Advanced PPP: multilink, fragmentation and interleaving, PPPoA and PPPoE

Where these technologies stand today

This note groups four PPP extensions. Two are mostly legacy: Multilink PPP over serial or ISDN lines, and PPP over ATM on ADSL. Faster Ethernet and fibre services replaced both. The other two ideas are still useful: fragmentation and interleaving explains how to protect voice on any slow link, and PPPoE remains the login method for many home and business broadband services. For PPP basics (LCP, NCPs, CHAP), see HDLC and PPP on Cisco serial links.

Multilink PPP

When one 2 Mbps line was not enough, the cheapest upgrade was often a second line. Multilink PPP (MLPPP, RFC 1990) bundles several physical links into one logical interface with one IP address. It splits packets across the members and adds sequence numbers so the far end can reassemble them in order. Unlike per-flow load sharing, even a single large transfer uses every link.

interface Multilink1
 ip address 10.255.2.1 255.255.255.252
 ppp multilink
 ppp multilink group 1
interface serial 0/1/0
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
interface serial 0/1/1
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1

Check the bundle with show ppp multilink.

Link fragmentation and interleaving

On a slow link, one 1500-byte data packet takes a long time to send: about 188 ms at 64 kbps. A voice packet stuck behind it suffers delay and jitter, even with priority queuing, because a packet that has started cannot be interrupted. LFI (link fragmentation and interleaving) cuts large packets into small fragments and lets high-priority packets jump in between them. A common target is about 10 ms of delay per fragment.

interface Multilink1
 ppp multilink fragment delay 10
 ppp multilink interleave
 service-policy output VOICE-FIRST

Interleaving only helps if a priority queue (for example LLQ in the policy above) puts voice first. On links of roughly 1 Mbps and above, serialisation delay is small and LFI is rarely needed. See QoS for voice, video and business applications.

PPPoA and PPPoE

Broadband providers wanted the features of PPP (a username and password per customer, address assignment and accounting) on DSL lines. Two methods appeared:

• PPPoA (PPP over ATM, RFC 2364): PPP carried directly in ATM cells on ADSL. Legacy: ATM-based DSL has largely been replaced by VDSL and fibre with Ethernet.
• PPPoE (PPP over Ethernet, RFC 2516): PPP inside Ethernet frames, with a discovery stage to find the access server. Still common on fibre (FTTP) and VDSL services.

PPPoE adds 8 bytes, so the MTU inside the session is 1492 bytes. Set the dialer MTU and clamp TCP MSS to avoid broken web pages. A Cisco PPPoE client looks like this:

interface GigabitEthernet0/0/0
 no ip address
 pppoe enable group global
 pppoe-client dial-pool-number 1
interface Dialer1
 ip address negotiated
 ip mtu 1492
 ip tcp adjust-mss 1452
 encapsulation ppp
 dialer pool 1
 ppp chap hostname customer123@isp.example
 ppp chap password Example-Pass-01
ip route 0.0.0.0 0.0.0.0 Dialer1

In a lab you can make another router the PPPoE server (the access concentrator). It uses a BBA group that points to a virtual template; each client session clones that template and is given an address from a local pool:

username customer123@isp.example password Example-Pass-01
ip local pool PPPOE-POOL 192.168.250.10 192.168.250.50
bba-group pppoe global
 virtual-template 1
interface Virtual-Template1
 ip unnumbered Loopback0
 peer default ip address pool PPPOE-POOL
 ppp authentication chap
interface GigabitEthernet0/0/1
 no ip address
 pppoe enable group global

Real-world example: a shop's new fibre line

A family-run hardware shop replaced its ADSL router with a Cisco ISR on a new fibre service. The ISP's welcome letter gave a PPPoE username and password and a VLAN ID for the Ethernet hand-off. The installer created a sub-interface with the VLAN, enabled the PPPoE client on it and built the dialer as above. The link came up, but the card terminal's web portal kept hanging. show interfaces dialer 1 showed the MTU was still 1500; setting it to 1492 and adding the MSS clamp fixed the stalls, because large packets were no longer being dropped.