 0.0.0.255
r5(config-router)#no auto
r2
--
r2#sh ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES TFTP up down
Serial0/0 unassigned YES NVRAM up up
Serial0/0.1 10.229.254.2 YES manual down down
Serial0/1 unassigned YES NVRAM down down
r2(config)#router eigrp 1
r2(config-router)#network 10.229.254.0 0.0.0.255
r2(config-router)#no auto
r2(config-router)#
r1 the hub
---
r1(config)#int s0/0
r1(config-if)#encap frame-relay
r1(config-if)#ip address 10.229.254.1 255.255.255.0
r1(config-if)#frame-relay map ip 10.229.254.2 102
r1(config-if)#frame-relay map ip 10.229.254.5 105
r1(config-if)#no shut
r1(config)#router eigrp 1
r1(config-router)#no auto
r1(config-router)#network 10.229.254.0 0.0.0.255
r1(config-router)#
i kept having problems with eigrp going up and down this is because i forgot the broadcast keyword in my map statements added and back working fine
I also added on r2 a loopback just so we would see it in the tables
r2(config)#int lo0
r2(config-if)#ip addres
*Mar 1 00:48:34.643: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to ups 22.22.22.22 255.255.255.255
r2(config-if)#exit
r2(config)#router eigrp 1
r2(config-router)#network 22.22.22.22 0.0.0.0
r2(config-router)#
ok on
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
22.0.0.0/32 is subnetted, 1 subnets
D 22.22.22.22 [90/2297856] via 10.229.254.2, 00:00:59, Serial0/0
10.0.0.0/24 is subnetted, 2 subnets
D 10.17.17.0 [90/2681856] via 10.229.254.5, 00:03:58, Serial0/0
C 10.229.254.0 is directly connected, Serial0/0
44.0.0.0/32 is subnetted, 1 subnets
D 44.44.44.44 [90/2809856] via 10.229.254.5, 00:03:58, Serial0/0
r1#
I am seeing all networks
on r2 i am not seeing any of r5 networks
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
22.0.0.0/32 is subnetted, 1 subnets
C 22.22.22.22 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 1 subnets
C 10.229.254.0 is directly connected, Serial0/0.1
r2#
r5#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
10.0.0.0/24 is subnetted, 2 subnets
C 10.17.17.0 is directly connected, Serial0/1/0
C 10.229.254.0 is directly connected, Serial0/0/0.1
44.0.0.0/32 is subnetted, 1 subnets
D 44.44.44.44 [90/2297856] via 10.17.17.4, 00:35:53, Serial0/1/0
r5#
same on r5 not seeing any of r2 networks
ok so back to r1 and i will disable split horizon
r1(config)#int s0/0
r1(config-if)#no ip split-?
split-horizon
r1(config-if)#no ip split-horizon
r1(config-if)#
this got me for while it was not working
r1#sh ip int
FastEthernet0/0 is administratively down, line protocol is down
Internet protocol processing disabled
Serial0/0 is up, line protocol is up
Internet address is 10.229.254.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Multicast reserved groups joined: 224.0.0.10
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Local Proxy ARP is disabled
Security level is default
Split horizon is disabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP fast switching on the same interface is enabled
IP Flow switching is disabled
IP CEF switching is enabled
IP CEF Fast switching turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Policy routing is disabled
Network address translation is disabled
BGP Policy Mapping is disabled
WCCP Redirect outbound is disabled
WCCP Redirect inbound is disabled
WCCP Redirect exclude is disabled
Serial0/1 is administratively down, line protocol is down
Internet protocol processing disabled
eventhough split horizon was disabled i still was not getting the routes
there is a inbuilt split horizon that needs to be disabled
r1(config-if)#int s0/0
r1(config-if)#no ip split-horizon eigrp 1
r1(config-if)#
*Mar 1 00:57:33.815: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.229.254.5 (Serial0/0) is resync: split horizon changed
*Mar 1 00:57:33.815: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.229.254.2 (Serial0/0) is resync: split horizon changed
I am not seeing all routes
r5#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
22.0.0.0/32 is subnetted, 1 subnets
D 22.22.22.22 [90/2809856] via 10.229.254.1, 00:01:26, Serial0/0/0.1
10.0.0.0/24 is subnetted, 2 subnets
C 10.17.17.0 is directly connected, Serial0/1/0
C 10.229.254.0 is directly connected, Serial0/0/0.1
44.0.0.0/32 is subnetted, 1 subnets
D 44.44.44.44 [90/2297856] via 10.17.17.4, 00:01:26, Serial0/1/0
r5#
I also done the no ip split-horizon eigrp 1 on the spokes r5 and r2
r5#sh ip eigrp top all-
IP-EIGRP Topology Table for AS(1)/ID(5.5.5.5)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 22.22.22.22/32, 1 successors, FD is 2809856, serno 15
via 10.229.254.1 (2809856/2297856), Serial0/0/0.1
P 44.44.44.44/32, 1 successors, FD is 2297856, serno 4
via 10.17.17.4 (2297856/128256), Serial0/1/0
via 10.229.254.1 (3321856/2809856), Serial0/0/0.1
P 10.17.17.0/24, 1 successors, FD is 2169856, serno 3
via Connected, Serial0/1/0
via 10.229.254.1 (3193856/2681856), Serial0/0/0.1
P 10.229.254.0/24, 1 successors, FD is 2169856, serno 14
via Connected, Serial0/0/0.1
via 10.229.254.1 (2681856/2169856), Serial0/0/0.1
if we look at the router on the directly connected r4 44.44.44.44
P 44.44.44.44/32, 1 successors, FD is 2297856, serno 4
via 10.17.17.4 (2297856/128256), Serial0/1/0
via 10.229.254.1 (3321856/2809856), Serial0/0/0.1
I am now getting it via r4 and also via r1 there is duplicate.
I have the route directly via r4 and i am also reciveing back the route
via r1 that i sent it. This is potential loop but the dual algorithm will not
allow this as backup route as the route via r4 is selected as the best route with a
feasible distance of 2297856 and the route via r1 is not a feasible successor as it advertised distance 2809856 is not less than the route via r4 feasible distance so this prevents a potential loop
The reason split horizon is on by default in eigrp is to stop these duplicate routes been recieved not from a loop prevention standpoint as we see dual will take care of the loop prevention but from an efficency of the routing protocol of having to process this additional route unneedless this of course unless you are is in the example in partial mesh nmba network
if SPLIT HORIZON is used for avoiding loop how does disabling prevents loop in eigrp in this scenario.
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