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Cisco BTS 10200 Softswitch

Field Notice: BTS 10200 Version 2.1 Fault When SDP Messages Exceed 500 Bytes


Revised November 5, 2002


Products Affected

Product

Comments

BTS 10200 Release 2.1

When interworking with MGCP gateways and access devices that send SDP messages larger than 500 bytes.

Problem Description

In VoIP networks using the BTS 10200 v2.1 call agent, if an MGCP gateway or access device sends an SDP message longer than 500 bytes, the BTS may stop processing incoming calls, stop pinging the devices with MGCP msgs, and stop responding to RSIP messages. The BTS will respond to Q.931 SETUP messages with DISCONNECT "Temporary Failure". This SDP message length may be seen in gateways running Cisco IOS® Release 12.2(11)T2 and later if default parameters are used in these devices. The problem is not limited to Cisco IOS devices. Any device sending SDP messages longer than 500 bytes may trigger the situation.

Background

When making basic MGCP calls using the BTS 10200 Version 2.1, if the SDP in the 200 OK response to the CRCX exceeds 500 bytes, the MGA process in the BTS will crash. No further calls can be made. The BTS will not respond to RSIPs, will not send AUEP msgs and will not ping the gateways. Q.931 setup messages will be rejected with Temporary Failure errors.

For gateways running Cisco IOS software, this will only happen if the following conditions are met. Under these conditions, the SDP will reach approximately 520 bytes.

  1. T38 is not inhibited. This is the default setting.

  2. Gateways are running Cisco IOS Software Release 12.2(11)T2 or later.

  3. The gateway interfaces are in high-complexity mode. This default on 5300 and 5400 gateways.

Other gateways which use the NM-HDV network module (e.g. 26xx/36xx) use medium-complexity mode by default. If these devices are configured for high-complexity mode this problem could occur.High-complexity mode enables more codecs which grows the SDP, causing this problem. Cisco IOS software-based gateways which are running in medium-complexity mode will not have this problem even if T38 is not inhibited because the SDP is under 500 bytes.For gateways not running Cisco IOS software it is also critical to ensure those gateways are configured to avoid SDP messages larger than 500 bytes.

Problem Symptoms

The BTS stops processing incoming calls, stops pinging the gateways with MGCP msgs, and stops responding to RSIP messages. The BTS responds to Q.931 SETUP messages with DISCONNECT Temporary Failure.

Workaround/Solution

If this situation happens in an active system, restart the BTS to recover. To prevent this issue on Cisco IOS software-based gateways, inhibit T38 fax using the following commands:

configure terminal

mgcp fax t38 inhibit

end

For non-Cisco IOS gateways and devices, customer should work to keep the length of SDP messages shorter than 500 bytes.

This issue is resolved in BTS 10200 software version 3.0 and higher.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

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