Distance Vector - Routing algorithm
A Distance Vector routing algorithm is a class of dynamic routing protocols where each router periodically shares its entire routing table with its directly connected neighbors. Routers calculate the best path to a destination based on the "distance" (often hop count) reported by neighbors, using algorithms like the Bellman-Ford algorithm to determine the shortest path. In the z/OS context, this allows the mainframe's TCP/IP stack to dynamically learn routes to other network segments.
Key Characteristics
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- Periodic Updates: Routers send their full routing table to neighbors at regular intervals (e.g., every 30 seconds for RIP).
- Hop Count Metric: Typically uses hop count as the primary metric for determining the "distance" or cost to a destination.
- "Bellman-Ford" Algorithm: The underlying mathematical algorithm used by routers to compute the shortest paths based on received distance vectors.
- Slow Convergence: Can be slow to adapt to network topology changes, especially in larger networks, leading to potential routing loops during convergence.
- "Count to Infinity" Problem: A known issue where routing loops can persist for extended periods, as routers slowly increment their distance to an unreachable network.
- Mitigation Techniques: Uses mechanisms like
split horizonandpoison reverseto help prevent routing loops and speed up convergence. - RIP (Routing Information Protocol): The most common example of a Distance Vector protocol, supported by the z/OS Communications Server.
Use Cases
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- Simple Internal Network Segments: Used for routing within small, stable, and non-complex network segments directly attached to the mainframe where minimal routing overhead is acceptable.
- Legacy Network Integration: Interacting with older network devices or segments that still rely on Distance Vector protocols (like RIP) for their routing infrastructure.
- Educational and Testing Environments: Employed in lab settings to demonstrate fundamental dynamic routing principles and the operation of routing protocols within a z/OS networking context.
- Specific Network Appliances: In scenarios where the z/OS system needs to exchange routes with specific, simple network appliances that only support basic routing protocols.
Related Concepts
Distance Vector protocols are a fundamental component of TCP/IP for z/OS, enabling the mainframe to participate in dynamic routing. They contrast with Static Routing, where routes are manually configured. More advanced and scalable routing protocols like OSPF (Open Shortest Path First), which is a Link-State protocol, and BGP (Border Gateway Protocol), a Path-Vector protocol, are generally preferred in modern, complex z/OS environments for their faster convergence and better scalability. Understanding Distance Vector algorithms provides a foundational understanding for these more complex routing solutions.
- Limit Scope: Use Distance Vector protocols like RIP only in small, stable, and simple network segments, avoiding their use in large or complex enterprise networks.
- Prefer Link-State/Path-Vector: For mission-critical and large-scale z/OS network deployments, prioritize OSPF (for interior routing) and BGP (for exterior routing) due to their superior scalability, faster convergence, and robustness.
- Implement Authentication: If using RIPv2, configure
MD5 authenticationto secure routing updates and prevent malicious or unauthorized route injections. - Route Filtering: Utilize route filtering mechanisms within the z/OS Communications Server to control which routes are advertised or accepted, enhancing security and network stability.
- Monitor Routing Tables: Regularly monitor the z/OS routing tables and protocol activity using
NETSTATcommands andVARY TCPIP,,OMP,DISPLAYto ensure correct routing behavior and identify potential issues.