Answers to Chapter 4 Troubleshooting Exercises

1:	Identify the mistake in the configuration in Example 4-33. Example 4-33 Configuration for Troubleshooting Exercise 1 ip nat pool EX1 192.168.1.1 192.168.1.254 netmask 255.255.255.0 type match-host ip nat pool EX1A netmask 255.255.255.240 address 172.21.1.33 172.21.1.38 address 172.21.1.40 172.21.1.46 ip nat inside source list 1 pool EX1 ip nat inside source static 10.18.53.210 192.168.1.1 ip nat outside source list 2 pool EX1A ! access-list 1 permit 10.0.0.0 0.255.255.255 access-list 2 permit 192.168.2.0 0.0.0.255
A:	The IG address in the static mapping overlaps with the pool EX1.
2:	RTR1 in Figure 4-30 connects two internetworks with overlapping addresses. Figure 4-30. The Internetwork for Troubleshooting Exercise 2 NAT is implemented on the router as configured in Example 4-34, but devices cannot communicate across the router. What is wrong? Example 4-34 Configuration for Troubleshooting Exercise 2 interface Ethernet0 ip address 172.16.10.1 255.255.255.0 ip nat inside ! interface Ethernet1 ip address 172.16.255.254 255.255.255.0 ip nat outside ! router ospf 1 redistribute static metric 10 metric-type 1 subnets network 10.0.0.0 0.255.255.255 area 0 ! ip nat translation timeout 500 ip nat pool NET1 10.1.1.1 10.1.255.254 netmask 255.255.0.0 ip nat pool NET2 192.168.1.1 192.168.255.254 netmask 255.255.0.0 ip nat inside source list 1 pool NET1 ip nat outside source list 1 pool NET2 ! ip classless ! ip route 10.1.0.0 255.255.0.0 Ethernet0 ip route 192.168.0.0 255.255.0.0 Ethernet1 ! access-list 1 permit 172.16.0.0 0.0.255.255
A:	The problem is not with the NAT itself, but with routing. All translations are dynamic, and there is no way for a host on either side to determine the initial address to which packets must be sent to reach the other side.
3:	Refer to the configurations of Cozumel and Guaymas in Figure 4-21. If the first line of access list 1 in both configurations is removed, what is the result? Can Guaymas and Cozumel still ping each other?
A:	When either router sends a packet to the other sourced from its E1 interface, the source address is translated to an address out of the IG pool. The two routers can still ping each other even if the source address is translated. If Cozumel pings Guaymas, for example, its source address of 10.255.13.254 might be translated to 206.100.176.50. Although Guaymas does not recognize this address as part of its directly connected subnet, it has a route to 206.100.176.0/20 pointing to Cozumel. When it sends a response to the ping, the response is forwarded to Cozumel, which translates the destination address back to 10.255.13.254.