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CCNP Practical Studies: Using DSL to Access a Central Site

Chapter Description

This sample chapter focuses on Digital Subscriber Line (DSL) technology, one of the most popular broadband access methods and a new topic on the CCNP exam.

DSL Access Architectures and Protocols

The following sections show you the different access architectures and protocols for the DSL service. Four types of access architectures and protocols are covered in this chapter.

  • IRB

  • RBE

  • PPPoA

  • PPPoE

RFC 1483 Bridging and IRB Overview

When configured for RFC 1483 bridging, the ATU-R acts as a half bridge, forwarding all MAC frames not present on the LAN side to the WAN interface. In the case of 1483 bridging, 802.3 MAC frames are encapsulated along with an LLC/SNAP header into cells using AAL5 segmentation. The LLC/SNAP header is used to identify the protocols encapsulated to the remote end. Bridge groups are defined by associating VCs with each other. Bridge groups can be defined in several ways. Bridge members can communicate only with a network host, between each member and use IRB to route out of the bridge.

Bridge Group Virtual Interface (BVI) is a virtual interface that resides in the Cisco 827 and NRP. It acts as an interface between a bridge group and a routed interface. When configured for IRB, the BVI is assigned a number that corresponds to the bridge group that is used to associate the bridge group with the BVI. BVI is used as routed interface with network-layer attributes such as IP address, filtering, and so on. On BVI, routing is enabled on a per-protocol basis. BVI allows you to route a given protocol between routed interfaces and bridge groups. Figure 8-5 illustrates the RFC 1483 bridging protocol stack.

Figure 5Figure 8-5 RFC 1483 Bridging Protocol Stack

To configure IRB, follow these steps:

Step 1 Enable IRB with the following code:

bridge irb

Step 2 Specify the bridge protocol to define the type of Spanning Tree Protocol:

bridge bridge-group protocol {ieee | dec}

Step 3 Specify a protocol to be routed in a bridge group:

bridge bridge-group route protocol

Step 4 Configure the ATM subinterface and aal5snap encapsulation:

interface atm slot/0.subinterface-number {multipoint | point-to-point}
 pvc [name] vpi/vci
 encapsulation aal5snap

Step 5 Assign a network interface to a bridge group:

bridge-group bridge-group

Step 6 Enables a bridge group virtual interface:

interface bvi bridge-group

Example 8-9 demonstrates the IRB configuration of the Cisco 6400 NRP.

Example 8-9 IRB Configuration

bridge irb
bridge 1 protocol ieee
 bridge 1 route ip
!
interface ATM0/0/0.133 point-to-point
 description Integrated
 no ip directed-broadcast
 pvc 1/33
 encapsulation aal5snap
 !
 bridge-group 1
!
interface BVI1
 ip address 10.1.1.1 255.255.255.0

RBE Overview

When configured for RBE, the CPE configuration remains the same as that in IRB. RBE is intended to address most of the RFC 1483 bridging issues, such as broadcast storms and security. The ATU-R behaves like the routed-bridge interface that is connected to an Ethernet LAN. For packets sending from the customer side, the destination IP address is examined, and the Ethernet header is skipped. If the destination IP address is in the route cache, the packet is fast-switched to the outbound interface. If the destination IP address is not the route cache, the packet is queued for process switching.

For packets destined for the customer devices, the destination IP address is examined first, and then the destination interface is determined from the IP routing table. To place a destination MAC address in the Ethernet header, the router checks the ARP table for that interface. If the MAC address is not found, the router generates an ARP request for the destination IP address and forwards the ARP request to the destination interface only. If an unnumbered interface is used and multiple subscribers are on the same subnet, the routed-bridge interface uses proxy ARP. All of these can be achieved without using a bridge group or BVI in the aggregation gateway and therefore are more scalable. Figure 8-6 illustrates the RBE protocol stack.

Figure 6Figure 8-6 RBE Protocol Stack

To configure RBE, follow these steps:

Step 1 Configure the ATM subinterface, and use aal5snap encapsulation. ip unnumbered is used when subscribers are on the same subnet and to conserve IP address space. You can use the ip address command if subscribers are on different subnets.

interface atm slot/0.subinterface-number {multipoint | point-to-point}
ip unnumbered interface-name-number
 pvc [name] vpi/vci
 encapsulation aal5snap

Step 2 Associate the RBE command with the ATM subinterface:

atm route-bridged ip

Step 3 Define the static host route. It is required if the IP unnumbered configuration is used.

ip route network-number [network-mask] {address | interface} [distance]
 [name name]

Example 8-10 demonstrates the RBE configuration of the Cisco 6400 NRP.

Example 8-10 RBE Configuration

interface Loopback0
 ip address 192.168.1.1 255.255.255.0
 no ip directed-broadcast
!

interface ATM0/0/0.1 point-to-point
ip unnumbered Loopback0
 no ip directed-broadcast
 atm route-bridged ip
 pvc 1/35
 encapsulation aal5snap
!
ip route 192.168.1.2 255.255.255.255 ATM0/0/0.1

PPPoA Overview

When configured for PPP over ATM, the ATU-R acts as a router, and additionally provides DHCP and NAT services to the LAN side. In the case of PPP routing, IP packets are encapsulated into a PPP frame and then are segmented into ATM cells through AAL5. The PPP sessions initiated by the subscriber are terminated at the service provider that authenticates users, either using a local database on the router or through a RADIUS server. After the user is authenticated, IPCP negotiation takes place, and then the IP address gets assigned to the CPE. Figure 8-7 illustrates the PPPoA protocol stack.

Figure 7Figure 8-7 PPPoA Protocol Stack

Follow the next steps to configure PPPoA. (Note that local authentication is used here and that the IP address for the CPE is assigned by the router. RADIUS can be used for these tasks.)

Step 1 Configure a username and password for local authentication:

username name password secret

Step 2 Create an ATM subinterface and PVC:

interface atm slot/0.subinterface-number {multipoint | point-to-point}
 pvc [name] vpi/vci

Step 3 Configure PPPoA encapsulation, and associate a virtual template with it:

encapsulation aal5mux ppp virtual-template number

aal5mux encapsulation is used for the PPPoA configuration. virtual-template serves as the template, and the virtual-access interface is cloned from the virtual template.

Step 4 Create a virtual template interface:

interface virtual-template number

Step 5 Conserve IP addresses by configuring the ATM subinterface as unnumbered, and assign the IP address of the interface type you want to leverage:

ip unnumbered interface-name-number

Step 6 Create the local IP address pool:

ip local pool name begin-ip-address-range [end-ip-address-range]

Step 7 Specify the pool for the interface to use:

peer default ip address pool poolname

Step 8 Enable CHAP or PAP authentication on the interface:

ppp authentication {chap | pap | chap pap | pap chap} [if-needed]
 {default | list-name} [callin]

Example 8-11 demonstrates the PPPoA configuration of the Cisco 6400 NRP.

Example 8-11 PPPoA Configuration

username cisco password 0 cisco
!
interface ATM0/0/0.133 point-to-point
 no ip directed-broadcast
 pvc 1/33
 encapsulation aal5mux ppp Virtual-Template1
 !
interface Virtual-Template1
 description PPPoATM
 ip unnumbered FastEthernet0/0/0
 no ip directed-broadcast
 peer default ip address pool ccnp
 ppp authentication chap
!
ip local pool ccnp 10.1.1.10 10.1.1.50

PPPoE Overview

For PPPoE, the ATU-R is transparent to this function, bridging the MAC/PPP frames across the WAN interface. The PPPoE feature allows a PPP session to be initiated on a simple bridging Ethernet-connected client. The session is transported over the ATM link via encapsulated Ethernet-bridged frames. The session can be terminated at either a local exchange carrier central office or an Internet service provider point of presence. The termination device is a Cisco 6400 UAC.

In the PPPoE architecture, the IP address allocation for the individual host running the PPPoE client is based on the same principle of PPP in dial mode—that is, via IPCP negotiation. Where the IP address is allocated from depends on the type of service the subscriber has subscribed to and where the PPP sessions are terminated. The PPPoE uses the dialup networking feature of Microsoft Windows. The IP address assigned is reflected with the PPP adapter. The IP address assignment can be either by the UAC or the home gateways if L2TP is used. The IP address is assigned for each PPPoE session. Figure 8-8 illustrates the PPPoE protocol stack.

Figure 8Figure 8-8 PPPoE Protocol Stack

To configure PPPoE, follow these steps. (Note that local authentication is used here, and the router assigns IP addresses for the hosts. RADIUS can be used for these tasks.)

Step 1 Make sure Cisco Express Forwarding is enabled. If it isn't, use the following command to enable it:

ip cef

Step 2 Configure the username and password for local authentication:

username name password secret

Step 3 Enable the virtual private dialup network (VPDN) configuration:

vpdn enable

Step 4 Configure the VPDN group to accept the dial-in and to be used to establish PPPoE sessions. Also specify the virtual template that will be used to clone virtual-access interfaces:

vpdn-group number
 accept-dialin
 protocol pppoe
 virtual-template template-number

Step 5 Create the ATM subinterface and PVC. Also configure AAL5SNAP encapsulation and specify the PPPoE protocol that the VPDN group will use:

interface atm slot/0.subinterface-number {multipoint | point-to-point}
 pvc [name] vpi/vci
 encapsulation aal5snap
 protocol pppoe

Step 6 Create the virtual template interface:

interface virtual-template number

Step 7 Conserve IP addresses by configuring the ATM subinterface as unnumbered, and assign the IP address of the interface type you want to leverage:

ip unnumbered interface-name-number

Step 8 Configure the maximum transmission unit (MTU):

ip mtu 1492

Because Ethernet has a maximum payload size of 1500 bytes, the PPPoE header is 6 bytes and the PPP ID is 2 bytes, so the PPP MTU must not be greater than 1492 bytes.

Step 9 Create a local IP address pool:

ip local pool name begin-ip-address-range [end-ip-address-range]

Step 10 Specify the IP address pool for the interface to use:

peer default ip address pool poolname

Step 11 Enable CHAP or PAP authentication on the interface:

ppp authentication {chap | pap | chap pap | pap chap} [if-needed]
 {default | list-name} [callin]

Example 8-12 demonstrates the PPPoE configuration of the Cisco 6400 NRP.

Example 8-12 PPPoE Configuration

username cisco password 0 cisco
!
vpdn enable
!
vpdn-group 1

 accept-dialin
 protocol pppoe
 virtual-template 1
!
ip cef
!
interface ATM0/0/0.133 point-to-point
 no ip directed-broadcast
 pvc 1/33
 encapsulation aal5snap
 protocol pppoe
 !
interface Virtual-Template1
 ip unnumbered FastEthernet0/0/0
 no ip directed-broadcast
 ip mtu 1492
 peer default ip address pool ccnp
 ppp authentication chap
!
ip local pool ccnp 10.1.1.10 10.1.1.50