Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 168

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Blocking —When a port is enabled, it first moves to a blocked state before listening to the
network. In this state, it does not participate in frame forwarding. It receives Bridge PDUs
and sends them for processing.

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Listening —When the bridge determines that the port should participate in frame
forwarding, it changes to the listening state. In this state, the bridge still does not forward
frames and does not learn of network MAC addresses. The bridge does receive and
process BPDUs and network management frames.

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Learning —The bridge port still discards incoming frames or frames switched over for
forwarding. The bridge begins to incorporate MAC addresses associated with this port.
BPDU and network management messages are still processed.

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Forwarding —This is the full functional state for a bridged port. In this state, the bridge
does not discard incoming frames and forwards them to other ports; the bridge also
forwards frames out the port. BPDUs and network management frames are still processed.

STP

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The selection of the root bridge is based on the lowest numerical bridge priority.

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The bridge with the lowest source MAC address breaks a tie.

CRB

CRB enables the router to both route and bridge the same protocol on separate interfaces. With
IRB, a protocol can be routed between routed interfaces, bridged interfaces, or different bridge
groups.

The access list numbers for MAC address filters are from 700 to 799. The access list for
EtherType filters are from 200 to 299.

Methods for Using SRBs Table 4-18

Bridge Type Function

Source route bridging
(SRB)

The source determines the route to use to reach the destination node before
sending an information frame to it. The bridge does not store MAC addresses.
Limited to 7 bridge hops; explorer frames used by stations.

Source route
transparent bridging
(SRT)

SRT bridges can forward traffic from both transparent and source-route end
nodes and form a common spanning tree with transparent bridges.

Source-route
translational bridging
(SR/TLB)

Bridge between SRB and transparently bridged networks. Look at MTU,
reorder bits, convert frames.