in-path asymmetric routing detection enable
Enables asymmetric route detection.
Syntax
[no] in-path asymmetric routing detection enable
Parameters
None
Usage
Asymmetric route detection automatically detects and reports asymmetric routing conditions and caches this information to avoid losing connectivity between a client and a server.
For details about asymmetric routing, see the SteelHead Management Console User’s Guide and the SteelHead Deployment Guide.
Types of asymmetry:
Complete Asymmetry - Packets traverse both SteelHeads going from client to server but bypass both SteelHeads on the return path.
• Asymmetric routing table entry: bad RST
• Log: Sep 5 11:16:38 amnesiac kernel: [intercept.WARN] asymmetric routing between 10.11.111.19 and 10.11.25.23 detected (bad RST)
Server-Side Asymmetry - Packets traverse both SteelHeads going from client to server but bypass the server-side SteelHead on the return path.
• Asymmetric routing table entry: bad SYN/ACK
• Log: Sep 7 16:17:25 amnesiac kernel: [intercept.WARN] asymmetric routing between 10.11.25.23:5001 and 10.11.111.19:33261 detected (bad SYN/ACK)
Client-Side Asymmetry - Packets traverse both SteelHeads going from client to server but bypass the client-side SteelHead on the return path.
• Asymmetric routing table entry: no SYN/ACK
• Log: Sep 7 16:41:45 amnesiac kernel: [intercept.WARN] asymmetric routing between 10.11.111.19:33262 and 10.11.25.23:5001 detected (no SYN/ACK)
Multi-SYN Retransmit- Probe-Filtered - Occurs when the client-side SteelHead sends out multiple SYN+ frames and does not get a response.
• Asymmetric routing table entry: probe-filtered(not-AR)
• Log: Sep 13 20:59:16 amnesiac kernel: [intercept.WARN] it appears as though probes from 10.11.111.19 to 10.11.25.23 are being filtered. Passing through connections between these two hosts.
Multi-SYN Retransmit- SYN-Rexmit - Occurs when the client-side SteelHead receives multiple SYN retransmits from a client and does not see a SYN/ACK packet from the destination server.
• Asymmetric routing table entry: probe-filtered(not-AR)
• Log: Sep 13 20:59:16 amnesiac kernel: [intercept.WARN] it appears as though probes from 10.11.111.19 to 10.11.25.23 are being filtered. Passing through connections between these two hosts.
You can use the following tools to detect and analyze asymmetric routes:
TCP Dump - Run a TCP dump on the client-side SteelHead to verify the packet sequence that is causing the asymmetric route detection. You can take traces on the LAN and WAN ports of the SteelHead and, based on the packet maps, look for the packet sequence that is expected for the type of warning message in the log. For example to obtain information on all packets on the WAN interface, sourced from or destined to 10.0.0.1, and with a source/destination TCP port of 80:
tcpdump -i wan0_0 host 10.0.0.1 port 80
You can use the following command to filter SYN, SYN/ACK, and reset packets. This command does not display the ACK packets but it can be useful if the link is saturated with traffic and the traces are filling quickly. The following command uses the -i parameter to specify the interface and the -w parameter to write to a file:
tcpdump -i wan1_0 'tcp[tcpflags] & (tcp-syn|tcp-fin|tcp-rst) = 0' -w lookingforasymwan
Trace Route - Run the trace route tool to discover what path a packet is taking from client to server and from server to client. Access the client and run the traceroute command with the IP address of the server, and then run the traceroute command from the server with the IP address of the client. For example for networking equipment:
#Client’s Address: 10.1.0.2
#Server’s Address: 10.0.0.4
client# traceroute 10.0.0.4 Type escape sequence to abort.
Tracing the route to 10.0.0.4
1 10.1.0.1 4 msec 0 msec 4 msec
2 10.0.0.2 4 msec 4 msec 0 msec
3 10.0.0.3 4 msec 4 msec 0 msec
4 10.0.0.4 4 msec 4 msec 0 msec
server# traceroute 10.1.0.2 Type escape sequence to abort.
Tracing the route to 10.1.0.2
1 10.0.0.6 4 msec 0 msec 4 msec
2 10.0.0.5 4 msec 4 msec 0 msec
3 10.1.0.1 4 msec 4 msec 0 msec
4 10.1.0.2 4 msec 4 msec 0 msec
The no command option disables asymmetric route detection and caching.
Example
amnesiac (config) # in-path asymmetric routing detection enable
Product
SteelHead CX, SteelHead EX, SteelHead-v, SteelHead-c
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