User's Manual

312
Users Manual of CS-6306R
29.1.1.2 Running Mechanism
UDLD is a L2 protocol running on the LLC layer, which uses 01-00-0c-cc-cc-cc as its destination MAC
address. SNAP HDLC is similar to 0x0111. When it runs with layer-1 FEFI and automatic negotiation, the
completeness of a link in the physical layer and the logical link layer can be checked.
UDLD can provide some functions that FEFI and automatic negotiation cannot conduct, such as checking and
caching the neighbor information, shutting down any mis-configured port and checking the faults and
invalidation on the logical ports except the point-to-point logical ports.
UDLD adopts two basic mechanisms: learn the information about neighbors and save it in the local cache.
When a new neighbor is detected or a neighbor applies for synchronizing the cache again, a series of UDLD
probe/echo (hello) packets will be transmitted.
UDLD transmits the probe/echo packets on all ports and, when a UDLD echo information is received on the
ports, a detection phase and an authentication process are triggered. If all effective conditions are satisfied
(port is connected in two directions and the cable is correctly connected), this port will be up. Otherwise, the
port will be down.
Once a link is established and labeled as bidirectional, UDLD will transmit a probe/echo message every 15
seconds.
29.1.1.3 State of the Port
The UDLD interface may be in one of the following states:
Port state Remark
Detection Means that the interface is in detection state.
Unknown Means that the interface is in unknown state,
that is, it may be in detection state or it has not
conducted detection.
Unidirectional Means that the unidirectional connection has
been detected.
Bidirectional Means that the bidirectional connection has
been detected.
29.1.1.4 Maintaining the Cache of the Neighbor
UDLD transmits the Probe/Echo packets regularly on each active interface to maintain the completeness of
the neighbor’s cache. Once a Hello message is received, it will be saved in the memory temporally and an
interval that is defined by hold-time will also be saved. If the hold-time times out, the corresponding cache is
fully cleared. If a new Hello message is received in the hold-time, the new Hello message will replace the old
one and the timer will be reset to zero.
Once a UDLD-running interface is disabled or the device on the interface is restarted, all the caches on the
interface will be removed to maintain the completeness of the UDLD cache. UDLD transmits at least one
message to notify the neighbor to remove the corresponding cache items.