PPPoE in MANET
PPPoE combines Ethernet and PPP, to provide an authenticated method of assigning IP addresses to client systems.
The Cisco MANET solution employs PPPoE sessions to enable intra-nodal communications between a router and its partner radio. Each radio initiates the PPPoE session as soon as the radio establishes a radio link to another radio. After the PPPoE sessions are active, a PPP session is established end-to-end (router-to-router).
This is duplicated each time a radio establishes a new radio link. The Virtual Multipoint Interface (VMI) on the device can aggregate multiple PPPoE sessions and multiplex them to look like a single interface to the routing processes. Underneath the VMI are virtual access interfaces that are associated with each of the PPP and PPPoE connections.
A PPPoE session is established between a device and a radio on behalf of every other device and radio neighbor located in the MANET. These Layer 2 sessions are the means by which radio network status gets reported to the Layer 3 processes in the device. The following figure shows the PPPoE session exchange between mobile devices and directional radios in a MANET network.
Link Control Protocol (LCP) forms part of the Point-to-Point Protocol (PPP), within the family of Internet protocols. The routers exchange IP Control Protocol (IPCP) messages to negotiate options specific to the protocol.
VMI Interface
The VMI on the device can aggregate all of the per-neighbor PPPoE sessions from the radio Ethernet connection. The VMI maps the sessions to appear to Layer 3 routing protocols and applications as a single point-to-multipoint, multiaccess, broadcast-capable network. However, the VMI preserves the integrity of the PPPoE sessions on the radio side so that each point-to-point connection can have its own quality of service (QoS) queue.
The VMI also relays the link-quality metric and neighbor up/down signaling from the radio to the routing protocols. The VMI signals are used by the Enhanced Interior Gateway Routing Protocol (EIGRP) for IPv4 and IPv6 neighbors and the Open Shortest Path First version 3 (OSPFv3) for IPv6 neighbors.
The VMI can operate in two modes: bypass or aggregate.
Bypass Mode
This is the recommended mode for PPPOE in a MANET network.
In bypass mode, the virtual-access interfaces are directly exposed to applications running above L2. In bypass mode, you must still define a VMI because VMI continues to manage presentation of cross-layer signals, such as, neighbor up, neighbor down, and metrics. However, applications will still be aware of the actual underlying virtual-access interfaces and send packets to them directly.
If you are running multicast applications that require virtual-access interfaces to be exposed to applications above L2 directly, you can configure VMI to operate in bypass mode. Most multicast applications require that the virtual-access interfaces be exposed directly to routing protocols for the multicast Reverse Path Forwarding (RPF) to operate as expected .
Aggregate Mode
In this mode, all the virtual-access interfaces created by PPPoE sessions are aggregated logically under the configured VMI. VMI on the router can aggregate multiple PPPoE sessions and multiplex them to look like a single interface to the routing processes. Applications above Layer 2 (L2), such as Enhanced Interior Gateway Routing Protocol (EIGRP) and OSPFv3, should be defined only on VMI. Underneath VMI are virtual access interfaces that are associated with each of the PPP/PPPoE connections. Packets sent to VMI are forwarded to the correct virtual-access interface(s).
Aggregate mode VMIs operate in Non-Broadcast Multiple Access (NBMA) mode. Multicast traffic is forwarded only to the NBMA neighbors where a listener for that group is present. This is the preferred mode when operating in PIM sparse mode.
Virtual Access Interface
The Virtual-Access interfaces are logically “underneath” the VMI interface. Each Virtual-Access interface represents a “destination” which is either a routing next-hop, or a multicast group. At the bottom of the interface hierarchy is the actual physical interface connecting the router and radio. The Virtual-Access interface funnels the traffic to the physical interface for transmission to the radio device.
PPPOE Packet Flow
The Packet Flow diagram below illustrates the packet flow for both control and data packets over a PPPOE session when the VMI interface is in bypass mode.
All control traffic is sent to by the datapath to IOS where it is handed over to the appropriate protocol to be processed. The incoming interface for this traffic is the virtual-access interface associated with the neighbor which is the source of the traffic. In this case, Virtual-access1 corresponds to Neighbor 1 and Virtual-Access 2 corresponds to Neighbor 2.
All data traffic is processed by the datapath and does not typically get sent to IOS.