204 Chapter 4: Bridges/Switches and LAN Design
1. Routers can forward IP multicast packets, if configured to do so. However, this does not mean that the LAN
multicast frame is forwarded.
Table 4-25 lists features that must be interpreted within the following context: "If I migrated
from a single Ethernet segment to a network with two segments separated by a bridge/switch/
router, and if traffic loads and destinations stayed constant, the result would be _______."
Table 4-26 summarizes the reasons why Spanning Tree places a port into forwarding or
blocking state:
OSI layer used when making forwarding decision
Layer 2
Layer 2
Layer 3
Internal processing variants
Store-and-
forward
Store-and-
forward, cut-
through,
FragmentFree
Store-and-
forward
Frame/packet fragmentation allowed?
No
No
Yes
Multiple concurrent equal-cost paths to same
destination allowed?
No
No
Yes
Table 4-25
Comparison of a Single Segment to Multiple Segments
Feature
Bridging
Switching
Routing
Greater cabling distances allowed
Yes
Yes
Yes
Decrease in collisions, assuming equal traffic loads
Yes
Yes
Yes
Decreased adverse impact of broadcasts
No
No
Yes
Decreased adverse impact of multicasts
No
Yes, with
CGMP
Yes
Increase in bandwidth
Yes
Yes
Yes
Filtering on Layer 2 header allowed
Yes
Yes
Yes
Filtering on Layer 3 header allowed
No
No
Yes
Table 4-26
Spanning Tree--Reasons for Forwarding or Blocking
Characterization
of Port
Spanning Tree State
Explanation
All root bridge's ports
Forwarding
The root bridge is always the designated bridge
on all connected segments.
Each nonroot bridge's
root port
Forwarding
The root port is the port receiving the lowest-
cost CBPDU from the root.
Table 4-24
Comparison of Segmentation Options (Continued)
Feature
Bridging
Switching
Routing
ch04.fm Page 204 Monday, March 20, 2000 5:02 PM