Question

What are the differences between the routing protocols?

a) DSR and OSPF

b) Intra and inter AS

Answer 1

a)

In principle, dynamic routing would maintain routes between mobile nodes on a continual basis, using periodic route request messages. Routing tables in each host are built and constantly updated. Whereas on demand routing, while it is also "dynamic," in that it computes routes available in real time (i.e. it does not rely on static routing tables), the route to a destination is only computed when needed. And then it is timed out. Which makes on demand routing more power efficient, and conserves bandwidth, but it is slower, because a route has to be established before each session.

Dynamic Source Routing (DSR) is in fact an on demand protocol. Whereas RIP and OSPF are dynamic, but not on demand.

b)

The term interdomain routing describes the process of routing prefixes between Autonomous Systems, whereis intradomain routing refers to routing inside a single AS.

Interdomain Routing

BGP, being the de-facto standard for interdomain routing, will not perform load-balancing by default. It can however be configured to do so. Two main options exist here:

1. Configuring BGP to pair using loopback interface and use some underlying routing (for example static routing or another internal routing protocol) to provide to equal paths to the opposite side, which will effectively turn on load-balancing
2. Using the BGP multipathing feature, which is however a little more complex. One has to consider whether load-balancing should be performed with an iBGP (internal) or eBGP (external) peer or possibly both. To be candidates for multipathing, the routes must be equal in the following attributes:
   Weight
   Local preference
   AS-PATH Length
   Origin
   MED

Intradomain Routing

Most of the internal routing protocols support easier load-balancing. As soon as the one metric being used is equal, multiple equal costs routes can be installed into the routing table. OSPF for example by default uses up to four equal cost routes. Other routing protocols such as EIGRP can even do unequal cost multipathing.