How to Identify Asymmetric Routing: Tools and Techniques

As a network engineer and tech blogger, I often face questions about asymmetric routing and its impact on network performance. In this article, we’ll explore how to find it and the tools that can help you detect them. Let’s dive in!

What is Asymmetric Routing?

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Signs of Asymmetric Routing

Before we discuss the tools, let’s look at some common indicators:

1. Different paths in forward and reverse traceroutes
2. Varying number of hops between source and destination
3. Inconsistent latency or packet loss in different directions

Tools to Find Asymmetric Routing

1. Traceroute

Traceroute is a fundamental tool for identifying asymmetric routes. Run traceroute from the source to the destination. Then, run it vice versa. You can compare the paths. Spot the differences between them.

# From source to destination
traceroute destination_ip

# From destination to source
traceroute source_ip

2. Visual Traceroute Tools

Visual traceroute tools offer a graphical representation of network paths, making it easier to spot asymmetries. These tools often offer side-by-side comparisons of forward and reverse paths.

Several visual traceroute tools are available to help network administrators visualize and analyze network paths:

1. Path Analyzer Pro: Offers a visual representation of routes with a line graph showing path speeds per link. It works on both Windows and macOS[1][3].
2. VisualRoute: Provides a graphical user interface that displays routes on a map along with a table of traceroute results. It offers features like statistics storage and reverse tracing[1][3].
3. SolarWinds TraceRoute NG is a free tool. It can run continuously across the network or to a remote location. It detects changes in the standard path[3].
4. NetPath (part of SolarWinds Network Performance Monitor): It offers a comprehensive visual analysis of network paths. The tool shows all devices, applications, networks, and vendors in a single-page view[4].
5. PingPlotter: A popular connectivity tester with graphical representations, available in multiple versions for individuals and businesses[3].
6. CountryTraceRoute: Provides a GUI that displays information like IP addresses, hostnames, and countries for each hop[3].
7. VisualRoute Lite is a free tool for non-business use. It shows routes with IP addresses. It also displays response times for each router in the path[3].

These tools offer various features like geographical mapping, hop-by-hop analysis, and performance metrics. These features help network engineers troubleshoot network paths. They also allow engineers to improve paths more effectively.

3. Network Monitoring Solutions

Comprehensive network monitoring tools, like Obkio’s Network Performance Monitoring Tool, offer continuous monitoring. They can detect asymmetric routing issues in real-time[2].

4. BGP Table Analysis

Examining BGP tables can reveal asymmetric routing configurations. Compare the next-hop IP addresses for traffic in both directions[1].

show ip bgp neighbors <ip_address> received-routes
show ip bgp neighbors <ip_address> advertised-routes

5. Extended Ping with Record Route Option

To execute an Extended Ping with Record Route Option, follow these steps:

1. Enter privileged EXEC mode on your Cisco device.
2. Type ping and press Enter to start the extended ping command.
3. When prompted, set up the following options:

• Protocol: Select IP (default)
• Target IP address: Enter the destination IP
• Repeat count: Specify the number of packets (default is 5)
• Datagram size: Set the packet size (default is 100 bytes)
• Timeout in seconds: Set the timeout value
• Extended commands: Enter ‘y’ to access extra options

1. When you reach “Loose, Strict, Record, Timestamp, Verbose[none]:”, type record to enable the Record Route option.
2. Specify the number of hops to record (maximum 9).
3. Finish any remaining prompts and press Enter to start the ping.

Example output:

R1#ping
Protocol [ip]: 
Target IP address: 192.168.1.1
Repeat count [5]: 
Datagram size [100]: 
Timeout in seconds [2]: 
Extended commands [n]: y
Source address or interface: 
Type of service [0]: 
Set DF bit in IP header? [no]: 
Validate reply data? [no]: 
Data pattern [0xABCD]: 
Loose, Strict, Record, Timestamp, Verbose[none]: record
Number of hops [ 9 ]: 
Loose, Strict, Record, Timestamp, Verbose[RV]: 
Sweep range of sizes [n]: 
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.1, timeout is 2 seconds:
Packet has IP options:  Total option bytes= 40, padded length=40
Record route: <hop1> <hop2> <hop3> <hop4> <hop5> <hop6> <hop7> <hop8> <hop9>

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

To interpret the results of Extended Ping with Record Route Option:

1. Examine the IP addresses listed in the “Record route” section of the output. These addresses represent the interfaces of routers along the path in both directions.
2. The first set of IP addresses shows the forward path from source to destination.
3. The second set of IP addresses (after the destination IP) shows the return path.
4. Compare the forward and return paths to find any asymmetric routing. If the paths differ, it indicates asymmetric routing is occurring.
5. Look for any missing hops or unexpected IP addresses, which could show routing issues or misconfigured devices.
6. Check the number of hops recorded. If it’s less than the limit (usually 9), it means the entire path was captured. If it reaches the limit, some hops may be missing from the output.
7. Analyze the round-trip times for each hop to find any latency issues along the path.
8. If certain hops are consistently missing or showing high latency, it may suggest network problems at those points.

By carefully examining these elements, you can gain valuable insights into the network path. You can find potential issues. Additionally, you can detect asymmetric routing scenarios.

6. Packet Capture Tools

Use packet capture tools like Wireshark to analyze traffic flows and find asymmetric routing patterns.

Visualizing Asymmetric Routing

To better understand asymmetric routing, let’s look at a simple network diagram:

   [Server A]
      |
   [Router 1]----[Router 2]
      |             |
   [Router 3]----[Router 4]
      |
   [Server B]

In this scenario, traffic from Server A to Server B might take the path:
Server A -> Router 1 -> Router 2 -> Router 4 -> Server B

While return traffic takes a different route:
Server B -> Router 3 -> Router 1 -> Server A

This asymmetry can be visualized using a table:

Direction	Path
A to B (Forward)	Server A -> R1 -> R2 -> R4 -> Server B
B to A (Reverse)	Server B -> R3 -> R1 -> Server A

Potential Issues with Asymmetric Routing

While asymmetric routing isn’t always problematic, it can cause issues in certain scenarios:

1. Firewall State Tracking: Firewalls that rely on seeing both directions of traffic may block connections[1].
2. Performance Inconsistencies: Different paths may have varying latencies and bandwidths.
3. Troubleshooting Complexity: Diagnosing network issues becomes more challenging with asymmetric routes[1].

Conclusion

Identifying asymmetric routing is crucial for maintaining a healthy network. You can detect and solve asymmetric routing issues by using tools like traceroute. Visual traceroute applications and comprehensive network monitoring solutions also help. This allows you to prevent these issues before they impact your network’s performance.

Remember, while asymmetric routing can sometimes be beneficial for load balancing, you must understand its implications. Make sure it doesn’t interfere with critical network functions or security measures.

Have you encountered asymmetric routing in your network? Share your experiences and troubleshooting techniques in the comments below!

Citations:
[1] https://www.noction.com/blog/bgp-and-asymmetric-routing
[2] https://obkio.com/blog/traceroutes-internet-traffic-is-asymmetrical/
[3] https://www.netlab.tkk.fi/tutkimus/ipana/paperit/QoSR/S130-QoSR-asymmetric.pdf
[4] https://community.cisco.com/t5/routing/verifying-asymmetric-routing-in-the-network/td-p/2554500
[5] https://docs.netgate.com/pfsense/en/latest/troubleshooting/asymmetric-routing.html
[6] https://www.auvik.com/franklyit/blog/asymmetric-routing-issue/
[7] https://live.paloaltonetworks.com/t5/general-topics/asymmetric-routing/td-p/18958
[8] https://www.linkedin.com/pulse/what-asymmetric-routing-causes-problems-networks-hector-cardenas