CS6250 CS 6250 Exam 1 Latest

CS6250 / CS 6250 Exam 1 (Latest Update 2025 / 2026) Computer Networks | Questions & Answers | 100% Correct | Grade A - Georgia Tech

Question:

• What is BGP?

Answer:

The border routers of the ASes use the Border Gateway Protocol (BGP) to exchange routing information with one another. In contrast, the Interior Gateway Protocols (IGPs) operate within an AS, and they are focused on "optimizing a path metric" within that network. Example IGPs include Open Shortest Paths First (OSPF), Intermediate System - Intermediate System (IS- IS), Routing Information Protocol (RIP), and E-IGRP. In this lesson, we will focus on BGP.

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Question:

• How does an AS determine what rules to import/export?

Answer:

Let's look at the different types of routes an AS (let's call it X) decides whether to export.

Routes learned from customers: These are the routes X receives as

advertisements from its customers. Since provider X is getting paid to provide reachability to a customer AS, it makes sense that X wants to advertise these customer routes to as many neighboring ASes as possible. This will likely cause more traffic toward the customer (through X) and, hence, more revenue for X.

Routes learned from providers: These are the routes X receives as

advertisements from its providers. Advertising these routes does not make sense since X has no financial incentive to carry traffic for its provider's routes.Therefore, these routes are withheld from X's peers and X's other providers, but they are advertised to X's customers.

Routes learned from peers: These are routes that X receives as advertisements

from its peers. As we saw earlier, it does not make sense for X to advertise to provider A the routes it receives from provider B. Because in that case, providers A and B will use X to reach the advertised destinations without X making revenue.

When an AS receives multiple route advertisements towards the same destination from multiple ASes, it needs to rank the routes before selecting which one to import. In order of preference, the imported routes are the customer routes, then the peer routes, and finally, the provider routes. The

reasoning behind this ranking is as follows:

An AS wants to ensure that routes toward its customers do not traverse other ASes, unnecessarily generating costs. 2 / 4

An AS uses routes learned from peers since these are usually "free" (under the peering agreement).An AS resorts to importing routes learned from providers only when necessary for connectivity since these will add to costs.

Question:

• What were the original design goals of BGP? What was considered later?

Answer:

Scalability: As the size of the Internet grows, the same is true for the number

of ASes, the number of prefixes in the routing tables, the network churn, and the BGP traffic exchanged between routers. One of the design goals of BGP is to manage the complications of this growth while achieving convergence in reasonable timescales and providing loop-free paths.

Express routing policies: BGP has defined route attributes that allow ASes to

implement policies (which routes to import and export) through route filtering and route ranking. Each ASes routing decisions can be kept confidential, and each AS can implement them independently.

Allow cooperation among ASes: Each AS can still make local decisions (which

routes to import and export) while keeping these decisions confidential from other ASes.

Security: Originally, the design goals for BGP did not include security.

However, the increase in size and complexity of the Internet demands security measures to be implemented. We need protection and early detection for malicious attacks, misconfiguration, and faults. These vulnerabilities still cause routing disruptions and connectivity issues for individual hosts, networks, and even entire countries. There have been several efforts to enhance BGP security ranging from protocols (e.g., S-BGP), additional 3 / 4

infrastructure (e.g., registries to maintain up-to-date information about which ASes own which prefixes ASes), public keys for ASes, etc. Also, there has been extensive research to develop machine learning-based approaches and systems. But these solutions have not been widely deployed or adopted for multiple reasons that include difficulties in transitioning to new protocols and a lack of incentives.

Question:

• What are the basics of BGP?

Answer:

A pair of routers, known as BGP peers, exchange routing information over a semi-permanent TCP port connection called a BGP session. In order to begin a BGP session, a router will send an OPEN message to another router. Then the sending and receiving routers will send each other announcements from their routing tables. The time it takes to exchange routes varies from a few seconds to several minutes, depending on the number of routes exchanged.A BGP session between a pair of routers in two different ASes is called an external BGP (eBGP) session, and a BGP session between routers that belong to the same AS is called an internal BGP (iBGP) session.

Question:

• What is the difference between iBGP and eBGP?

Answer:

eBGP (for sessions are between border routers of neighboring ASes) and iBGP (for sessions between internal routers of the same AS).

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