Cisco 3600 Series Multiservice Platforms

Cisco 3600 High-Speed Serial Interface (HSSI) Performance Improvements

October 27, 1998

Products Affected




HSSI network module for the Cisco 3600 Series

Problem Description

Cisco has engineered two performance improvements for the HSSI interface on the Cisco 3600 Series. Prior to these performance enhancements, the HSSI interface would experience packet underruns with bi-directional traffic at 22 Mbps. With these performance improvements, the rate of packet underruns has been reduced. Cisco has now measured a packet underrun rate of less than 1 percent with 30 Mbps bi-directional traffic. If a customer application demands higher performance with fewer dropped packets, a Cisco 7200 Series router is a better solution.


The HSSI network module has two serial controllers. One controller is used for transmit packets, and the other for receive packets. With Cisco part number 800-03616-01 (as shown in the Cisco IOS show diag command), the HSSI bus arbitration scheme gave preference to the transmit controller. This could lead to the receive controller being unable to access the bus in order to transfer packets into memory, leading to an overrun (which results in the packet being dropped).

With Cisco part number 800-03616-02, the bus arbitration scheme gives equal priority to the receive controller. This will reduce the number of receive overflows. However, since the transmit controller has lower priority that before, it may increase the number of transmit underruns.

In order to get the NM-1HSSI version 800-03616-02, you must place an RMA for the NM-1HSSI network module.

The second change to improve HSSI performance is described in DDTS CSCdj95459. This change improves the ability of the HSSI network module to access the protocol control information (PCI) bus in the Cisco 3600. This change improves the access to the PCI bus for both the transmit and receive controllers, thus it can improve both receive overruns and transmit underruns. This change is implemented in a Cisco IOS software upgrade.

Problem Symptoms

The problem is seen as a high number of receive overruns or transmit underruns as shown in blue text in the Cisco IOS show interface command output below:

hssi-3640# show interface hs 3/0
Hssi3/0 is up, line protocol is up 
Hardware is M1T-HSSI-B 
Internet address is
MTU 4470 bytes,
BW 8192 Kbit,DLY 200 usec, rely 255/255,load 254/255
Encapsulation FRAME-RELAY,loopback not set,keepalive set (10 sec)
LMI enq sent  460, LMI stat recvd 460, LMI upd recvd 0, DTE LMI up
LMI enq recvd 0, LMI stat sent  0,LMI upd sent  0
LMI DLCI 1023    LMI type is CISCO  frame relay DTE
Broadcast queue 0/100, broadcasts sent/dropped 0/0, interface broadcasts 0
Last input 000007, output 000000, output hang never
Last clearing of "show interface" counters 011645
Queueing strategy fifo
Output queue 0/40, 0 drops; input queue 0/500, 0 drops
2 minute input rate 24757000 bits/sec, 3041 packets/sec
2 minute output rate 24757000 bits/sec, 3041 packets/sec
    13958540 packets input, 1319252671 bytes, 0 no buffer
    Received 768 broadcasts, 0 runts, 0 giants, 0 throttles,  0 parity
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
    13955072 packets output, 1315642787 bytes, 14790 underruns
    14790 output errors, 0 applique, 0 interface resets
    0 output buffer failures, 0 output buffers swapped out
    0 carrier transitions     TM=down  CA=up  LC=down


If you experience a high number of underruns, you can do the following:

  1. Install a version of Cisco IOS 11.3(4.3)T, 12.0(0.12), or 11.3(4.3)NA

  2. If you have a NM-1HSSI with part number 800-03616-01, you can exchange it with part number 800-03616-02 or later.

Request for Change Notification (RCN)




36xx: HSSI Arbiter with Equal Priority Algorithm

Bug Details

If you are a registered CCO user and you have logged in, you can view this BugID CSCdj95459.

For More Information

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