notes: Frame Relay Address Resolution

Frame-relay networks are multi-access networks, which means that more than two devices can attach to the network, similar to LANs.
- Unlike LANs, you cannot send a data link layer broadcast over frame-relay. Therefore frame-relay networks are often called NBMA (nonbroadcast multi-access) network.
- Because frame-relay is a multi-access technology, it always needs layer3-to-layer2 address resolution to identify to which remote router a frame is destined too.
- The exceptions are frame-relay point-to-point sub-interface and PPP-over-frame-relay.

- Broadcast Replication

- Frame-relay does not have the capability to send a single frame over multiple PVC's to multiple destinations.
- But the broadcast functionality is still sometimes required by routing protocols.
- Also known a pseudo-broadcast, frame-relay can make duplicate copies of a packet and send one on each PVC.
- Frame-relay can thus send copies of layer3 broadcasts over VCs, if configured to do so.

Note: at the hub it is essential to use, to send broadcast/multicat, osfp and rip can send unicast to its neighbor.

Static Mappings

- Are used to statically resolve the REMOTE layer3 address(IP) to a LOCAL Layer2 address(DLCI).
- Are manually configured with the command "frame-relay map".
- Require broadcast to be enabled manually if needed.
- Static frame-relay mappings (frame-relay map) override dynamic mappings (via InARP).
- 0vveride dynamic mappings
- broadcast support must be manually configured with the "broadcast" keyword. ( this is important for routing protocols)

command:

frame-relay map ip  100.100.100.4 204 broadcast

Interface DLCI

- use on point to point and point to multipoint
- not used in conjunction w frame-relay map
- physical interface may receive  many DLCIs but a subinterface needs to know which DLCI to use.

command:

frame-relay interface-dlci 101

 

note: if need to enable TE, map-class applied to an interface it will be applied to all DLCI.

frame-relay interface-dlci 112
   class map-class name


InARP (Inverse ARP)

- Is used to dynamically resolve the REMOTE layer3 address(IP) to a LOCAL Layer2 address(DLCI).
- Is enabled automatically when an IP address is configured.
- Has auto-broadcast enabled by default.
- The InARP status query request can be disabled per DLCI or for all DLCIs on a interface. The InARP reply cannot be disabled!!
- The command "no frame-relay inverse-arp" configured on a physical interface stops the InARP query messages only for the physical interface, not the sub-interfaces. It must be configured on the sub-interfaces is needed.
- When a point-to-point interface is connected to a InARP disabled interface, the InARP disabled interface will still reply, provided an IP address is configured on that interface. On the querying router the
"sh frame-relay map" will still show that mapping as dynamic.
- when a dlci is placed on a subinterface it will not initiate inverse arp request, but it will still respond however.

Note: dynamically learned arp is always learned via broadcast.

CCIE LAB SCENARIO: not allowed to use inverse-arp

show frame-relay map ( you will see dynamic listed learned via inversearp)

if you dont turn-off inverse-arp at the beginning of the lab you will see magical connectivity you didnt expect. because on a fr switch perspective it is fully meshed.

issue the following commands sets

int s0/0
shutdown
encapsulation frame-relay
no frame-relay inverse-arp
no shut

also you can clear the fr in-arp

clear frame-relay inarp

commands:

Shows the DLCI mapping, status, dynamic/static, type, broadcast

sh frame-relay map

Disables InARP requests only for the DLCIs specified

no frame-relay inverse arp ip {dlci}

Statically map a remote IP address to a local DLCI [broadcast] Enables frame-relay broadcast relay across the PVC

frame-relay map ip {ip} {dlci} [broadcast]