Download Cisco.200-105.1e.83q.vcex

Configuring a loopback interface on RouterA with an address of 192.168.1.50/24 would cause Router A to become the designated router (DR). The designated router (DR) is determined by the router with the highest interface priority number. If the priority numbers are tied, then the router with the highest router ID (RID) becomes the DR.  
The default priority number is 1, and can be configured as high as 255. Changing the priority to 0 would make the router ineligible to become the DR or the backup designated router (BDR). The ip ospf priority # command is used to manually configure a priority on a specific interface.  
Router IDs are determined first by the highest loopback IP address, followed by the highest IP address on an active physical interface. Thus, in the case of a priority tie, the router with the highest loopback IP address will have the highest RID, and will become the DR for the network segment.  
The current Router ID for a router can be determined by executing the show ip interface brief command. In the sample output of the show ip interface brief command below, the RID will be 10.108.200.5.  
Router# show ip interface brief 
Interface IP-Address OK? Method Status Protocol  
Ethernet0 10.108.00.5 YES NVRAM up up  
Ethernet1 unassigned YES unset administratively down down 
Loopback0 10.108.200.5 YES NVRAM up up  
Serial0 10.108.100.5 YES NVRAM up up  
Serial1 10.108.40.5 YES NVRAM up up  
Serial2 10.108.100.5 YES manual up up  
Serial3 unassigned YES unset administratively down down 
Neither Router B nor C will be the DR because the IP addresses on their physical interfaces are lower than 192.168.1.50/24. 
Router A will not be the backup designated router. Since it is the DR, it cannot also be the BDR. 
Router C will not be the BDR because its IP address is lower than that of Router B. Router B will be the BDR.  
References:
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

PPP is a Layer 2 protocol encapsulation type that supports both synchronous and asynchronous circuits and provides built-in security mechanisms. The encapsulation ppp interface configuration mode command is used to configure encapsulation for a PPP (Point to Point Protocol) serial link on a Cisco router. PPP encapsulation provides for router-to-router and host-to-network connections over both synchronous and asynchronous circuits. Serial links are configured to use Cisco High Level Data Link Control (HDLC) encapsulation, by default, on Cisco routers. The Cisco version of HDLC is incompatible with the industry standard version used on other router brands because it contains a type field that identifies the underlying network protocol being encapsulated by HDLC. This is a beneficial feature of Cisco HDLC but makes it incompatible with other router brands.  
For this reason, a Cisco router that is going to be connected to a non-Cisco router should be configured to use PPP instead of the default. The encapsulation ppp interface configuration mode command will do this. If you set one of the routers for PPP and leave the other router at the default encapsulation for a serial connection, the connection will fail due to incompatible encapsulation.  
You would use the show run command to verify matching encapsulation types. In the partial output of the show run command for two routers shown below, it can be seen that although one of the routers has the encapsulation ppp command in its configuration, the other does not. The absence of the encapsulation ppp command means that the default HDLC is being used. This incompatibility will cause both routers to report a serial interface up, line protocol down condition since the connection is live, but the Layer 2 framing is misconfigured. 
router1#show run                    router2#show run 
<output omitted>                    <output omitted> 
interface serial 0/0               interface serial 0/1 
encapsulation ppp  
If authentication between the routers is also required, the authentication pap, authentication ms-chap, or authentication chap commands could be used to apply Password Authentication Protocol (PAP), Microsoft Challenge Authentication Protocol (MS-CHAP), or Challenge Authentication Protocol (CHAP) authentication to the connection, respectively. 
A full configuration of a serial link for using PPP with authentication is as shown below:
Router1(config)#interface Serial0 
Router1(config-if)#encapsulation ppp 
Router1(config-if)#ppp authentication pap 
Note above that the third line enables PAP authentication, which is not secure. Alternately, you can use CHAP authentication (which is secure) with the ppp authentication chap command. Regardless of which authentication mechanism you choose, these authentication commands will only be accepted on an interface where PPP encapsulation has been enabled, which rules out any non-serial interfaces. 
The third type of encapsulation that can be configured on a serial WAN link is Frame Relay, which can be selected with the encapsulation frame relay command under the interface.  
In summary, the three encapsulation types available for WAN serial links are PPP, HDLC, and Frame Relay. The command for each is as follows, executed under the interface configuration prompt:
encapsulation ppp 
encapsulation hdlc 
encapsulation frame relay 
All other options are invalid commands. 
References:
http://docwiki.cisco.com/wiki/Point-to-Point_Protocol

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) port number 53 is assigned to Domain Name Service (DNS), which is used to convert hostnames into Internet Protocol (IP) addresses. 
Some common TCP and UDP port number assignments are as follows:
- port 25: Assigned to Simple Mail Transfer Protocol (SMTP), a TCP protocol used to send and receive e-mail messages.
- port 23: Assigned to Telnet to allow remote logins and command execution.
- port 21: Assigned to File Transfer Protocol (FTP). It is used to control FTP transmissions. Port number 20 is also used by FTP for FTP data.
- port 80: Assigned to Hypertext Transfer Protocol (HTTP), which is the base for transferring Web pages over the Internet.
References:
http://docwiki.cisco.com/wiki/Internetworking_Basics#Multiplexing_Basics

There are two authentication methods available when implementing a PPP connection: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).
Challenge Handshake Authentication Protocol (CHAP) uses a one-way hash function based on the Message Digest 5 (MD5) hashing algorithm to hash the password. This hashed value is then sent across the wire. In this situation, the actual password is never sent. No one tapping the wire will be able to reverse the hash to come up with the original password. This is why MD5 is referred to as a one-way function. It cannot be reverse engineered. CHAP uses a three-way handshake process to perform the authentication. Moreover, CHAP periodically repeats the authentication process after link establishment. 
When configuring PPP with CHAP authentication, both routers must be configured with a username that will be presented by the other router with a password. Therefore, the username to configure on Router A will be the username of Router B. The password should be the same on both machines. If these settings are not correct, then authentication will fail. The authentication process can be displayed as it happens with the debug PPP authentication command.  
Link Control protocol (LCP) is defined in Request for Comments (RFCs) 1548 and 1570 and has primary responsibility to establish, configure, authenticate, and test a PPP connection. LCP negotiates the following when setting up a PPP connection:
- Authentication method used (PAP or CHAP), if any 
- Compression algorithm used (Stacker or Predictor), if any 
- Callback phone number to use, if defined 
- Multilink; other physical connections to use, if configured 
Network Control Protocol (NCP) defines the process for how the two PPP peers negotiate which network layer protocols, such as IP and IPX, will be used across the PPP connection. LCP is responsible for negotiating and maintaining a PPP connection whereas NCP is responsible for negotiating upper-layer protocols that will be carried across the PPP connection. 
Password authentication Protocol (PAP) is simpler than CHAP, but less secure. During the authentication phase, PAP goes through a two-way handshake process. In this process, the source sends its user name (or hostname) and password in clear text, to the destination. The destination compares this information with a list of locally stored user names and passwords. If it finds a match, the destination returns an accept message. If it does not find a match, it returns a reject message. 
References:
http://docwiki.cisco.com/wiki/Point-to-Point_Protocol
https://www.cisco.com/c/en/us/support/docs/wan/point-to-point-protocol-ppp/25647-understanding-ppp-chap.html

To complete the configuration, you should run the command username Lansing password keypass. This command creates a user account for the Lansing router with a password of keypass.  
When creating an authenticated connection between the routers, a user account must be created for the other router. The password configured must match on both ends. 
When examining the output produced by the show running-configuration command for two routers, the output should read as below:

  
  
The lines that display enable password cisco and enable password cisco1 represent local passwords to enable privileged mode on the local router. These passwords do not have to match. The lines of output that must display matching passwords are username Lansing password keypass and username Tacoma password keypass. 
You should not run the command username Tacoma password keypass. The username Tacoma portion of the command will create an account named Tacoma. You need an account for the other router, Lansing. 
You should not run the command username Lansing keypass password. The password portion of the command must follow the syntax password [correct_password]. 
You should not run the command username Tacoma keypass password. The username Tacoma portion of the command will create an account for the wrong router, and the password portion of the command must follow the syntax password [correct_password]. 
References:
https://www.cisco.com/c/en/us/support/docs/wan/point-to-point-protocol-ppp/25647-understanding-ppp-chap.html