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Default split-horizon on frame-relay interface?

Hi All,

I am confusing about the split-horizon in frame-relay interface. I remember, enable frame-relay will disable the split-horizon in the frame-relay interface, that including the sub-interface in the hub-and-spoke topology. But I am not sure. Can anybody give me some idea? Thank you!

Waimen

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Default split-horizon on frame-relay interface?

IP split horizon checking is disabled by default for Frame Relay encapsulation so routing updates will come in and out the same interface. The routers learn the data-link connection identifiers (DLCIs) they need to use from the Frame Relay switch via Local Management Interface (LMI) updates. The routers then use Inverse ARP for the remote IP address and create a mapping of local DLCIs and their associated remote IP addresses. Additionally, certain protocols such as AppleTalk, transparent bridging, and IPX cannot be supported on partially meshed networks because they require "split horizon," in which a packet received on an interface cannot be transmitted out the same interface, even if the packet is received and transmitted on different virtual circuits. Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows us to overcome split horizon rules. Packets received on one virtual interface can now be forwarded out another virtual interface, even if they are configured on the same physical interface.

http://www.cisco.com/en/US/tech/tk826/tk365/technologies_tech_note09186a0080093fdd.shtml

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca569.html#4863

http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a00800942ab.shtml

Nilesh

1 REPLY
Cisco Employee

Re: Default split-horizon on frame-relay interface?

IP split horizon checking is disabled by default for Frame Relay encapsulation so routing updates will come in and out the same interface. The routers learn the data-link connection identifiers (DLCIs) they need to use from the Frame Relay switch via Local Management Interface (LMI) updates. The routers then use Inverse ARP for the remote IP address and create a mapping of local DLCIs and their associated remote IP addresses. Additionally, certain protocols such as AppleTalk, transparent bridging, and IPX cannot be supported on partially meshed networks because they require "split horizon," in which a packet received on an interface cannot be transmitted out the same interface, even if the packet is received and transmitted on different virtual circuits. Configuring Frame Relay subinterfaces ensures that a single physical interface is treated as multiple virtual interfaces. This capability allows us to overcome split horizon rules. Packets received on one virtual interface can now be forwarded out another virtual interface, even if they are configured on the same physical interface.

http://www.cisco.com/en/US/tech/tk826/tk365/technologies_tech_note09186a0080093fdd.shtml

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca569.html#4863

http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a00800942ab.shtml

Nilesh

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