Nexus 3000 – System
class MTU
Nexus 3000 operates in cut-through
mode by default as a result the packet cannot be fragmented thus the need to
have same Ethernet MTU is important. This can be changed to store-forward mode by
using the command “[no] Switching –mode store-forward”. Enabling store and
forward switching mode might impact port to port latency.
The system class MTU sets the MTU for
all packets in the class and cannot be configured larger than the jumbo MTU
(9216 bytes). This is a system
wide change and not specific to particular interface unless
specific interface has an overriding service-policy configuration.
Config example:
policy-map type network-qos jumbo
class type
network-qos class-default
mtu 9216
system qos
service-policy type
network-qos jumbo
- · Please do note that class-default is used, thus it applies to all traffic types.
- · Cut-through on N3K starts from packets at 560 bytes onwards. Smaller packets will be handles in a store and forward fashion.
Verify if
the JUMBO MTU has been enabled/disabled
n3k# show hardware internal cpu-mac inband counters
eth3 Link encap:Ethernet HWaddr 02:10:18:BE:7E:79
UP BROADCAST RUNNING MULTICAST
MTU:9312
Metric:1
RX packets:0 errors:0
dropped:0 overruns:0 frame:0
TX packets:3 errors:0
dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 b) TX bytes:192 (192.0 b)
eth4 Link encap:Ethernet HWaddr 02:10:18:BE:7E:7A
UP BROADCAST RUNNING MULTICAST
MTU:9312 Metric:1
RX packets:4242 errors:0
dropped:0 overruns:0 frame:0
TX packets:2838 errors:0
dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:315148 (307.7 KiB) TX bytes:288965 (282.1 KiB)
Nexus 3000
Switching Mode
In
|
Out
|
N3K Switching mode
|
1Gig
|
1Gig
|
cut-through
|
1Gig
|
10Gig
|
store and forward
|
1Gig
|
40Gig
|
store and forward
|
10Gig
|
1Gig
|
cut-through
|
10Gig
|
10Gig
|
cut-through
|
10Gig
|
40Gig
|
store and forward
|
40Gig
|
1Gig
|
cut-through
|
40Gig
|
10Gig
|
cut-through
|
40Gig
|
40Gig
|
cut-through
|
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