Question

Suppose a network \(N\) within a larger organization \(A\) acquires its own direct connection to an Internet service provider, in addition to an existing connection via A. Let \(R 1\) be the router connecting \(N\) to its own provider, and let \(R 2\) be the router connecting \(N\) to the rest of \(A\). (a) Assuming \(\mathrm{N}\) remains a subnet of A, how should R1 and R2 be configured? What limitations would still exist with N's use of its separate connection? Would A be prevented from using N's connection? Specify your configuration in terms of what R1 and R2 should advertise, and with what paths. Assume a BGP-like mechanism is available. (b) Now suppose \(N\) gets its own network number; how does this change your answer in (a)? (c) Describe a router configuration that would allow A to use N's link when its own link is down.

Short answer

R1 should advertise N’s subnet with a higher preference, and R2 with a lower. If N gets its own network number, R1 and R2 advertise independently. To allow A to use N's link, configure a backup route on R2.

Step-by-step solution

Identifying Configuration Requirements

For part (a), consider that network N remains a subnet of A. To configure R1 and R2, let's assume BGP is used. R1 should advertise N’s subnet with a route that has a higher preference (lower cost), while R2 should advertise the same subnet with a lower preference (higher cost). This setup ensures that traffic destined for N would prefer R1's direct connection to the provider over R2's path through A. The limitation here is that N, being a subnet of A, could still face routing policy restrictions imposed by A. Also, A can use N's connection if configured such that A recognizes R1's advertisements as valid paths.

Considering N with its own Network Number

For part (b), if N receives its own distinct network number, it is no longer a subnet of A. In this case, R1 and R2 should independently advertise the respective network numbers with their own paths. This separation allows N to fully control its traffic independently of A. However, this scenario also implies that A cannot use N's connection unless configured through explicit routing policies that recognize N’s network number.

Allowing A to Use N’s Link when A’s Link is Down

For part (c), to allow A to use N's link when A's own link is down, configure R2 to advertise a failover route. This can be done using BGP by setting a backup route with a higher cost through N's link. When A's main link fails, traffic will be routed through N’s connection via R2. This requires proper configuration of BGP attributes such as route preference.

Key concepts

Network Subnetting

Network subnetting is the practice of dividing a larger network into smaller, more manageable sub-networks, or subnets.
By doing this, administrators can more efficiently control traffic, improve security, and allocate IP addresses.
In the given exercise, Network N is initially a subnet of a larger organization A.
A subnet, by definition, inherits security policies and configurations from its parent network.
This means that while N can have some level of autonomy, it might still face limitations imposed by A, such as routing policies and access controls.
If N decides to obtain its own network number, effectively becoming an independent network, these limitations would no longer apply.
This would give N the freedom to manage its own traffic and security policies.

• Subnets help manage large IP spaces.
• They improve network performance and security.
• Subnet limitations are based on parent network policies.

In conclusion, whether to remain a subnet or become an independent network depends on the desired level of autonomy and control.

BGP Protocol

The Border Gateway Protocol (BGP) is crucial for managing how packets are routed across the internet.
It determines the best paths for data transmission between autonomous systems (ASes).
In our exercise, BGP is used to set up routing preferences between the routers (R1 and R2).
Typically, BGP assigns a cost or preference to each route, where a lower cost indicates a higher preference.
By manipulating these values, network administrators can guide the flow of traffic through preferred paths. For instance:

• R1 might advertise Network N's subnet with a lower cost.
• R2 might advertise the same subnet with a higher cost.
• This ensures that traffic prefers R1's direct connection.

If N becomes its own network, R1 and R2 would then independently advertise N's distinct network number.
This configuration allows N to fully control its inbound and outbound traffic.
In other words, BGP facilitates the designation of preferred pathways, enabling efficient network management and traffic control.

Router Advertisement Configuration

Router advertisement configuration is essential for defining how network paths are communicated to other routers.
This involves setting up routers to share route information, allowing for dynamic path adjustments.
In our scenario:

• R1 and R2 need to advertise Network N's information properly.
• R1 should broadcast a high-preference (low-cost) route to N via its ISP connection.
• R2 should broadcast a lower-preference (higher-cost) route through A.

This configuration directs traffic efficiently to ensure optimal routing.
If N becomes its own network, instead of a subnet, both R1 and R2 would advertise N's new independent network number. This independence enhances traffic control and flexibility.
Router advertisement configurations in BGP require careful planning to avoid conflicts and inefficiencies.
Properly setting advertisement preferences ensures that routers can dynamically adapt to changes in network topology, such as a failing link or congestion.

Network Failover

Network failover ensures business continuity by rerouting traffic through alternative paths when primary routes fail.
It's a critical aspect for maintaining network uptime and reliability.
In the given exercise, if organization A's main link fails, the aim is to reroute traffic through Network N's ISP link.

• R2 can be configured to advertise a backup route through N's link.
• This backup route will have a higher cost indicated by BGP attributes.
• Only when A’s main link fails, the traffic will use this higher-cost path.

This setup uses BGP attributes such as Local Preference or Multi-Exit Discriminator (MED) to influence path selection.
By properly configuring these attributes, organizations can ensure an effective failover strategy, prioritizing alternate routes during failures.
In summary, network failover configurations are vital for robust network planning, providing seamless continuity even in case of primary route disruptions.