For PPP or HDLC, there are only two devices on each link. These protocols
have no need to specifically address the far end device as there is only one
possible recipient of the frame. In the case of PPP the frame always contains
the all 1’s broadcast address and is therefore intended for any device at the
far end of the link. Because of the nature of point-to-point protocols, it is
unlikely that a Layer 2 to Layer 3 address mapping error will occur.
In
topologies such as point-to-multipoint, Frame Relay, or broadcast Ethernet, it
is essential that an appropriate Layer 2 destination address be given to the
frame. This will ensure its arrival at the correct destination. To achieve
this, the network device must match a destination Layer 3 address with the
correct Layer 2 address. Two mechanisms can be used to achieve this:
- Static maps
- Dynamic maps (ARP)
When using static maps, an incorrect map is a common mistake. In an
Ethernet environment this can easily occur when a NIC is replaced, since the
NIC determines the MAC address. In a Frame-Relay environment it is possible for
DLCIs to be incorrectly assigned by the Telco. In either case, simple
configuration errors can result in a mismatch of Layer 2 and Layer 3 addressing
information.
In a dynamic environment, the mapping of Layer 2 and Layer
3 information can fail for the following reasons:

- Devices may not respond to ARP or Inverse-ARP requests because this has
been specifically configured, as with the command no frame-relay
inverse-arp.
- The Layer 2 or 3 information that is cached may have physically changed. A
router caches MAC addresses for 14400 seconds, or 4 hours, by default.
- Invalid ARP replies are received which could be due to misconfiguration, or
as part of a Denial of Service (DoS) or Man-in-the-middle security attack.
Useful commands for troubleshooting suspected Layer 2 – Layer 3 mapping
problems include:
-
show arp – examine ARP cache.

-
show cdp neighbor detail – determine neighbors IP
address.
-
clear arp-cache – force relearning of MAC addresses
when a NIC has been changed.
-
show frame-relay map – determine DLCI to IP address
mapping.
