[j-nsp] High latency and slow connections

[Blaz Zupan]

I once again have access to my voodoo doll, so the black magic is over - I've
figured out the cause of my problems with high latency and slow connections
through the gigabit ethernet on our Juniper box.

After replacing the complete hardware (M5 and E-FEB with M10 and non-E-FEB,
also new gigabit ethernet PIC and SDH PIC), the problem persisted. Thinking
logically, something apparently must be telling the box to slow down. How do
we call that? Flow control of course.

Looking at the settings on the Cisco 3550 switch, we had the defaults: output
flow control on, input flow control off. A bit of reading in the Cisco
documentation told me what that actually means. The Cisco sends PAUSE frames
to the Juniper when it thinks the Juniper is sending too much traffic and
overwhelming other ports (probably the 24 100baseTX ports).

So, I configured "flowcontrol send off" on the Cisco 3550 and "set interfaces
ge-0/0/0 gigether-options no-flow-control" on the Juniper. As soon as I did
this, latency dropped considerably. LaterI successfully moved nearly 200 Mbps
of traffic from one VLAN through the Juniper to another VLAN without any
noteworthy increase in latency, while previously latency would increase above
100 ms when we were moving around 130 Mbps of traffic. Case closed.

Why the Cisco was sending the PAUSE frames is a question for cisco-nsp, but I
suspect a note in the Cisco documentation which claims that you have to turn
off flowcontrol when you have QoS active (mls qos). QoS was indeed turned on,
although not used.