CCIE R&S Written 1.40 Implement Ethernet Technologies

Speed and Duplex

First, let’s cover what auto-negotiation does not do: when auto-negotiation is enabled on a port, it does not automatically determine the configuration of the port on the other side of the Ethernet cable and then match it. This is a common misconception that often leads to problems.

Auto-negotiation is a protocol, and as with any protocol, it only works if it’s running on both sides of the link. In other words, if one side of a link is running auto-negotiation, and the other side of the link is not, auto-negotiation cannot determine the speed and duplex configuration of the other side. If auto-negotiation is running on the other side of the link, the two devices decide together on the best speed and duplex mode. Each interface advertises the speeds and duplex modes at which it can operate, and the best match is selected (higher speeds and full duplex are preferred).

The confusion exists primarily because auto-negotiation always seems to work. This is because of a feature called parallel detection, which kicks in when the auto-negotiation process fails to find auto-negotiation running on the other end of the link. Parallel detection works by sending the signal being received to the local 10Base-T, 100Base-TX, and 100Base-T4 drivers. If any one of these drivers detects the signal, the interface is set to that speed.

Parallel detection determines only the link speed, not the supported duplex modes.

Using auto-negotiation to your advantage is as easy as remembering one simple rule:
Make sure that both sides of the link are configured the same way.If one side of the link is set to auto-negotiation, make sure the other side is also set to auto-negotiation. If one side is set to 100/full, make sure the other side is also set to 100/full.


Stretch’s article here

Leased Line DataLink (L2) Protocols – HDLC and PPP
HDLC is the default on all Cisco RoutersHDLC is Cisco Proprietary, Low Overhead, No Features

PPP Industry StandardModerate OverheadFeature-iffic! e.g. Authentication, Compression, Multilink
PAP Clear Text AuthenticationCHAP Clear Text Username, MD5 hashing of password
Older PPP examples use VPDN (Virtual Private Dial Network)Newer examples use BBA (Broad Band Access) aka PPPoE Profiles
PPPoE Active Discovery Initiation/PPPoE Active Discovery Request (PADI/PADR)
PPPoE Active Discovery Offer (PADO) and PPPoE Active Discovery Session (PADS) frames

Configuration CLIENT (BBA)

interface Dialer1
ip address dhcp
encapsulation ppp
dialer-pool 6

interface Ethernet0/0
pppoe enable
pppoe-client dial-pool-number 6

Configuration SERVER (BBA)

ip dhcp pool VLAN146

interface virtual-template 1
ip add
peer default ip address dhcp-pool VLAN146

bba-group pppoe MYPPP
virtual-template 1

interface fastethernet 0/0
pppoe enable group MYPPP

To add authentication to the configuration,


interface virtual-template 1
ppp authentication chap

username R4 password cisco


interface dialer 1
ppp chap hostame R4
ppp chap password cisco

Configuration Client (VPDN)

interface Ethernet0/0
pppoe enable
pppoe-client dial-pool-number 1
interface Dialer1 ip address
encapsulation ppp
dialer-pool 1
dialer persistent

Configuration Server (VPDN)

vpdn enable
vpdn-group CISCO
protocol pppoe
virtual-template 1
interface Ethernet0/0
pppoe enable
interface Virtual-Template1
ip address

To view configuration and use of the dialer interface, use:-

show ip route
show pppoe session
show ip interface brief
show ip dhcp binding
show run | sec bba-group

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