Configure PPPoE Dial-up Connection on Cisco Router

Preconditions

For convenience, this article sets the following example configs:

  • Connect the WAN port of the router to the modem, dial up to the Internet through pppoe
  • The username obtained from the ISP is user_id and the password is user_pass
  • There is a Vlan on the internal network, the subnet is 192.168.1.0/24
  • The Vlan's gateway address is 192.168.1.254
  • Use 192.168.1.1 to 192.168.1.100 as dynamically assigned addresses, leaving the rest for static assignment
  • MTU of the ISP network is 1492

About MTU MRU and MSS:

The default values ​​commonly used in Ethernet networks (also the maximum values ​​without using extension standards such as JumboFrame) are as follows:

MTU (Maximum Transmission Unit): 1500byte
MRU (Maximum Receive Unit): 1500byte
MSS (Maximum Segment Size): MTU minus IP header (20byte), TCP header (20byte): 1460byte
With a PPPoE dial-up connection, the PPPoE header needs to occupy 8 bytes, so the maximum MTU / MRU is 1492 and the maximum mss is 1452
The actual value varies according to the ISP. For example, the MTU of the most widely used network in Japan is set to 1454.

Configuration for basic internet connection

Use the enable command to gain administrative privileges, and then enter the configuration mode through the console.
The "router-config#" indicates that this is the root of configuration hierarchy. The configuration should starts from the root hierarchy, and finally returns to root with "exit" command.

router> enable
router# config terminal
router-config#

PPPOE dial-up virtual interface

Create dialer1 virtual dialer interface and enter interface configuration

interface dialer1
  mtu 1494
  ip address negotiated
  ip tcp adjust-mss 1454
  ip nat outside
  dialer pool 1
  dialer-group 1
  encapsulation ppp
  ppp authentication chap callin
  ppp chap hostname user_id
  ppp chap password user_pass
  ppp pap sent-username user_id password user_pass
  ppp ipcp dns request accept
  exit

There are two similar commands: mtu and ip mtu .
ip mtu affects ipv4 packets through this port, while mtu commands affects all packets. so use mtu command to match mtu with the ISP's network.
Regarding the two authentication methods of chap and pap, if you know which method your ISP uses, you only need to set the corresponding authentication information. If not sure, write both.

Wait for a few seconds, and display the port information with the following command:

show interface dialer1

If you see that the port has obtained an external IP address, it indicates that the PPPoE dial-up connection has been successfully established.

Configure VLAN

Now the external network is already connected to the Internet. Next, we need to configure the internal network so that our devices can connect to the external network through the internal LAN.
The router automatically generates a Vlan1 by default and automatically assigns all 8 LAN ports in Vlan1. If there is no special requirement, you can just configure on the Vlan1.

interface Vlan1
  ip address 192.168.1.254 255.255.255.0
  ip nat inside
  no shutdown
  exit

In Cisco's IOS configuration commands, almost all commands can be preceded by a "no" to cancel or perform the opposite operation.
Because the default state of interfaces are shutdown, you need to ensure that the interface enters the UP state with no shutdown command.

NAT and routing rules

It is the same as the default state of the interface is shutdown, Enterprise routers does nothing by default for security reasons.
So although Vlan is configured, routing between Vlan and WAN ports is not performed by default. You need to manually configure internal routing rules:

ip nat inside source list 1 interface dialer 1 overload
dialer-list 1 protocol ip permit
ip route 0.0.0.0 0.0.0.0 dialer 1 permanent

Since we have only one external network interface, we simply route everything to dialer 1. permanent indicates that this routing rule will be retained even if the dialer1 link is down.

Configure DHCP

PC, NAS, and other fixed devices can be assigned to static IP for easy management. But we also need to connect mobile devices such as mobile phones, tablets and some IoT devices. So it's necessary to allocate a part of the IP from the address pool as an automatic allocation segment through DHCP.

ip dhcp pool lan
  network 192.168.1.0 255.255.255.0
  default-router 192.168.1.254
  dns-server 8.8.8.8 8.8.4.4
  exit

ip dhcp excluded-address 192.168.1.101 192.168.1.254

It should be noted here that the last command is for EXCLUDED IPs, not IPs for dhcp allocation.

Save Configs

At this point, our router should be able to dial up to the Internet automatically like a normal home router. But the current configuration is saved in the system RAM, so don't forget to use the write command to write the configuration to NVRAM, otherwise all the configuration will disappear after the next restart due to power outage or other reasons!

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