Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 131

Configuration of Bridge Between Ethernet 0 and Ethernet 1 (Continued) Example 4-4

bridge-group 1
!
interface ethernet 1
bridge-group 1

Tr ansparent Bridge Modes
Bridges and switches can operate in two basic modes: store-and-forward and cut-through . The
legacy mode is store-and-forward. The bridge stores the entire frame and verifies the CRC
before forwarding the frame. If a CRC error is detected, the frame is discarded.

With the evolution of bridges and switches with fast integrated circuits, a new mode of
operation is usually available: cut-through. In this mode, the switch does not wait for the entire
frame to enter its buffer; instead, it forwards the frame just after it reads the destination MAC
address. The advantage is that the switching operation is faster. The disadvantage is that the cut-
through operation does not check for CRC errors, which increases the amount of runt and error
frames on the network. Because of this problem, most switches provide the option to switch the
mode back to store-and-forward if an error threshold is met. Switches in cut-through can track
the CRC errors of forwarded frames to determine if there are too many errors. Other switches
implement modified cut-through, which waits to receive 64 bytes before it begins transmission,
which prevents the switch from sending runt frames.

STP

When multiple bridges connect the same LAN segments, bridge loops can occur. The STP is a
Layer-2 link management protocol that discovers a loop free topology for connectivity between
LANs. As shown in Figure 4-16, STP might place some redundant bridged interfaces in a
blocked state. STP provides recovery from bridge failures by changing blocked interfaces to a
forwarding state, if a primary link fails. Although a DEC and IBM version are available, the
IEEE 802.1d standard is the default protocol.

Root Bridge Election
A root bridge is elected as the root of the spanning-tree topology. All ports that are not needed
to reach the root bridge are placed in blocking mode. The selection of the root bridge is based
on the lowest numerical bridge priority. The bridge priority ranges from 0 to 65535. If all
bridges have the same bridge priority, the bridge with the lowest MAC address is selected as the
root. Physical changes of the network force spanning-tree recalculation.