228 Chapter 7: Using OSPF Across Multiple Areas
Scenario 7-3 Answers
1. There are problems with Router B. There is inconsistency in the routing table, and the system
is extremely slow. What commands would be used to identify the problem? In examining
the diagram and configuration, what problems can you see?
Router B has been configured to be the designated router for the LAN, which means that it is
dealing with all the traffic on the LAN associated with the management of OSPF. Given that
the system is an older Cisco router, it is a poor choice for a designated router. A better choice
would be Router A, which is a larger system that connects directly to Area 0, making it a better
choice from the standpoint of the network design. If Router B were a larger system, there
could be an argument for making it the designated router to elevate Router A, which would
otherwise be functioning as both the ABR and the designated router.
The router has not been configured as a stub, so the communication between Router A and
Router B will be confused, preventing any communication between the two routers.
2. Router A is having problems connecting to Area 0. This is causing problems in other areas
because Router A is used to connect to Area 0. What commands would be used to identify the
problem? In examining the diagram and configuration, what problems can you see?
Router A is configured incorrectly. The command that would show the problem would be
either show ip route, show ip protocols, or show ip ospf database. The lack of LSA traffic
would indicate a configuration problem. When examining the configuration, you would see
that the mask on the configuration of the network command for Area 0 is wrong. The mask
for 140.100.32.0 should be 0.0.0.15. Using the mask of 0.0.0.0 will place only interfaces with
the IP address 140.100.32.0, which is a subnet address not an interface address. Therefore,
there will be no communication of OSPF LSAs between the areas.
3. Issue the commands that would be used to correct the configuration problems that you see in
the example configuration for Routers A and B.
The commands that would solve these problems are as follows:
On Router A:
rr
rr
oo
oo
uu
uu
tt
tt
ee
ee
rr
rr
oo
oo
ss
ss
pp
pp
ff
ff
11
11
00
00
00
00
nn
nn
ee
ee
tt
tt
ww
ww
oo
oo
rr
rr
kk
kk
11
11
44
44
0
0
00
..
..
11
11
00
00
00
00
..
..
33
33
22
22
..
..
00
00
00
00
..
..
00
00
..
..
00
00
..
..
11
11
55
55
aa
aa
rr
rr
ee
ee
aa
aa
00
00
ii
ii
nn
nn
tt
tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee
ff
ff
aa
aa
ss
ss
tt
tt
ee
ee
tt
tt
hh
hh
ee
ee
rr
rr
nn
nn
ee
ee
tt
tt
11
11
nn
n
n
oo
oo
ii
ii
pp
pp
oo
oo
ss
ss
pp
pp
ff
ff
cc
cc
oo
oo
ss
ss
tt
tt
11
11
00
00
ii
ii
pp
pp
oo
oo
ss
ss
pp
pp
ff
ff
pp
pp
rr
rr
ii
ii
oo
oo
rr
rr
ii
ii
tt
tt
yy
yy
11
11
00
00
00
00
On Router B:
rr
rr
oo
oo
uu
uu
tt
tt
ee
ee
rr
rr
oo
oo
ss
ss
pp
pp
ff
ff
22
22
00
00
00
00
nn
nn
ee
ee
tt
tt
ww
ww
oo
oo
rr
rr
kk
kk
11
11
44
44
0
0
00
..
..
11
11
00
00
00
00
..
..
11
11
33
33
..
..
00
00
00
00
..
..
00
00
..
..
00
00
..
..
11
11
55
55
aa
aa
rr
rr
ee
ee
aa
aa
22
22
aa
aa
rr
rr
ee
ee
aa
aa
22
22
ss
ss
tt
tt
uu
uu
bb
bb
ii
ii
nn
nn
tt
tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee
FF
FF
aa
aa
ss
ss
tt
t
t
EE
EE
tt
tt
hh
hh
ee
ee
rr
rr
nn
nn
ee
ee
tt
tt
00
00
nn
nn
oo
oo
ii
ii
pp
pp
oo
oo
ss
ss
pp
pp
ff
ff
pp
pp
rr
rr
ii
ii
oo
oo
rr
rr
ii
ii
tt
tt
yy
yy
11
11
00
00
00
00
Scenario 7-3 Answers 229
4.
When you issue the show ip ospf interface command, you notice that there is a discrepancy
in the timers on the link between Routers A and B. The transmit timer on Router A is set to 5,
and the retransmit timer is set to 1. What problems would this cause? What command
would be used to change the timers, and what are the default settings?
The default setting for the transmit timer is set to 1 second, and the retransmit timer is set
to 5 seconds. The transmit timer determines the estimated number of seconds that it takes to
send an LSA to a neighbor. The retransmit timer states the number of seconds to wait for
an acknowledgment before retransmitting an LSA.
If the transmit timer is not smaller than the retransmit timer, the interface retransmits in the
belief that the other side did not receive the LSA. This leads to excess traffic, confusion
in the topology database, and the possibility of flapping links. To correct the settings, issue
the following subinterface commands:
ii
ii
pp
pp
oo
oo
ss
ss
pp
pp
ff
ff
rr
rr
ee
ee
tt
tt
rr
rr
aa
aa
nn
nn
ss
ss
mm
mm
ii
ii
tt
tt
--
--
ii
ii
nn
nn
tt
tt
ee
ee
rr
rr
vv
vv
aa
aa
ll
ll
ss
ss
ee
ee
cc
cc
oo
oo
nn
nn
dd
dd
ss
ss
ii
ii
pp
pp
oo
oo
ss
ss
pp
pp
ff
ff
tt
tt
rr
rr
aa
aa
nn
nn
ss
ss
mm
mm
ii
ii
tt
tt
--
--
dd
dd
ee
ee
ll
ll
aa
aa
yy
yy
ss
ss
ee
ee
cc
cc
oo
oo
nn
nn
dd
dd
ss
ss
This chapter covers the
following topics:
■ Special OSPF Areas—Describes how
OSPF design uses stub, totally stubby, and
not-so-stubby areas to optimize resource
utilization.
■ Configuring OSPF Authentication—
Describes the crucial topic of configuring
specialized OSPF area types and MD5
authentication for OSPF. It also describes a
few basic OSPF show and debug commands,
which are sufficient for most verification and
troubleshooting needs.
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