06-18-2008 07:12 AM - edited 03-05-2019 11:42 PM
My remote site customer has a router that is having an unusually high number of
discard packets compared to other Internet routers with much higher utilization.
Their engineers are thinking that this is due to the fact that they have a 10 Gig
network and Gigabit-connected servers behind the router with a default 40-packet que-depth on the OC3 FIFO interface. They are thinking that it may not be deep enough to hold the packets during brief bursts coming in the Gig interface before going out their 155Mbps WAN interface.
What would be your opinion on changing the output queue depth on router's POS2/0 to a higher value,like 400? The max setting is 4096.
The configuration would be:
int pos2/0
hold-queue 400
!
Their engineers are wanting to know if this would be an acceptable remedy to this
problem or if there is a Cisco-approved number for the hold-queue for an 155Mbps
circuit behind a 10-gig network.
06-18-2008 08:47 AM
40 packets (assuming 1500 coming from Ethernet), would not allow a single TCP flow to ramp up to full OC3 line rate for much WAN latency. Theoretically, the FIFO queue should accommodate the BDP (bandwidth delay product). However, such a large size isn't required for multiple flows, assuming they don't all start at the same time, and such a large queue could introduce highly variable latency.
What you might try would be either activation of WRED on the POS interface and/or CBWFQ FQ.
e.g.
interface pos 2/0
random-detect
or
policy-map CBWFQ
interface pos 2/0
service-policy output CBWFQ
or
policy-map CBWFQ
class class-default
random-detect
fair-queue
interface pos 2/0
service-policy output CBWFQ
Although WRED queue size often defaults about the same as FIFO default queue size, WRED's is elastic. I.e. bursts should pass, only sustained congestion should cause packet drops.
FQ usually defaults to larger queue size than FIFO, but more importantly, each flow gets its own queue. I.e. one large flow burst should have minimal impact on other flows.
06-19-2008 07:15 AM
Joseph,
Thanks for the great input. Will look into your suggestions and give feedback later on.
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