Asymmetric routing gets blamed for everything. Traffic flowing in through one path and returning via a different path? Must be bad. Firewalls dropping connections? Asymmetric routing's fault. Performance degradation? Probably asymmetric routing. The networking community has spent decades treating asymmetric routing like a disease that needs to be eradicated, a misconfiguration to be fixed, a failure mode to be avoided. Network engineers configure complex routing policies, deploy stateful failover mechanisms, and architect entire networks around enforcing symmetric flow to eliminate this supposed problem.

Here's the uncomfortable truth: asymmetric routing isn't the problem. It's the solution. The Internet wouldn't scale to billions of users and zettabytes of traffic if every packet had to return on the exact reverse path it arrived on. BGP doesn't enforce symmetric routing because symmetric routing would kill the Internet's ability to adapt to failures, optimize for cost and latency, and utilize redundant paths efficiently. Asymmetric routing is how networks actually work at scale, and the sooner we stop treating it as a bug and start treating it as a feature, the better our networks will perform.

The real problem isn't asymmetric routing. The real problem is stateful devices that weren't designed for asymmetric flows, deployed in locations where state synchronization is impossible, breaking connections that would otherwise work fine. Fix the stateful devices or accept symmetric routing as a constraint in specific locations, and asymmetric routing becomes what it should be: a natural consequence of optimal path selection in a distributed system.

Let's explore what asymmetric routing actually is, why it's necessary, where it actually causes problems (and where it doesn't), and how to build networks that embrace asymmetry instead of fighting it.

What Asymmetric Routing Actually Is

Asymmetric routing is simple: the path that packets take from source to destination differs from the path that return packets take from destination back to source. In a connection between Host A and Host B, traffic from A to B might go A → Router1 → Router2 → Router3 → B, while return traffic goes B → Router4 → Router5 → A.

This is completely normal. In fact, it's expected behavior in any network with multiple paths between endpoints. Consider:

The Internet: When you connect to a server halfway around the world, your packets might transit through New York to reach London, while return packets take a shorter path through a submarine cable. Different ISPs, different paths, both optimized for their direction's traffic engineering policies.

Enterprise networks with redundant uplinks: A company with connections to two ISPs might prefer ISP1 for outbound traffic (cheaper) but receive inbound traffic primarily via ISP2 (better peering). Asymmetric by design, optimized for cost and performance.

ECMP environments: Equal-Cost Multi-Path routing distributes traffic across multiple paths based on flow hashing. Forward and reverse flows might hash differently, taking different paths through the same network. This is working as intended.

Datacenter spine-leaf fabrics: Modern datacenters use leaf-spine topologies with dozens of equal-cost paths. Hash-based load balancing means forward and reverse flows routinely take different paths. Nobody considers this a problem, it's fundamental to achieving high bandwidth utilization.

Asymmetric routing is the natural result of independent routing decisions at each hop. The forward path is determined by routing tables and policies on devices between source and destination. The reverse path is determined by completely different routing tables and policies on devices between destination and source. Unless you explicitly constrain routing to enforce symmetry (usually through inferior paths or complex policy), asymmetry is what happens.

Why Asymmetric Routing Has a Bad Reputation

The hatred of asymmetric routing comes from a specific, real problem: stateful devices that expect to see both directions of a flow. Firewalls, NAT devices, load balancers, and some intrusion prevention systems maintain state for connections they forward. When a SYN packet passes through Firewall A, it creates a state entry allowing the SYN-ACK and subsequent packets for that connection. If the SYN-ACK returns through Firewall B instead, it arrives at a device with no state for that connection. Firewall B drops it because it looks like an unsolicited incoming connection attempt. The connection fails, users complain, and network engineers blame asymmetric routing.

Let's be precise about what actually breaks:

Stateful Packet Filtering

Stateful firewalls track connections through a state table. The first packet of a connection (TCP SYN, first UDP datagram) creates a state entry. Subsequent packets matching that state are allowed through with minimal processing. This works beautifully when all packets for a connection traverse the same firewall. When asymmetric routing sends return packets through a different firewall, that firewall has no state entry and drops the packet.

The firewall isn't broken. It's doing exactly what stateful filtering is designed to do: drop packets that don't match established connections. The asymmetry violates the firewall's assumption that both directions of a flow will traverse it.

Network Address Translation

NAT maintains a mapping between internal private addresses and external public addresses/ports. When an internal host initiates a connection, the NAT device creates a mapping (e.g., 192.168.1.10:5000 maps to 203.0.113.5:12345) and translates the source address/port for outbound packets. Return packets must traverse the same NAT device so it can reverse the translation using its state table.

If return packets arrive at a different NAT device, that device has no translation state. It doesn't know that 203.0.113.5:12345 should be translated to 192.168.1.10:5000. The packet either gets dropped (no route to 203.0.113.5 internally) or delivered to the wrong host. NAT fundamentally requires seeing both directions of a flow.

TCP State Tracking and Sequence Number Validation

Some security devices track TCP state deeply, not just connection existence but actual sequence numbers, window sizes, and protocol compliance. They maintain per-connection state about expected sequence numbers and validate that TCP segments are legitimate parts of an established connection. Asymmetric routing breaks this because only one direction of traffic is visible to each device. Half the sequence number space is invisible, making proper validation impossible.

Load Balancers and Persistence

Load balancers distribute connections across backend servers, often using session persistence (same client always reaches same server). This requires state: the load balancer remembers that client 192.168.1.50 was sent to server 10.0.0.5 and must continue sending that client's traffic there. If asymmetric routing sends some of that client's connections through a different load balancer with different state, the client ends up on different backend servers, breaking session state.

Notice the pattern: the problem is state, not asymmetry. Stateless devices (routers performing simple forwarding, switches moving frames, cables carrying bits) work perfectly fine with asymmetric flows. The issue is devices that must see both directions of a connection to function correctly, deployed in a topology where asymmetry can occur.

The Geography of State Synchronization

Here's where it gets interesting: the viability of asymmetric routing through stateful devices depends entirely on whether those devices can synchronize state faster than flows return. This creates a stark divide based on physical location and latency.

The Co-Located Cluster: Asymmetry That Works

Consider a datacenter with a cluster of four firewalls sitting in the same rack, connected via a 10 Gbps or faster synchronization link. When Firewall 1 receives a SYN packet and creates state for a new connection, it immediately synchronizes that state to Firewalls 2, 3, and 4. This synchronization happens over a dedicated link with microsecond latency.

Now consider the timing of a connection establishment. The TCP three-way handshake involves:

1. Client sends SYN to server, passes through Firewall 1

2. SYN propagates to server (maybe 1-50 ms depending on distance)

3. Server processes SYN, sends SYN-ACK (processing adds microseconds)

4. SYN-ACK propagates back to client (another 1-50 ms)

Total round-trip time: 2-100 milliseconds, depending on network distance.

Meanwhile, state synchronization between co-located firewalls: 10-100 microseconds. The state replicates to all cluster members in 0.1 milliseconds, while the SYN-ACK won't arrive for at least 2 milliseconds (and likely much longer). By the time the SYN-ACK comes back (potentially through Firewall 2, 3, or 4 due to ECMP load balancing), all firewalls already have state for this connection. Asymmetric routing works perfectly because state synchronization vastly outpaces packet round-trip time.

This is why you can deploy a cluster of 8, 16, or even 32 firewalls in a single datacenter, load balance traffic across them using ECMP, allow fully asymmetric flows, and have everything work correctly. The clustering protocol (VRRP, proprietary vendor clustering, or custom state sync) keeps all devices synchronized well within the timing constraints of network round-trip times.

The Distributed Disaster: When Distance Defeats State Sync

Now consider a geographically distributed deployment: firewalls in Chicago and Dallas, 1,000 miles apart. The speed of light gives us a fundamental limit: 1,000 miles of fiber is roughly 10 milliseconds of latency (assuming fiber's roughly 2/3 light speed and reasonable routing efficiency). Round-trip latency between Chicago and Dallas is 20+ milliseconds.

When Chicago's firewall receives a SYN packet, it must synchronize state to Dallas. This takes 10+ milliseconds one-way. But look at our connection timing again:

1. Client sends SYN, arrives at Chicago firewall

2. Chicago firewall processes SYN, forwards to server

3. Chicago firewall initiates state sync to Dallas (10+ ms in flight)

4. SYN reaches server, server sends SYN-ACK

5. SYN-ACK routes back, but asymmetrically arrives at Dallas firewall

If the server is close to the firewalls (same datacenter), the SYN-ACK returns in 1-2 milliseconds, but state synchronization to Dallas takes 10+ milliseconds. The SYN-ACK arrives at Dallas before Dallas knows about the connection. Dallas drops the packet. Connection fails.

You can't solve this with faster networking or better protocols. Physics wins. State cannot propagate faster than light in fiber, and latency increases linearly with distance. The farther apart your stateful devices, the wider the window where asymmetric flows will arrive before state synchronizes.

The Distance Threshold

This creates a practical threshold for state synchronization:

Same rack/building (< 1 km): 10-100 microsecond state sync, works perfectly for any realistic connection timing. Asymmetric routing is fine.

Same metro area (< 50 km): 0.5-1 millisecond state sync. Still faster than most connection round-trip times. Asymmetric routing usually works but might see occasional issues with very fast local connections.

Regional (50-500 km): 1-10 millisecond state sync. Now competing with connection establishment timing. Asymmetric routing becomes problematic for servers close to the firewalls.

Cross-country (500+ km): 10-50+ millisecond state sync. Slower than typical connection establishment even for distant servers. Asymmetric routing fails consistently.

This is why you can cluster firewalls in one datacenter and allow asymmetric flows, but can't do the same across datacenters in different cities. The latency of state synchronization fundamentally limits where stateful clustering can work.

Why Asymmetric Routing Is Necessary for Scale

Let's address the elephant in the room: if asymmetric routing causes so many problems with stateful devices, why not just enforce symmetric routing everywhere? Because it doesn't scale. The Internet exists because we allow asymmetric routing.

BGP Doesn't Do Symmetric Routing

Border Gateway Protocol, the protocol that makes the Internet work, makes completely independent routing decisions for each direction of traffic. When AS 64500 advertises a route to 203.0.113.0/24, it's telling its neighbors "here's how to reach this prefix from you." It says nothing about how return traffic will flow.

Consider a simple scenario: AS 64500 has two upstream providers, ISP A and ISP B. AS 64500 prefers ISP A for outbound traffic (cheaper transit) but ISP B has better peering arrangements, so inbound traffic naturally prefers ISP B. This is optimal routing, minimizing cost for outbound traffic and latency for inbound traffic. It's also completely asymmetric.

To enforce symmetric routing, AS 64500 would need to constrain routing so both directions use the same ISP. Either send outbound through ISP B (paying more for transit) or receive inbound through ISP A (accepting worse latency and connectivity). Both options are suboptimal. Multiply this by tens of thousands of autonomous systems making independent decisions, and enforced symmetry becomes impossible without globally degrading routing.

Load Balancing Requires Asymmetry

Modern networks achieve high throughput through parallelism: multiple equal-cost paths used simultaneously. ECMP (Equal-Cost Multi-Path) routing is fundamental to datacenter fabrics, ISP core networks, and Internet exchange points. ECMP uses hash-based load balancing, taking a flow's 5-tuple (source IP, destination IP, source port, destination port, protocol) and hashing it to select one of N equal paths.

Here's the catch: forward and reverse flows hash differently. A flow from 192.168.1.5:50000 to 203.0.113.10:443 hashes based on those values. The reverse flow from 203.0.113.10:443 to 192.168.1.5:50000 hashes the values in opposite order. Different hash input, potentially different path selection. Unless your hash algorithm is carefully symmetric (and many aren't), ECMP naturally creates asymmetric routing.

You could enforce symmetric hashing, but that's complex and reduces flexibility. More importantly, it doesn't help when different parts of the network have different numbers of equal-cost paths. If the forward path has 8 ECMP links but the reverse path has 4, perfect symmetry is mathematically impossible. Something will be asymmetric.

Failure Recovery Depends on Asymmetry

When a link or router fails, routing protocols reconverge, computing new paths around the failure. This reconvergence doesn't happen simultaneously across the entire network. BGP might take minutes to fully converge, and even fast interior gateway protocols like OSPF take seconds. During convergence, different routers have different views of topology, creating temporary asymmetry.

If we required symmetric routing during failures, we'd have two bad options: drop all traffic until symmetry is restored (unacceptable) or pause routing updates until all routers converge simultaneously (impossible in a distributed system). We accept temporary asymmetry because the alternative is extended outages.

Traffic Engineering Optimizes Each Direction Independently

Real networks optimize for different goals in different directions. Outbound traffic might optimize for cost (use cheaper transit). Inbound traffic might optimize for latency (use better-connected peers). Uploads might take one path (optimized for throughput), downloads another (optimized for latency). These optimizations make networks faster and cheaper. They also make routing asymmetric.

Enforcing symmetric routing means sacrificing optimization. Pick one path that's decent for both directions instead of great paths for each direction. At Internet scale, this adds up to massive inefficiency.

Stateless Flow Hashing vs Stateful Flow Management

There are two fundamental approaches to handling flows in networks: stateless hashing and stateful tracking. Understanding the difference is critical to understanding why asymmetric routing works in some contexts and fails in others.

Stateless Hashing: The Asymmetry-Friendly Approach

Stateless load balancing uses a hash of packet header fields to select a path. Take the 5-tuple (source IP, dest IP, source port, dest port, protocol), run it through a hash function, modulo by number of paths, select that path. No state required. Each packet's path is computed independently based purely on its headers.

This approach has beautiful properties:

No memory required: The device doesn't need to remember previous packets. Every decision is made fresh from packet headers.

Scales infinitely: Adding more paths is trivial. Change the modulo, done. No state to migrate.

Survives restarts: A device can crash and reboot, and when the first packet of an existing flow arrives, it makes the same decision it made before because the hash is deterministic.

Handles asymmetry naturally: Forward and reverse flows are different 5-tuples, they hash differently, they can take different paths. The device doesn't care. It just hashes and forwards.

ECMP uses this approach. So do many hardware load balancers when configured for simple distribution. Routers doing per-flow load balancing use this. It's fast (hash computation is cheap), scalable (no state limits), and naturally handles asymmetric routing.

The downside is limited intelligence. You can't do session persistence (same client always to same server) without state. You can't do smart health checking or gradual draining of servers. You can't detect elephant flows and move them. You're making the same decision every time based purely on the hash.

Stateful Tracking: Power with a Price

Stateful devices maintain a table of active flows. When a new flow arrives, they create an entry: this 5-tuple goes to this backend server (or this path, or this security policy). Subsequent packets matching that 5-tuple use the stored decision instead of recomputing.

This enables sophisticated features:

Session persistence: Once a client reaches Server A, all subsequent connections from that client go to Server A until the session expires.

Connection tracking: Firewalls can enforce that SYN-ACKs only come after SYNs, that sequence numbers progress correctly, that connections close properly.

Smart load balancing: Distribute connections based on server load, response times, or application-layer information.

Graceful server removal: Stop sending new connections to a server while letting existing connections complete.

The cost is state. Every active flow consumes memory. State tables have limits (run out of memory = start dropping new connections). State must be managed (timeouts for idle connections, cleanup for closed connections). And critically, state creates the asymmetric routing problem.

A stateful device expects to see both directions of a flow. If it sees the SYN but not the SYN-ACK, it has orphaned state. If it sees the SYN-ACK but not the SYN, it has no state to match against. Asymmetric routing breaks stateful devices that can't synchronize state.

Hybrid Approaches and Practical Compromises

Real deployments often use both approaches at different layers:

Network core uses stateless ECMP: Massive scale, simple forwarding, asymmetry is fine.

Edge uses stateful load balancers: Need session persistence and smart distribution, willing to constrain topology to ensure symmetric flow through load balancers.

Security perimeter uses clustered stateful firewalls: State synchronization within cluster handles local asymmetry, but enforce symmetric routing between geographically distributed sites.

The key is understanding where you need state (and accepting symmetric routing constraints) versus where stateless hashing suffices (and embracing asymmetry).

Linux Networking and the Fast Path Problem

Here's a fun asymmetric routing failure mode that has nothing to do with firewalls or NAT: Linux's receive side scaling and queue handling. Modern NICs have multiple receive queues, and Linux distributes packet processing across CPU cores to achieve high throughput. This works great until asymmetric routing sends packets from the same flow to different interfaces, triggering a subtle but painful performance problem.

How RSS and RPS Work

Receive Side Scaling (RSS) is a NIC feature that hashes incoming packets and distributes them across multiple hardware queues. Each queue has a dedicated interrupt line to a specific CPU core. When a packet arrives, the NIC hashes the 5-tuple, selects a queue, and that queue's CPU core processes the packet. This parallelizes packet processing across cores, dramatically improving throughput.

Receive Packet Steering (RPS) is the software equivalent, used when NICs don't support RSS. The kernel receives packets on one queue but distributes them across CPUs based on flow hash.

The critical optimization: each CPU core has caches (L1, L2, sometimes L3) with socket structures, routing tables, connection state, and application data for flows it's processing. When packets for a flow consistently arrive at the same core, that core's caches are hot with relevant data. Processing is fast (fast path) because everything needed is in cache.

What Asymmetry Breaks

Now introduce asymmetric routing at the interface level: forward packets arrive on eth0, reverse packets arrive on eth1. The NICs hash independently using their own RSS configurations. Forward packets hash to CPU 2. Reverse packets hash to CPU 5. Different CPUs handling the same TCP connection.

CPU 2 processes outbound packets. It loads the socket structure into cache, updates TCP state, processes application data, generates ACKs. Its caches are optimized for this connection.

CPU 5 processes inbound packets. It has no cache for this connection. It must fetch the socket structure from main memory (slow), load routing table entries (cache miss), access application buffers (another miss). This is slow path processing: tens of thousands of cycles instead of hundreds.

Worse, there's now contention. Both CPUs need to access and modify the same socket structure. Cache line bouncing between cores adds latency. Lock contention (Linux uses per-socket locks for many operations) causes one core to wait while the other holds the lock. A flow that should be handled entirely in one core's cache is now split across cores with memory and locking overhead.

The performance impact is measurable. A 10 Gbps interface might achieve 9.5 Gbps with symmetric routing (fast path processing) but only 6-7 Gbps with asymmetric routing causing cross-core traffic (slow path). Not because the network path is wrong, but because CPU cache behavior punishes the asymmetry.

Solutions and Workarounds

Configure consistent RSS hashing: Use the same hash algorithm and CPU distribution on all interfaces. This doesn't eliminate asymmetry but reduces the chance of flows splitting across CPUs.

Bond interfaces: Use Linux bonding to combine multiple physical interfaces into one logical interface. Now all packets for a flow arrive on the same logical interface, hit the same RSS hash, land on the same CPU. This doesn't prevent network-level asymmetry but hides it from the host's perspective.

Use symmetric routing: If possible, configure the network to ensure both directions of flows arrive on the same interface. This might mean suboptimal network paths but optimal host performance.

Accept the performance hit: For many workloads, the penalty is acceptable. A 30% throughput reduction might not matter if you're far from capacity. The cost of enforcing symmetric routing (operational complexity, suboptimal paths) might exceed the benefit.

This Linux-specific issue illustrates a broader point: asymmetric routing can cause problems in unexpected places. It's not just firewalls and NAT. Any system that optimizes based on the assumption of consistent flow paths can be disrupted by asymmetry.

When Asymmetric Routing Is Actually Bad

Let's be honest about where asymmetric routing genuinely causes problems:

Through geographically distributed stateful devices: You cannot cluster firewalls or NAT devices across cities and expect asymmetric routing to work. State synchronization is too slow. This is physics, not a configuration problem.

Through uncoordinated stateful devices: Two independent firewalls with no clustering, each seeing one direction of traffic, will drop connections. If you must put stateful devices in the path, either synchronize state or enforce symmetric routing.

With protocol-inspecting middleboxes: Deep packet inspection devices, application layer gateways, and some intrusion prevention systems need to see both directions of a connection to understand protocol behavior. Asymmetry breaks their analysis.

For performance-sensitive hosts: As the Linux RSS example showed, asymmetric routing to different interfaces on the same host can degrade performance significantly. This matters most for high-throughput servers.

During network debugging: Asymmetric routing makes troubleshooting harder. Packet captures on one path don't show return traffic. Latency measurements become complex. Flow tracking is difficult. This isn't a technical failure but an operational pain point.

Notice the pattern: asymmetric routing is bad when it violates assumptions baked into devices or systems. The routing itself isn't the problem. The problem is architectural decisions (deploying stateful devices, enabling deep inspection, relying on protocol analysis) that assume symmetry.

When Asymmetric Routing Is Fine (Or Even Good)

Now the flip side, where asymmetric routing is perfectly acceptable and often beneficial:

Through stateless routers and switches: Simple forwarding devices don't care about flow direction. Asymmetric routing through a pure L3 network works perfectly.

Across the Internet: BGP creates asymmetric routing naturally, and the Internet depends on it. Any attempt to enforce symmetry would break global routing.

Within ECMP fabrics: Modern datacenter networks use ECMP extensively. Asymmetric routing is expected behavior and causes no problems for properly designed applications.

Through co-located stateful clusters: A cluster of firewalls in the same rack with fast state synchronization handles asymmetry fine. The stateful devices aren't the problem; the geographic distribution is.

For traffic engineering: Optimizing outbound and inbound paths independently improves performance and reduces cost. This is good network design, enabled by accepting asymmetry.

During failover: When paths fail and routing reconverges, temporary asymmetry is inevitable and harmless. Enforcing symmetry during failover extends outages.

The pattern here: asymmetric routing is fine when devices and applications don't depend on seeing both directions of flows. Which is most of the network, most of the time.

Building Networks That Embrace Asymmetry

If we accept that asymmetric routing is normal and beneficial, how do we build networks that handle it correctly?

Design Principle: Minimize Statefulness

The less state in your network, the fewer places asymmetric routing can cause problems. Use stateless devices where possible:

Prefer routing to NATing: If you can use public IP addresses and avoid NAT, do it. One less stateful device in the path.

Use stateless load balancing: For simple distribution, hash-based load balancing works fine and handles asymmetry naturally.

Limit deep inspection: Protocol inspection and application-aware firewalling are powerful but create state. Use them where needed, not everywhere.

Push state to endpoints: Rather than stateful middleboxes, let applications and hosts manage session state. They already do this; don't duplicate it in the network.

Design Principle: Co-locate Stateful Devices

When you need stateful devices, cluster them in a single location with fast state synchronization. This allows asymmetric routing through the cluster:

Deploy firewall clusters in datacenters, not distributed across WAN sites.

Use centralized NAT at Internet edge, not distributed across branches.

Build load balancer clusters with dedicated low-latency sync links.

The key metric is state sync latency versus connection round-trip time. If sync is 10x faster than RTT, asymmetric routing works. If sync is slower than RTT, you need symmetric routing or will drop connections.

Design Principle: Accept Constraints Where Needed

For truly distributed stateful devices (disaster recovery firewalls in different cities, geo-distributed NAT), accept that you must enforce symmetric routing for connections through those devices:

Use policy-based routing to pin connections to specific paths.

Configure AS-path prepending or MEDs to influence BGP routing.

Use active-passive failover instead of active-active to eliminate competing paths.

Limit symmetric enforcement to paths through stateful devices. The rest of the network can remain asymmetric.

Design Principle: Build for Failure Modes

Plan for asymmetry during failures:

Test failover scenarios with asymmetric routing. Don't assume paths remain symmetric when links fail.

Use stateful firewall clustering that survives individual device failures without dropping established connections.

Configure connection tracking timeouts appropriately. Short timeouts reduce orphaned state during asymmetric events.

Monitor for asymmetry and state synchronization lag. Alert when sync latency approaches connection establishment time.

Design Principle: Document and Educate

Make asymmetric routing an explicit architectural decision, not an accident:

Document where asymmetric routing is expected (Internet paths, ECMP fabrics) versus where it must be symmetric (through distributed stateful devices).

Train operations teams to understand asymmetry. Stop blaming it reflexively for every problem.

Include asymmetry in troubleshooting procedures. Capture traffic on both paths, expect different routes for forward and reverse flows.

Update runbooks to check for stateful device issues rather than assuming symmetric routing is required.

The Uncomfortable Truth About Network Design

The networking industry has spent decades treating asymmetric routing as a pathology to be eliminated. Conference talks discuss how to prevent it. Vendor documentation warns about it. Network engineers configure elaborate policy routing to avoid it. This is backwards.

Asymmetric routing is how networks naturally behave when you let routing protocols optimize paths independently. It's how the Internet achieves scale, resilience, and performance. Fighting it means suboptimal routing, complex policy, and operational overhead. All to accommodate stateful devices that weren't designed for distributed systems.

The real problem isn't asymmetric routing. The real problem is:

Deploying stateful devices in places where state can't synchronize fast enough (geographically distributed firewalls, cross-continent NAT).

Using stateful inspection where it's not needed (application layer firewalling for protocols that don't need it).

Failing to cluster stateful devices with proper state synchronization.

Assuming symmetry in systems (like Linux RSS) that perform better with it but don't require it.

Blaming routing instead of fixing the actual bottleneck (slow state sync, poor clustering, suboptimal device placement).

Fix those problems and asymmetric routing stops being a problem. Better yet, it becomes an asset: your network can optimize paths independently, handle failures gracefully, and scale efficiently because you're not fighting natural routing behavior.

The Path Forward: Embracing Reality

Moving forward, we need to stop treating asymmetric routing as the enemy:

Default to allowing asymmetry: Design networks that work with asymmetric routing by default. Only enforce symmetry where actually required.

Cluster stateful devices properly: If you need firewalls, NAT, or load balancers, deploy them in clusters with fast state sync. Don't distribute them geographically and expect things to work.

Use stateless approaches where possible: Hash-based load balancing, stateless packet filtering, routing without inspection all work fine with asymmetry and scale better.

Acknowledge latency physics: State cannot synchronize faster than light in fiber. Design around this constraint, don't pretend it doesn't exist.

Stop blaming asymmetry for unrelated problems: When connections fail, check state synchronization, device configuration, and firewall rules before assuming asymmetric routing is the culprit.

Educate the industry: Conference talks should discuss how to handle asymmetric routing correctly, not how to eliminate it. Vendor documentation should explain state synchronization requirements, not warn vaguely about asymmetry.

Update best practices: RFC 1925 rule 11 says "Every old idea will be proposed again with a different name and a different presentation, though this too is a consequence of the Fundamental Theorem." The old idea here is that packets should follow the same path in both directions. The presentation has changed (from circuit switching to stateful inspection) but it's still wrong for the same reasons: it doesn't scale, it limits optimization, and it fights natural routing behavior.

Living with Asymmetry

Asymmetric routing is not a bug. It's a feature of distributed systems where independent agents (routers, BGP processes, ECMP hashing) make optimal local decisions without global coordination. The Internet scales to billions of devices and zettabytes of traffic precisely because we don't require packets to follow symmetric paths.

The problems attributed to asymmetric routing are actually problems with:

Stateful devices deployed without adequate clustering and state synchronization

Geographic distribution of stateful infrastructure beyond the latency threshold where state sync can work

Assumptions about packet paths baked into systems (CPU core affinity, cache optimization) that don't fundamentally require symmetry

Lazy troubleshooting that blames asymmetry instead of investigating actual root causes

Fix those problems and asymmetric routing works beautifully. Co-locate your stateful devices in fast-sync clusters. Use stateless approaches where statefulness isn't required. Accept symmetric routing constraints only where physics (state sync latency vs RTT) demands it. Design networks that embrace asymmetry as the natural consequence of optimal distributed routing.

The next time someone blames asymmetric routing for a network problem, ask them: is the routing actually wrong, or did we deploy stateful devices that can't handle the reality of how networks work? Usually it's the latter. Asymmetric routing is doing its job, optimizing paths independently. The stateful middlebox expecting symmetry is the actual problem.

Stop fighting asymmetric routing. Start building networks that handle it correctly. The Internet will thank you, your performance will improve, and your operations will simplify. Because asymmetric routing isn't the problem. It's the solution we've been using all along, just with a bad reputation from decades of misplaced blame.