notes: Frame Relay Interface Types & Split Horizon

Frame-relay interfaces carry one of two characteristics: point-to-point or multipoint.

1. Physical interfaces

- Are treated as multipoint interfaces.
- Multipoint means the interface can terminate multiple PVC's(layer2 circuits).
- Requires layer3-to-layer2 resolution through either InARP or manual mapping. (Refer to previous section)
- Manual mapping per PVC is done with the "frame map ip" command.
- To manually assign just one PVC on the interface use "frame-relay interface-dlci".
- may need to disable ip split horizon depending on the routing protocol.
- dont use point-to-point or point-to-multipoint subinterface
- all dlcis are recieved on physical interface, n need an interface-dlci command.


2. Point-to-Point sub-interfaces

- Can only terminate one PVC.
- Do not require layer3-to-layer2 resolution, since there is only one PVC.
- Inverse-arp is disable when in Point-to-point. Do not send InARP status queries, but will respond to an InARP status query request.
- require frame-relay interface dlci command
- will have a type mismatch with physical & point to multipoint interfaces.
- broadcast will work since it is point to point.


3. Multipoint sub-interfaces

- Are treated as multipoint interfaces.
- Can terminate multiple PVCs.
- Requires layer3-to-layer2 resolution through either InARP or manual mappings.
- Manual mapping per PVC is done with the "frame map ip" command.
- To manually assign just one PVC on the interface use "frame-relay interface-dlci".
- may need to disable split-horizon
- will have a type mismatch with point-to-point sub interface


Split Horizon

- multipoint interface will need to think about split horizon ( FR, ATM & Ethernet)
- used by RIP & EIGRP


Interface states

1. The physical interface connecting to a frame-relay switch will be up/up, once it receives LMI from that frame-relay switch, regardless of the DLCI it is learning or not learning.
- This means a physical interface can be up/up, even though there is no layer2 communication.

2. But with a point-to-point sub-interface, the sub-interface will only show up/up, when LMI is received and one of the received DLCIs matches the DLCI configured on the sub-interface.

3. When a multipoint sub-interface has multiple DLCI's defined, all DLCI's must be down before the interface will show down/down. If one DLCI is up, the interface will be up/up.


TIPS:

1. When removing a frame-relay sub-interface configuration, the configuration is removed off the interface, but the sub-interface will only be deleted after a reboot.
- This can be seen with a "sh ip int brief" when the interface is listed as DELETED.
- Thus to change a sub-interface from point-to-point to multipoint, delete the sub-interface and reload the router. Then create new multipoint interface.

2. Always do "show frame-relay map" when starting a lab and after configuration is complete to verify layer2 connectivity.
If there are 0.0.0.0 frame-relay mappings, save the configuration and reload. It is the only way to get rid of it.

3. To ping a locally configured IP on a frame-relay interface, layer3-to-layer2 resolution is required. This is needed because the frame actually exits the router to the other side of the link only to get redirected back because of the remote IP. If the mapping is not done, the ping reply is dropped by the router on the other side of the link.