Sunday, 19 April 2015

Quick build - PPPoE Client on Cisco IOS

In this quick-build guide I'll show you how to set up a very basic IOS-based PPPoE client. This example is from a Cisco 819 router, however the config is pretty much the same on most ISR type devices. As usual, the build will cover the most simple common use case (no VLAN tags, dynamic AC selection, negotiated IP).

Note, if you want a PPPoE access concentrator to go with your client, you may find the Quick Build: Cisco IOS PPPoE Server with RADIUS Authentication post useful.

The Setup



The PPPoE client is basically set up in two parts - the first being the physical interface which will connect towards the access concentrator, the second being a dialer interface that will become instantiated when the PPPoE session comes up. We'll build the physical interface first, as follows:

interface GigabitEthernet0
 description To AC
 pppoe enable pppoe-client dial-pool-number 1
 no shutdown
!

Pretty minimal... turn PPPoE on, and tell it which dialer pool to use. Note, in older versions of IOS the command was simply "pppoe-client dial-pool-number 1". Next, we have to configure the dialer interface, as follows:

interface Dialer1
 ip address negotiated
 encapsulation ppp
 dialer pool 1
 dialer-group 1
 ppp authentication chap callin
 ppp chap hostname user@domain
 ppp chap password 0 b0dges
!
dialer-list 1 protocol ip permit
ip route 0.0.0.0 0.0.0.0 Dialer1

This creates the dialer interface that we will use, tells it to use PPP and to pick up its IP address dynamically.

The "dialer pool" command places this dialer into the pool where the physical interface was set to look, while the "dialer-group" command specifies which dialer-list will be used to decide what traffic is interesting (i.e. will bring or keep the PPPoE session up).

The PPP commands force the authentication type to CHAP, specify that we will not make the AC authenticate to us (generally not supported) and set the CHAP hostname (think username) and password.

Finally, the dialer-list referred to in the earlier "dialer-group" command is defined to match any IP traffic at all, before a static route is used to force traffic out of the dialer interface.

That really is all that you need! In real life you will probably need to add NAT statements and you will definitely need at least one other interface, but that's the PPPoE part done.

Debugging


There's an entire post dedicated to this subject, but the short version is as follows:

  • Verify that you are getting PPPoE control traffic between your client and the server (debug pppoe packet, debug pppoe event). The sequence should be PADI, PADO, PADR, PADS. PADT indicates someone is pulling down the session, the debugs should show you who!
  • Check the static route has installed in your routing table as traffic will only trigger the PPP up if it hits the interface (show ip route)
  • Verify that there is at least one "up" IP interface on the box other than the dialer. If there's no source address usable then any test traffic will fail to encapsulate and you won't be able to bring PPP up. (show ip interfaces brief)
  • If your client can't authenticate, check the credentials (both hostname and password under the Dialer interface) and ensure that the authentication type is CHAP in "callin" mode.
  •  Check your PPP is negotiating OK (debug ppp negotiation)

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