754 Appendix A: Answers to the "Do I Know This Already?" Quizzes and Q&A Sections
38
Assume that a router is configured to allow only one route in the routing table to each
destination network. If more than one route to a particular subnet is learned, and if each
route has the same metric value, which route is placed into the routing table if the routing
protocol uses distance vector logic?
In this scenario, the first route learned is placed into the table. If that route is removed at
a later time, the next routing update received after the original route has been removed is
added to the routing table.
39
Describe the purpose and meaning of route poisoning.
Route poisoning is the distance vector routing protocol feature in which a newly bad route
is advertised with an infinite metric. Routers receiving this routing information then can
mark the route as a bad route immediately. The purpose is to prevent routing loops.
40
Describe the meaning and purpose of triggered updates.
A triggered update is the routing protocol feature in which an update is sent immediately
when new routing information is learned rather than waiting on a timer to complete before
sending another routing update.
Answers to the Chapter 7 "Do I Know This Already?" Quiz
1
Configure a numbered IP access list that would stop packets from subnet 134.141.7.0,
255.255.255.0, from exiting serial 0 on some router. Allow all other packets.
access-list 4 deny 134.141.7.0 0.0.0.255
access-list 4 permit any
interface serial 0
ip access-group 4
The first access list statement denied packets from that subnet. The other statement is
needed because of the default action to deny packets not explicitly matched in an access
list statement.
2
How would a user who does not have the enable password find out what access lists have
been configured and where they are enabled?
The show access-list command lists all access lists. The show ip interfaces and show ipx
interfaces commands identify interfaces on which the access lists are enabled.
3
How many IP extended access-list commands are required to check a particular port
number on all IP packets?
Two statements are required. If the protocol type IP is configured, the port number is not
allowed to be checked. So, the TCP or UDP protocol type must be used to check the port
numbers. Therefore, if port 25 needs to be checked for both TCP and UDP, two statements
are needed: one for TCP and one for UDP.
apA.fm Page 754 Monday, March 20, 2000 5:24 PM