Home > Articles > Branch Connections

Branch Connections

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 30, 2017.

Chapter Description

Broadband solutions provide teleworkers with high-speed connection options to business locations and to the Internet. In this sample chapter from Connecting Networks v6 Companion Guide, identify branch connection options for small to medium-sized businesses, basic configuration for a PPPoE connection on a client router, and more.

From the Book

Connecting Networks v6 Companion Guide

Connecting Networks v6 Companion Guide

$66.00 (Save 10%)

PPPoE (3.2)

In this section, you configure a Cisco router with PPPoE.

PPPoE Overview (3.2.1)

In this topic, you learn how PPPoE operates.

PPPoE Motivation (

In addition to understanding the various technologies available for broadband Internet access, it is also important to understand the underlying data link layer protocol that the ISP uses to form a connection.

A data link layer protocol commonly used by ISPs is Point-to-Point Protocol (PPP). PPP can be used on all serial links, including those links created with dialup analog and ISDN modems. To this day, the link from a dialup user to an ISP, using analog modems, likely uses PPP.

Figure 3-9 shows a basic representation of that analog dial connection with PPP.

Figure 3-9

Figure 3-9 PPP Frames over Legacy Dialup Connection

Additionally, ISPs often use PPP as the data link protocol over broadband connections. There are several reasons for this. First, PPP supports the ability to assign IP addresses to remote ends of a PPP link. With PPP enabled, ISPs can use PPP to assign each customer one public IPv4 address. More importantly, PPP supports CHAP authentication. ISPs often want to use CHAP to authenticate customers because during authentication, ISPs can check accounting records to determine whether the customer’s bill is paid prior to letting the customer connect to the Internet.

These technologies came to market in the following order, with varying support for PPP:

    1. Analog modems for dialup that could use PPP and CHAP

    2. ISDN for dialup that could use PPP and CHAP

    3. DSL, which did not create a point-to-point link and could not support PPP and CHAP

ISPs value PPP because of the authentication, accounting, and link-management features. Customers appreciate the ease and availability of the Ethernet connection; however, Ethernet links do not natively support PPP. PPP over Ethernet (PPPoE) provides a solution to this problem. As shown in Figure 3-10, PPPoE sends PPP frames encapsulated inside Ethernet frames.

Figure 3-10

Figure 3-10 PPP Frames over an Ethernet Connection (PPPoE)

PPPoE Concepts (

As shown in Figure 3-11, the customer’s router is usually connected to a DSL modem using an Ethernet cable. PPPoE creates a PPP tunnel over an Ethernet connection. This allows PPP frames to be sent across the Ethernet cable to the ISP from the customer’s router. The modem converts the Ethernet frames to PPP frames by stripping the Ethernet headers. The modem then transmits these PPP frames on the ISP’s DSL network.

Figure 3-11

Figure 3-11 Tunneling to Create a PPP Link over Ethernet

Implement PPPoE (3.2.2)

In this topic, you implement a basic PPPoE connection on a client router.

PPPoE Configuration (

With the ability to send and receive PPP frames between the routers, the ISP could continue to use the same authentication model as with analog and ISDN. To make it all work, the client and ISP routers need additional configuration, including PPP configuration, as shown in Figure 3-12.

Figure 3-12

Figure 3-12 Steps for a PPPoE Customer Configuration

To understand the configuration, consider the following:

    1. To create a PPP tunnel, the configuration uses a dialer interface. A dialer interface is a virtual interface. The PPP configuration is placed on the dialer interface, not the physical interface. The dialer interface is created using the interface dialer number global configuration command. The client can configure a static IP address but will more likely be automatically assigned a public IP address by the ISP.

    2. The PPP CHAP configuration usually defines one-way authentication; therefore, the ISP authenticates the customer. The hostname and password configured on the customer router must match the hostname and password configured on the ISP router. Notice in Figure 3-12 that the CHAP username and password match the settings on the ISP router.

    3. The physical Ethernet interface that connects to the DSL modem is then enabled with the pppoe enable interface configuration command. This command enables PPPoE and links the physical interface to the dialer interface. The dialer interface is linked to the Ethernet interface with the dialer pool number and pppoe-client dial-pool-number number interface configuration commands, using the same number. The dialer interface number does not have to match the dialer pool number.

    4. The maximum transmission unit (MTU) should be lowered to 1492, versus the default of 1500, to accommodate the PPPoE headers.

PPPoE Verification (

As shown in Figure 3-13, the customer’s router is connected to the ISP router using DSL. Both routers have been configured for PPPoE.

Figure 3-13

Figure 3-13 Verifying the PPPoE Configuration

In Example 3-1, the show ip interface brief command is issued on R1 to verify the IPv4 address automatically assigned to the dialer interface by the ISP router.

Example 3-1 Verifying the Dialer Interface Is Up

R1# show ip interface brief
Interface                   IP-Address  OK?  Method  Status                 Protocol
Embedded-Service-Engine0/0  unassigned  YES  unset   administratively down  down
GigabitEthernet0/0          unassigned  YES  unset   administratively down  down
GigabitEthernet0/1          unassigned  YES  unset   up                     up
Serial0/0/0                 unassigned  YES  unset   administratively down  down
Serial0/0/1                 unassigned  YES  unset   administratively down  down
Dialer2               YES  IPCP    up                     up
Virtual-Access1             unassigned  YES  unset   up                     up
Virtual-Access2             unassigned  YES  unset   up                     up

As shown in Example 3-2, the show interface dialer command on R1 verifies the MTU and PPP encapsulation configured on the dialer interface.

Example 3-2 Verifying the MTU Size and Encapsulation

R1# show interface dialer 2
Dialer2 is up, line protocol is up (spoofing)
  Hardware is Unknown
  Internet address is
  MTU 1492 bytes, BW 56 Kbit/sec, DLY 20000 usec,
    reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Closed, loopback not set
  Keepalive set (10 sec)
  DTR is pulsed for 1 seconds on reset
<output omitted>

Example 3-3 displays the routing table on R1.

Example 3-3 Verifying the R1 Routing Table

R1# show ip route | begin Gateway
Gateway of last resort is to network
S* is directly connected, Dialer2 is subnetted, 2 subnets
C is directly connected, Dialer2
C is directly connected, Dialer2

Notice that two /32 host routes for have been installed in R1’s routing table. The first host route is for the address assigned to the dialer interface. The second host route is the IPv4 address of the ISP. The installation of these two host routes is the default behavior for PPPoE.

As shown in Example 3-4, the show pppoe session command enables you to display information about currently active PPPoE sessions.

Example 3-4 Viewing the Current PPPoE Sessions

R1# show pppoe session
    1 client session
Uniq ID  PPPoE  RemMAC          Port                VT   VA         State
           SID  LocMAC                                   VA-st      Type
     N/A     1  30f7.0da3.1641 Gi0/1                Di2  Vi2        UP
                30f7.0da3.0da1                           UP

The output displays the local and remote Ethernet MAC addresses of both routers. The Ethernet MAC addresses can be verified by using the show interfaces command on each router.

PPPoE Troubleshooting (

After you ensure that the client router and DSL modem are connected with the proper cables, the cause of a PPPoE connection not functioning properly is usually one or more of the following reasons:

  • Failure in the PPP negotiation process

  • Failure in the PPP authentication process

  • Failure to adjust the TCP maximum segment size (MSS)

PPPoE Negotiation (

Verify PPP negotiation using the debug ppp negotiation command. Example 3-5 displays part of the debug output after R1’s G0/1 interface has been enabled.

Example 3-5 Examining the PPP Negotiation Process

R1# debug ppp negotiation
*Sep 20 19:05:05.239: Vi2 PPP: Phase is AUTHENTICATING, by the peer
*Sep 20 19:05:05.239: Vi2 LCP: State is Open
<output omitted>
*Sep 20 19:05:05.247: Vi2 CHAP: Using hostname from interface CHAP
*Sep 20 19:05:05.247: Vi2 CHAP: Using password from interface CHAP
*Sep 20 19:05:05.247: Vi2 CHAP: O RESPONSE id 1 len 26 from "Fred"
*Sep 20 19:05:05.255: Vi2 CHAP: I SUCCESS id 1 len 4
<output omitted>
*Sep 20 19:05:05.259: Vi2 IPCP:    Address (0x03060A010302)
*Sep 20 19:05:05.259: Vi2 IPCP: Event[Receive ConfAck] State[ACKsent to Open]
*Sep 20 19:05:05.271: Vi2 IPCP: State is Open
*Sep 20 19:05:05.271: Di2 IPCP: Install negotiated IP interface address
*Sep 20 19:05:05.271: Di2 Added to neighbor route AVL tree: topoid 0, address
*Sep 20 19:05:05.271: Di2 IPCP: Install route to
R1# undebug all

The output is an example of what should be generated when PPP is correctly configured.

The four main points of failure in a PPP negotiation are as follows:

  • No response from the remote device (the ISP)

  • Link Control Protocol (LCP) not open

  • Authentication failure

  • IP Control Protocol (IPCP) failure

PPPoE Authentication (

After confirming with the ISP that it uses CHAP, verify that the CHAP username and password are correct. Example 3-6 shows the CHAP configuration on the dialer2 interface.

Example 3-6 Verify the CHAP Configuration

R1# show running-config | section interface Dialer2
interface Dialer2
 mtu 1492
 ip address negotiated
 encapsulation ppp
 dialer pool 1
 ppp authentication chap callin
 ppp chap hostname Fred
 ppp chap password 0 Barney

Re-examining the output of the debug ppp negotiation command in Example 3-7 verifies that the CHAP username is correct.

Example 3-7 Verify the CHAP Username

R1# debug ppp negotiation
*Sep 20 19:05:05.239: Vi2 PPP: Phase is AUTHENTICATING, by the peer
*Sep 20 19:05:05.239: Vi2 LCP: State is Open
<output omitted>
*Sep 20 19:05:05.247: Vi2 CHAP: Using hostname from interface CHAP
*Sep 20 19:05:05.247: Vi2 CHAP: Using password from interface CHAP
*Sep 20 19:05:05.247: Vi2 CHAP: O RESPONSE id 1 len 26 from "Fred"
*Sep 20 19:05:05.255: Vi2 CHAP: I SUCCESS id 1 len 4
<output omitted>
*Sep 20 19:05:05.259: Vi2 IPCP:    Address (0x03060A010302)
*Sep 20 19:05:05.259: Vi2 IPCP: Event[Receive ConfAck] State[ACKsent to Open]
*Sep 20 19:05:05.271: Vi2 IPCP: State is Open
*Sep 20 19:05:05.271: Di2 IPCP: Install negotiated IP interface address
*Sep 20 19:05:05.271: Di2 Added to neighbor route AVL tree: topoid 0, address
*Sep 20 19:05:05.271: Di2 IPCP: Install route to
R1# undebug all

If the CHAP username or password were incorrect, the output from the debug ppp negotiation command would show an authentication failure message such as shown in Example 3-8.

Example 3-8 Authentication Failure Message

*Sep 20 19:05:05.247: Vi2 CHAP: I FAILURE id 1 Len 26 MSG is "Authentication

PPPoE MTU Size (

Accessing some web pages might be a problem with PPPoE. When the client requests a web page, a TCP three-way handshake occurs between the client and the web server. During the negotiation, the client specifies the value of its TCP maximum segment size (MSS). The TCP MSS is the maximum size of the data portion in the TCP segment.

A host determines the value of its MSS field by subtracting the IP and TCP headers from the Ethernet maximum transmission unit (MTU). On an Ethernet interface, the default MTU is 1500 bytes. Subtracting the IPv4 header of 20 bytes and the TCP header of 20 bytes, the default MSS size will be 1460 bytes, as shown in Figure 3-14.

Figure 3-14

Figure 3-14 MTU and MSS

The default MSS size is 1460 bytes, when the default MTU is 1500 bytes; however, PPPoE supports an MTU of only 1492 bytes to accommodate the additional 8-byte PPPoE header, as shown in Figure 3-15.

Figure 3-15

Figure 3-15 Adjusted MSS with PPPoE Header

You can verify the PPPoE MTU size in running configuration, as shown in Example 3-9. This disparity between the host and PPPoE MTU size can cause the router to drop 1500-byte packets and terminate TCP sessions over the PPPoE network.

Example 3-9 Verifying the MTU Size on the Dialer Interface

R1# show running-config | section interface Dialer2
interface Dialer2
 mtu 1492
 ip address negotiated
 encapsulation ppp
<output omitted>

The ip tcp adjust-mss max-segment-size interface configuration command helps prevent TCP sessions from being dropped by adjusting the MSS value during the TCP three-way handshake. In most cases, the optimum value for the max-segment-size argument is 1452 bytes. Example 3-10 shows this configuration on R1’s LAN interface.

Example 3-10 Adjusting the TCP MSS

R1(config)# interface g0/0
R1(config-if)# ip tcp adjust-mss 1452

The TCP MSS value of 1452 plus the 20-byte IPv4 header, the 20-byte TCP header, and the 8-byte PPPoE header adds up to a 1500-byte MTU, as illustrated previously in Figure 3-15.

6. VPNs (3.3) | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020