Asymmetric Route Detection Meaning

Summary

The "Asymmetric Route Detected" alert shows when the Exinda detects that it is only receiving part of the communication path between hosts. For shaping, this is harmless, but for acceleration and edge cache, the asymmetric route needs to be resolved for proper operation.

Overview

There are many alerts that the Exinda can issue, including an "Asymmetric Route Detection" alert. This alert triggers whenever it detects that there is an asymmetric route that it is a part of, namely using the fact that it is seeing conversations happen out of order or that it is missing part of conversations that are passing through it (that they had taken a different route from source -> destination or destination -> source).

Cause

Asymmetric routing in general is a normal, but unwanted situation in an IP network. Asymmetric routing is a situation where for one reason or another packets flowing in TCP connections flow through different routes to different directions. As a rough example: Host A and B located in different continents are communicating through a TCP connection. Segments sent from host A to host B reach the destination through an ISP 1 link but segments sent from B to A reach the destination through an ISP 2 link.

Asymmetric routing as described above is not a problem for current TCP/IP implementations, since TCP connection does not care which route a certain IP packet takes as long as it reaches its destination in a reasonable time.

Asymmetric routing can occur in a smaller scale also. It could happen in a situation where an organization uses two different routes, like an SSL VPN and an MPLS leased line, for instance, to their branch office. In an Exinda technology context, we speak about asymmetric routing when we have a situation where segments on a single connection come in to the network through one interface or node and go out through another interface or node.

The "Asymmetric Route Alert" in the Exinda firmware alerts administrators to the fact that it detects an asymmetric route.

Resolution

For environments with acceleration, and shaping environments with Edge Cache:

1.- If the IP's alerted are not involved in the acceleration process, the alerts are harmless and can be disabled by going to System-->Setup-->Alerts and disabling the "Asymmetric Route Detection" alert.
2.- If the IP's involved in the alert are potentially being accelerated and some issues in the network are being noticed, investigate the routes that these IPs are taking to reach its destinations. If the root cause of asymmetric routing is found, fix it is possible.
3.- If the routes cannot be modified, the first step would be to make sure all the possible asymmetric links for these flows are going through the same Exinda appliance. In case the appliance only contains one physical bridge, an upgrade to a new device or to a new network card with a higher number of bridges will be necessary. Then go to System-->Optimization-->TCP and disable the option "Dual Bridge Bypass". This procedure works for both Edge Cache and x800 Acceleration scenarios. Note: Connecting several links to the same Exinda can potentially and dramatically increase the number of connections that the hardware can handle, before taking this step, please refer to your regional Exinda SE for suggestions.
4.- If the routes cannot be modified and an upgrade to an appliance and/or to a network card that can host more bridges is not possible, we could place an identical appliance (or appliances) bridging the other link (or links) and connect all appliances together as a cluster. To do that please follow the instructions for HA/Clustering: http://docs.exinda.com/ga-released/exos/6.4/PDFs/cluster-ha.pdf. Then, go to System-->Optimization-->TCP and disable the option "Dual Bridge Bypass" (this change will be replicated among all of the exindas in the cluster if it is applied in the Master Exinda, for more detail follow the above link). However, this solution is only applicable for x800 environments and not for Edge Cache scenarios.

For Exindas not running Edge Cache or acceleration:

The alerts are harmless and can be disabled by going to System-->Setup-->Alerts and disabling the "Asymmetric Route Detection" alert.
The alert is not necessary and harmless, simply disable

Additional Information

There is a command which will show what IPs are causing the Asymmetric Route alerts. From an SSH session with the Exinda, use the following commands:

en
conf t
show monitor asymmetric-route