SPLIT HORIZON A router will NOT advertise a route out the same interface it
received it in on. Try sub-interfaces.
Excerpt from Cisco
http://www.cisco.com/univercd/cc/td/doc/product/software/ssr83/rpc_r/53992.h
tm#xtocid2008062
Enabling and Disabling Split Horizon for IP Networks
Normally, routers that are connected to broadcast-type IP networks and that
use distance vector routing protocols employ the split horizon mechanism to
prevent routing loops. Split horizon blocks information about routes from
being advertised by a router out any interface from which that information
originated. This behavior usually optimizes communications among multiple
routers, particularly when links are broken. However, with nonbroadcast
networks such as frame relay and SMDS, situations can arise for which this
behavior is less than ideal.
Use the no ip split-horizon interface subcommand to disable the split
horizon mechanism.
ip split-horizon
no ip split-horizon
For all interfaces except those for which either frame relay or SMDS
encapsulation is enabled, the default condition for this command is ip
split-horizon; in other words, the split horizon feature is active. If the
interface configuration includes either the encapsulation frame-relay or
encapsulation smds commands, the default is for split horizon to be
disabled. Split horizon is not disabled by default for interfaces using any
of the X.25 encapsulations.
Note For networks that include links over X.25 PSNs, the neighbor interface
subcommand can be used to defeat the split horizon feature. You can as an
alternative explicitly specify the no ip split-horizon command in your
configuration. However, if you do so, you must similarly disable split
horizon for all routers in any relevant multicast groups on that network.
If split horizon has been disabled on an interface and you wish to enable
it, use the ip split-horizon interface subcommand to restore the split
horizon mechanism.
Note In general, Cisco recommends against changing the state of the default
for this interface subcommand unless you are certain that your application
requires doing so to properly advertise routes. Remember that if split
horizon is disabled on a serial interface (and that interface is attached to
a packet-switched network), you must disable split horizon for all routers
in any relevant multicast groups on that network.
Example:
The following illustrates a simple example of disabling split horizon on a
serial link. In this example, the serial link is connected to an X.25
network.
interface serial 0
encapsulation x25
no ip split-horizon
Example of Implicit Split Horizon Conditions
A typical situation in which the no ip split-horizon command would be useful
is illustrated in Figure 14-10a. This figure depicts two IP subnets that are
both accessible via a serial interface on Router C (connected to frame relay
network). In this example, the serial interface on Router C accommodates one
of the subnets via the assignment of a secondary IP address.
The Ethernet interfaces for Router A, Router B, and Router C (connected to
IP networks 12.13.50.0, 10.20.40.0, and 20.155.120.0) all have split horizon
enabled by default, while the serial interfaces connected to networks
128.125.1.0 and 131.108.1.0 all have split horizon disabled by default. The
partial interface configuration specifications for each router that follow
Figure 1-11 illustrate that the ip split-horizon interface subcommand is not
explicitly configured under normal conditions for any of the interfaces.
In this example, split horizon must be disabled in order for network
128.125.1.0 to be advertised into network 131.108.1.0, and vice versa. These
subnets overlap at Router C, interface S0. If split horizon were enabled on
serial interface S0, it would not advertise a route back into the frame
relay network for either of these subnets.
-----Original Message-----
From: Thomas [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, July 18, 2001 10:19 AM
To: [EMAIL PROTECTED]
Subject: EIGRP on NBMA of Frame Relay - help! [7:12834]
Hi All,
I have a hub-and-spoke topology with 3 routers. Router A acts as a hub
router with pure dynamic multipoint on the Frame Relay network (NBMA).
Router B and router C are spoke ones with no static mapping. The three
router running EIGRP protocol with the same Autonomous System number.
Problem: On router A, I can see all LAN/WAN subnets on router B and C
learning through EIGRP. However, on router B (the spoke) I cannot see the
subnets of router C (the other spoke of the hub), which it supposes to learn
throught EIGRP. Vice versa, router C cannot see routes to subnets on router
B. What's the problem with this picture? Any solution for this? The IP
scheme for the 3 routers as follow. By the way, I turned off the
"auto-summary" on EIGRP
Router A: WAN IP = 192.168.1.1 255.255.255.248
LAN IP = 10.1.1.1 255.255.0.0
Router B: WAN IP = 192.168.1.2 255.255.255.248
LAN IP = 10.2.1.1 255.255.0.0
Router C: WAN IP = 192.168.1.3 255.255.255.248
LAN IP = 10.3.1.1 255.255.0.0
Thanks All in advance!
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