Routing Fundamentals
Bit the Chipmunk, AWS Expert published on
Hello network adventurers! Bit here, perched on a branch overlooking the meeting of your on-premises data center and your AWS cloud environment. Today, we’re diving into the plumbing of connectivity: routing fundamentals. Because before you pick a service or a link, you need to understand how traffic finds its way from your on-premises network into AWS — and back — using the right routing strategy.
Let’s map the paths together! 🌰
🧭 1. What Routing Means in Hybrid Networks
In a hybrid connectivity scenario (e.g., using AWS Site-to-Site VPN, AWS Direct Connect, or a combination), routing determines which network traffic goes where, how fast, and via which path. Key decisions you’ll face:
- Do you use static routes (manually configured)?
- Or dynamic routing (automatic route exchange, such as with BGP)?
- How do you influence traffic flows (load-sharing, active/passive) using route attributes?
- How do you account for physical / Layer-2 interconnects (LAGs, optics, VLANs) and overlays (GRE, IPsec)?
These are fundamental questions for Task 1.5 of the exam.
🔀 2. Static vs. Dynamic Routing — Quick Comparison
Static Routing
- Routes are manually configured and fixed.
- Best for simple networks, fixed connectivity, known paths.
- Pros: Predictable, low overhead, simpler to secure.
- Cons: Doesn’t adapt to link failures or topology changes automatically.
Dynamic Routing (e.g., BGP)
- Uses protocols to exchange routing information automatically.
- Ideal for large/hybrid networks where links may fail, change, or you want redundancy/load-sharing.
- Pros: Automatic failover, route scaling, path adaptation.
- Cons: More complex, requires management of routing attributes and security.
👀 Exam Clue: If the scenario mentions “links may fail or we need active/active traffic across Regions”, lean toward dynamic routing (BGP). If it says “single link, simple stub network”, static might suffice.
🌐 3. BGP & Route Attributes in Hybrid Designs
When you pick dynamic routing with BGP (commonly used for on-prem ↔ AWS via Direct Connect or VPN), you should know how BGP lets you influence traffic flows using attributes and policies:
- AS_Path: The shorter the path, the more preferred.
- Local Preference: Higher values are preferred within an AWS Region.
- MED (Multi-Exit Discriminator): Lower values are preferred if all other attributes are equal — but note that AWS gives very low priority to MED for Transit/Private VIF path selection. It’s best not to rely on it for traffic engineering.
- Community Tags: AWS-specific BGP community tags control routing preferences and scope — see below.
🧩 AWS BGP Community Tags
AWS defines two main groups of BGP community tags that influence routing between your on-premises environment and AWS:
| Community Tag | VIF Type | Purpose |
|---|---|---|
| 7224:7300 | Private & Transit VIFs | High preference (preferred path) |
| 7224:7200 | Private & Transit VIFs | Medium preference (load sharing) |
| 7224:7100 | Private & Transit VIFs | Low preference (backup path) |
| 7224:9100 | Public VIFs | Local Region only |
| 7224:9200 | Public VIFs | Continental scope (e.g., all North American regions) |
| 7224:9300 | Public VIFs | Global scope (all public AWS Regions) |
💡 Exam Tip: These “local preference” communities (the 7x00 series) are used by AWS to decide which path to use when sending traffic from AWS to you. The higher the number, the higher the preference.
🚦 VPN Routing and ECMP
For AWS Site-to-Site VPNs (especially those terminating at a Transit Gateway), AWS supports Equal-Cost Multi-Path (ECMP) routing when BGP attributes (AS_Path, Local Pref, etc.) are identical. This allows active/active routing across redundant tunnels — great for resilience and load-balancing.
🧭 4. The AWS BGP Path Selection Priority
When multiple routes exist, AWS follows this exact order of evaluation for choosing the best path for traffic leaving the AWS network (for example, from AWS to your data center):
- Longest Prefix Match (LPM) — The most specific prefix always wins. A /27 route will beat a /24 route every time, regardless of community tags or other attributes.
- BGP Local Preference — Higher local preference (for example, via 7224:7300) is preferred.
- AS_Path Length — Shorter AS paths are preferred.
- MED (Multi-Exit Discriminator) — Lower MED wins, but AWS may not consider MED in many cases, particularly on Public VIFs.
- Tie-Breaker — If all attributes are identical, AWS picks the path with the lowest peering IP address.
🎓 Exam Reminder: Always remember that LPM beats everything else — even your fancy community tags.
🏗️ 5. Hybrid Connectivity Architecture Considerations
When designing connectivity between on-premises and AWS, remember routing is just one piece — you must also think about physical and overlay layers:
- Layer 1 & 2: For example, a Link Aggregation Group (LAG), optical fibre light-levels, jumbo frames.
- Encapsulation/Encryption: GRE tunnels, IPsec over VPN, Transit Gateway Connect overlays.
- Resource Sharing: Multi-account, multi-Region topologies require careful routing design (avoiding overlapping CIDRs, using route-tables, propagation).
🧠 6. Exam Traps & Study Tips
🚧 Exam Traps
- Trap: “Use static routing for all hybrid links because it’s simpler.” Truth: In large/hybrid networks with fail-over or dynamic links, static won’t scale or adapt.
- Trap: “BGP automatically solves everything — you don’t need policies.” Truth: BGP gives control but you must design attributes (Local Pref, AS_Path, Communities) to shape traffic.
- Trap: “Routing alone makes connectivity redundant.” Truth: You must design both physical/overlay connectivity + routing strategy.
- Trap: “Using one Direct Connect link is enough if you configure routing.” Truth: Redundancy often requires multiple links, dynamic routing, and fail-over paths.
- Trap: “Use the High Preference Community Tag (7224:7300) to guarantee traffic always uses Link A.” Truth: LPM always wins — if Link B advertises a more specific prefix, traffic will prefer Link B even if Link A has higher local preference.
✅ Study Tips
- Memorize the AWS BGP path selection order — it’s often tested.
- Be able to explain when to use BGP (hybrid, multi-link, active/active) vs static.
- Understand AWS-specific community tags for local preference and scope.
- Know that MED is weak in AWS path selection — don’t rely on it for critical routing.
- Remember ECMP is supported on VPN tunnels for load sharing.
- Always think about LPM before any other attribute when predicting traffic paths.
📚 Further Reading
- Dynamic Routing using Amazon VPC Route Server – AWS Blog
- AWS Direct Connect BGP Communities – AWS Documentation
🐿️ Bit’s Final Nut
Think of routing like your forest trail system. Static trails are fixed paths you build and maintain; dynamic trails adjust when trees fall or new branches grow. But even in Bit’s forest, the narrowest, most specific trail (LPM) always gets you there fastest! So, when you see community tags, AS paths, or MEDs, remember — specificity rules the woods. 🌲
Stay routed, stay ready, and keep scrambling toward that certification! 🌰