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Answers to Scenario 6-3: IP Addressing and Subnet Derivation 449
Answers to Task 4 for Scenario 6-2
Task 4 for Scenario 6-2 requires consideration of the effects of split horizon. Split horizon logic
considers subinterfaces to be separate interfaces, in spite of the fact that several subinterfaces
share the same physical interface. Boston advertises about 10.1.4.0 in its RIP update only out
its subinterface 1. All other routes in Boston's routing table were learned through RIP updates
from Atlanta, via updates entering that same subinterface; therefore, Boston will not advertise
about those routes in updates it sends on that same subinterface.
The RIP updates from Atlanta to Charlotte, out Atlanta's subinterface 1, advertise about all
subnets not learned from RIP updates entering that same subinterface. All subnets except
10.1.2.0 (learned from Charlotte) and 10.2.1.0 (subinterface 1's subnet) will be listed in
Atlanta's RIP update to Charlotte. Subnet 10.1.4.0, learned from Boston, will indeed be
included in updates to Charlotte; split horizon considers subinterfaces as separate interfaces.
Answers to Scenario 6-3: IP Addressing and Subnet
Derivation
Refer back to the network illustrated in Figure 6-25 and Example 6-24, Example 6-25, and
Example 6-26 to establish the Scenario 6-3 design details and the context of the answers to the
three tasks for this scenario.
Answers to Task 1 for Scenario 6-3
Task 1 for Scenario 6-3 asks you to complete a table with the subnet numbers and broadcast
addresses used in this scenario's network after examining the show commands on the various
routers in Example 6-24, Example 6-25, and Example 6-26. Table 6-38 lists the subnet numbers
and broadcast addresses requested in this task.
Table 6-38
Subnets and Broadcast Addresses
Location of Subnet
Geographically
Subnet Mask
Subnet Number
Subnet Broadcast
Ethernet off router in
Mayberry
255.255.255.0
170.1.101.0
170.1.101.255
Ethernet off router in Mount
Pilot
255.255.255.0
170.1.102.0
170.1.102.255
Ethernet off router in
Raleigh
255.255.255.0
170.1.103.0
170.1.103.255
VC between Mayberry and
Mount Pilot
255.255.255.0
170.1.1.0
170.1.1.255
continues
ch06.fm Page 449 Monday, March 20, 2000 5:11 PM