Bridging, Switching, and Spanning Tree 157
Table 4-7 lists several features relating to segmenting LANs with bridges, switches, and routers.
Essentially, this chart summarizes features that could differ among the three devices. Table 4-8
lists features that describe how each device performs when compared to using a single Ethernet
segment with no bridge, switch, or router. The two tables together provide the necessary details
when comparing the three types of devices.
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 4-8 lists features that should 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 4-7
Comparison of Segmentation Options
Feature
Bridging
Switching
Routing
Forwards LAN broadcasts?
Yes
Yes
No
Forwards LAN multicasts?
Yes
Yes; can be
optimized with
CGMP
No
1
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 4-8
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
ch04.fm Page 157 Monday, March 20, 2000 5:02 PM