A Deep Dive into Inter-area and Intra-area Routes, External Routes, and Link Costs
Introduction: Open Shortest Path First (OSPF) is a robust and widely used routing protocol in computer networks, particularly in large-scale enterprise environments. One of the key aspects of OSPF is its path selection mechanism, which determines how routers choose the best path to reach a destination. In this blog post, we’ll delve into OSPF path selection, focusing on inter-area and intra-area routes, type 1 and type 2 external routes, and the significance of link costs in the routing decision process.
Understanding OSPF Routing Hierarchy: OSPF organizes routers and networks into areas to scale efficiently. Routers within the same area share a common link-state database, which contains information about the network topology. OSPF defines two types of areas: backbone area (Area 0) and non-backbone areas (other numbered areas). Inter-area and intra-area routes are determined based on the area boundaries.
Intra-Area Routes: Intra-area routes refer to routes within the same OSPF area. Routers exchange link-state advertisements (LSAs) within the area to build a complete map of the network topology. OSPF routers use the Shortest Path First (SPF) algorithm to calculate the shortest path to reach destinations within the area. The metric used for OSPF intra-area routes is typically based on the link cost, which is inversely proportional to the bandwidth of the link.
Inter-Area Routes: Inter-area routes involve communication between routers in different OSPF areas. When a router needs to reach a destination in another area, it relies on a designated router called the Area Border Router (ABR). The ABR summarizes routes from its attached areas and advertises them to other areas in the OSPF domain. Inter-area routes are determined based on the cost of reaching the ABR, along with the cost advertised by the ABR for reaching the destination.
Type 1 and Type 2 External Routes: External routes in OSPF represent routes to destinations outside the OSPF domain. OSPF supports two types of external routes: Type 1 (E1) and Type 2 (E2).
- Type 1 External Routes: Type 1 external routes maintain the same metric as the intra-area routes within the OSPF domain. When an external route enters the OSPF domain, the ABR adds the cost of reaching the external destination to the total path cost.
- Type 2 External Routes: Type 2 external routes have a fixed metric assigned by the ABR, regardless of the internal OSPF metrics. The ABR advertises the external route with a predefined cost, providing a simpler path selection process within the OSPF domain.
Link Costs and Path Selection: Link costs play a crucial role in OSPF path selection. OSPF assigns costs to links based on their bandwidth, with higher bandwidth links having lower costs. Routers prefer paths with lower cumulative costs, aiming to minimize latency and maximize throughput.
Conclusion: OSPF path selection is a complex yet integral part of network routing, ensuring efficient and reliable communication across interconnected routers and networks. Understanding the principles of intra-area and inter-area routes, type 1 and type 2 external routes, and the significance of link costs empowers network engineers to design and optimize OSPF-based networks for optimal performance and scalability.
Let’s consider a simplified network topology to illustrate OSPF path selection, including intra-area and inter-area routes, as well as type 1 and type 2 external routes.
Network Topology:
R1
|
(10) / \ (10)
/ \
/ \
R2 R3
/ \ / \
(5) \ / (5)
\ (1) (1) /
\ | | /
\ | | /
R4--(1) R5---R6 R7
| / (1) |
| / | |
(1) (1) (1) (1)
R8---R9 |
|
|
R10
Assumptions:
- Each link has a bandwidth of 100 Mbps.
- OSPF is configured on all routers.
- Link costs are determined using the formula: cost = 100 / bandwidth (in Mbps).
Example: Let’s say a packet needs to travel from Router R1 to Router R9. We’ll explore the path selection process step by step:
- Intra-Area Route Selection:
- R1, R2, and R3 are in Area 0.
- OSPF uses SPF algorithm to find the shortest path.
- The cost of each link is calculated:
- R1-R2: cost = 100 / 10 = 10
- R2-R4: cost = 100 / 5 = 20
- R4-R7: cost = 100 / 1 = 100
- R3-R5: cost = 100 / 10 = 10
- R5-R8: cost = 100 / 1 = 100
- R8-R9: cost = 100 / 1 = 100
- Total cost via R2-R4-R7: 20 + 100 = 120
- Total cost via R3-R5-R8: 10 + 100 = 110
- Hence, the packet will follow the path R1-R2-R4-R7-R8-R9 (110 total cost).
- Inter-Area Route Selection:
- R6 is the ABR between Area 0 and another area (not shown).
- R6 advertises summarized routes to Area 0.
- Cost to reach R6:
- R1-R2: cost = 10
- R2-R4: cost = 20
- R4-R6: cost = 100 / 5 = 20
- Total cost via R1-R2-R4-R6: 10 + 20 + 20 = 50
- Hence, the packet could alternatively follow the path R1-R2-R4-R6-R9 (50 total cost) for inter-area routing.
- External Route Selection:
- Suppose R9 represents an external destination.
- Type 1 (E1) external route: OSPF adds the internal cost to the ABR cost.
- Total cost via R1-R2-R4-R6-R9: 50 + 100 = 150
- Type 2 (E2) external route: ABR assigns a fixed cost for the external route.
- Suppose ABR advertises R9 with a fixed cost of 200.
- Total cost via R1-R2-R4-R6-R9: 50 + 200 = 250
Conclusion: In this example, we’ve demonstrated OSPF path selection, considering intra-area, inter-area, and external routes. The selection process involves evaluating link costs to determine the most efficient path for packet forwarding, ensuring optimal network performance and reliability.
Summary Table:
| Concept | Description |
|---|---|
| Intra-Area Routes | Routes within the same OSPF area. Calculated using SPF algorithm. |
| Inter-Area Routes | Routes between OSPF areas. Determined via ABRs. |
| Type 1 (E1) External Routes | External routes with internal OSPF costs added. |
| Type 2 (E2) External Routes | External routes with fixed costs advertised by ABRs. |
| Link Costs | Costs assigned to links based on bandwidth. Inverse relationship: higher bandwidth, lower cost. |
This table provides a concise overview of the key concepts involved in OSPF path selection, helping to understand the routing decisions made by OSPF routers in complex network environments.