PPPoE stands for Point-to-Point Protocol over Ethernet. It is a means of establishing a point-to-point communications channel over an Ethernet network. But why do we need PPPoE? To understand the reason of PPPoE, we need to understand about Ethernet and PPP.
As we know, Ethernet is not a point-to-point but a multipoint technology (even when two devices are connected back to back). Ethernet is designed to allow multiple devices to share a common medium, called a “broadcast domain”.
While Ethernet is dominating at customer sides, Internet Service Provider (ISP) still like PPP because of authentication (PPP supports CHAP), accounting (checking customer’s bill), link management (ISP can use PPP to assign a public IP address for customer).
However, Ethernet and PPP do not support each other natively. To utilize the benefits of both Ethernet and PPP, a protocol was created: PPPoE, which allows computers to connect to an ISP via a Digital Subscriber Line (DSL) modem.
One of the most popular network topology in practical nowadays is shown below with one HeadQuarter connecting to branch offices at some locations. The main enterprise resources are located in the HeadQuarter.
The router at the HeadQuarter undertakes the role of a Hub while branch routers take the role of Spokes. In this Hub-and-Spoke topology, each Branch can access some resources on the HeadQuarter. But there are some disadvantages with this topology:
+ When a spoke wants to communicate with another Spoke, it must go through the Hub which increases the traffic passing through the Hub, increase CPU and memory usage on Hub and can create bottle-neck problem. This also increases latency for time-sensitive applications such as VoIP, video conference…
+ Each site requires a static public IP address if the environment between them are public (like the Internet).
+ The configuration is complex, especially with large network. When a new Spoke is added, additional configuration is required on Hub
OSPF uses Link State Advertisement (LSA) to build up Link State Database (LSDB) so understanding how LSA works is the key point to grasp how OSPF operates.
In the topology above:
+ R3 and R4 only belong to Area 1. R1 only belongs to Area 0. R6 and R7 only belong to Area 2. These are known as Internal Routers.
+ R2 belongs to both Area 0 and Area 1. R5 belongs to both Area 0 and Area 2. These routers are known as Area Border Routers (ABRs).
+ Area 0 is known as Backbone Area. Every router which has an interface in Area 0 can be considered a Backbone Router. All other areas must have a connection to Area 0 (except using virtual-link). Without Area 0, routers can only function within that area.
+ When a change occurs in the network topology, the router experiencing the change creates a link-state advertisement (LSA) concerning that link.
OSPF has 11 LSA Types from 1 to 11 but some of them are not used like Type 6 (Multicast LSA), 8 (used for BGP), 9, 10, 11 (Opaque LSAs). In this tutorial we will learn more about other LSA Types (types 1 to 5 and 7).