4480 - Chapter 5 - Midterm Review

Why are different inter-AS and intra-AS protocols used in the internet?

1. Policy issues and how each organization may want to choose different rules for their AS. 2. Scalability and performance. Some systems may want to get bigger/smaller or be super fast or some other stuff like that.

What is the BGP route selection?

1.Prefer the highest local-preference value. 2. Prefer the shortest AS-path length. 3. Prefer the lowest origin value. 4. Prefer the lowest MED value. 5. Prefer routes learned from an EBGP peer over an IBGP peer. 6. Prefer best exit from AS. 7. For EBGP-received routes, prefer the current active route.

Compare and contrast the properties of a centralized and a distributed routing algorithm. Give an example of a routing protocol that takes a centralized and a decentralized approach.

A centralized routing algorithm computes the least-cost path between a source and destination by using complete, global knowledge about the network. The algorithm needs to have the complete knowledge of the connectivity between all nodes and all links' costs. The actualcalculation can be run at one site or could be replicated in the routing component of each andevery router. A distributed routing algorithm calculates the lease-cost path in an iterative,distributed manner by the routers. With a decentralized algorithm, no node has the completeinformation about the costs of all network links. Each node begins with only the knowledge of the costs of its own directly attached links, and then through an iterative process of calculationand information exchange with its neighboring nodes, a node gradually calculates the least-costpath to a destination or a set of destinations.OSPF (Open Shortest Path First) protocol is an example of centralized routing algorithm, and BGP (Border Gateway Protocol) is an example of a distributed routing algorithm.

What is meant by a control plane that is based on logically centralized control? In such cases, are the data plane and the control plane implemented within the same device or in separate devices? Explain. SECTION 5.2.

Logically centralized control means that a logically central routing controller computes and distributes the forwarding tables to be used by each and every router, and each router does not compute its forwarding tabel, unlike the per-router control. In the case of logically centralized control, the data plane and control plane are implemented in separate devices; the control plane is implemented in a central server or multiple servers, and the data plane is implemented in each router.

How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH attribute?

Next-Hop: The next IP address that a packet will be sent to in order to reach its destination.

Is it necessary that every autonomous systems use the same intra-AS routing algorithm? Why or why not?

No, this is because the inter-AS protocols will connect the intra-AS systems, allowing them to run independent protocols.

What is meant by a control plane that is based on per-router control?

Per-router control means that a routing algorithm runs in each and every router; both forwarding and routing function are constrained within each router. Each router has a routing component that communicates with the routing components in other routers to compute the values for its forwarding table. In such cases, we say that the network control and data planes are implemented monolithically because each router works as an independent entity that implements its own control and data planes.

Define and contract the following terms: subnet, prefix, and BGP route.

Subnet: A network inside a network. This can make networks more efficient and also allow for more network scaling. Prefix: A part of a word that comes before the rest of the word. For example, Inter means between. BGP Route: Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information between autonomous systems (AS) on the Internet. The protocol is often classified as a path vector protocol but is sometimes also classed as a distance-vector routing protocol.

What is the "count to infinity" problem in distance vector routing?

This is a routing loop where two routers both send erroneous routing tables to each other (such as that they can get to a router after a specific cost but the router has been cut off suddenly from the system) and they will just keep sending packets back and fourth, thinking that the other router can get the packet to the destination faster than they can.