(ASCEND) Poor TNT performance?

[Simon Rainey <srainey@rmplc.net>]

Hi,

Question:
Does anyone have any experience with a TNT handling ISDN calls only and
running close to the theoretical throughput of 45Mbps?

Background:
We are using TNTs in an ISDN-only environment. A typical config is:

3 x 8-port E1
4 x HDLC-2
1 x 10/100 ethernet

The hardware is all the latest revision and we're running 7.0.1 firmware.
We authenticate using RADIUS and run OSPF. We typically see around one call
coming up or going down per second on each primary rate. All routing is
dynamic, with updates performed via OSPF.

With this config a TNT should be able to handle 720 ISDN calls.

In our application the remote sites primarily have either a 64kbps service
or a 128kbps service using multilink. Each site is an IP routed network
with between 30 and 200 stations. The main application is Internet access,
although there is some videoconferencing using IP. Every site demands their
full bandwidth and our observations are that most of them do indeed
saturate their links almost continously during the day.

Until recently we had fewer than 10 E1 circuits terminating on each TNT.
When we increased this we noticed that the throughput began to flat top at
around 11Mbps. Performing a throughput test revealed that we could only get
20kbps over a 64kbps connection. This was from a local server on the same
100Mbps switched backbone. Repeating the test via a Cisco AS5300 confirmed
that we could achieve 64kbps across the link.

If the TNT really can support 720 ISDN calls then I would expect it to be
able to deliver 720 x 64kbps = 45Mbps. I suspect that this is a pipedream
.... I remember only too clearly that the MAX 4000 could never cope with 60
ISDN calls.

When the throughput begins to flat top the console becomes very slow and
erratic. I've also seen some SNMP timeouts being reported. This all smacks
of the CPU running out of steam, and I suspect it is the packet routing
that is causing the problem (am I right in assuming that the TNT's IP
routing is all done by the main CPU rather than by a hardware routing
engine?).

Any comments would be most appreciated.

Regards,
Simon.

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