High latency when idle, low latency when passing traffic

Mark Williams · Guest

We are using ppp multilink bundle connections throughout out network to
aggregate bandwidth from multiple T1s. One of these connections is
exhibitying bizarre behavior.

At night, when the traffic rate is low, latency on the line increases
to about 300 ms. As soon as data starts coming down the line, latency
drops down to sub-80 ms. There are 7 T1s in the multilink bundle. I
checked the DSU settings on both ends for each of the lines, and they
all look correct. We are using ESF / B8ZS.

We have three other connects using similar routers, IOS versions, and
numbers of T1s in the multilink bundle;they all show what you'd expect:
really low latency at night, with increases under heavy load. It's not
an issue of ARPing adding to the latency.

Could this be a timing issue?

Guest

Some things to look at:
1. CPU - show proc cpu
2. Interface drops
3. Interface queue . Are there packets in the hold-queue on the
interface waiting to be sent? (show interface)
4. Interface errors on a particular T1.

Steve Griffin
www.blueconsole.com (Bluetooth Wireless Console Cable)

clubfoot · Guest

Mark,

Can you swap the suspect T1 with another T1 at both ends. This could prove
the T1 is at fault. Also, look at the NIU's. If you see an "L1" or "L2" LED
that is orange, red or not lit then the problem is with telco. If the "DS1"
LED is yellow or red then the problem is in house. When we had problems with
latency, sometimes it was the router or CSU/DSU but most often it was due to
a T1 issue. Sometimes one loop of the T1 followed a different path through
the crossboxes than the other loop and this caused problems - this is hard
to prove though since it may not show up as an error on the LED's and you
would need to physically get into the T1 circuit with a TDR and check the
distance of the two cable pairs and then get telco to match the pairs. Other
issues could be half-taps on the circuit between you and telco - again they
may only be detectable by getting into telco's circuits and they probably
would not show errors on the LED's on the NIU, but can cause errors that
only show up during specific situations. Do you have access to a TBERD (T1
tester)? If so then you can run tests on the T1 (quasi, all ones, all zeros,
3 in 8, etc.) the different tests will identify different issues with the
T1. You may have a problem with a circuit that has an exposed wire someware
between you and telco and at night moisture builds up and causes errors due
to high impedance shorts - when you send a lot of traffic through the
circuit then the current dries out the moisture and the circuit cleans up.
This may sound like sci-fi but I have had more then one T1 tech tell me that
this is more common then one may think. To prove this you may have to check
the NIU LED's late at night to see if they change from what they are during
the day. I couldn't tell you how many times I was at a site at 2-3-4 am
trying to prove or disprove telco as the problem. God I'm glad I got out of
hi-tech. To put things in reference - I worked on a project (many years)
where among other things we maintained 96 sites and over 150 T1's, OC-3
through OC-48, microwave, etc. and Cisco routers, Nortel voice and Marconi
ATM, and over 90% of the problems we encountered were with the telco T1's.
5-9's my @$$!
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