對於普通的switch，都會有這個學習的過程，當一個包到來的時候，由於包里面有MAC，VLAN Tag，以及從哪個口進來的這個信息。於是switch學習后，維護了一個表格port –> MAC –> VLAN Tag。
這樣以后如果有需要發給這個MAC的包，不用ARP，switch自然之道應該發給哪個port，應該打什么VLAN Tag。
OVS也要學習這個，並維護三個之間的mapping關系。
在我們的例子中，無論是從port進來的本身就帶Tag的，還是從port 2, 3, 4進來的后來被打上Tag的，都需要學習。
sudo ovs-ofctl add-flow helloworld "table=2 actions=learn(table=10, NXM_OF_VLAN_TCI[0..11], NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[], load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]), resubmit(,3)"
這一句比較難理解。
learn表示這是一個學習的action
table 10，這是一個MAC learning table，學習的結果會放在這個table中。
NXM_OF_VLAN_TCI這個是VLAN Tag，在MAC Learning table中，每一個entry都是僅僅對某一個VLAN來說的，不同VLAN的learning table是分開的。在學習的結果的entry中，會標出這個entry是對於哪個VLAN的。
NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[]這個的意思是當前包里面的MAC Source Address會被放在學習結果的entry里面的dl_dst里面。這是因為每個switch都是通過Ingress包來學習，某個MAC從某個 port進來，switch就應該記住以后發往這個MAC的包要從這個port出去，因而MAC source address就被放在了Mac destination address里面，因為這是為發送用的。
NXM_OF_IN_PORT[]->NXM_NX_REG0將portf放入register.
一般對於學習的entry還需要有hard_timeout，這是的每個學習結果都會expire，需要重新學習。
我們再來分析一個實踐中，openstack中使用openvswitch的情況，這是br-tun上的規則。
cookie=0x0, duration=802188.071s, table=10, n_packets=4885, n_bytes=347789, idle_age=730, hard_age=65534, priority=1 actions=learn(table=20,hard_timeout=300,priority=1,NXM_OF_VLAN_TCI[0..11],NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],load:0->NXM_OF_VLAN_TCI[],load:NXM_NX_TUN_ID[]->NXM_NX_TUN_ID[],output:NXM_OF_IN_PORT[]),output:1
cookie=0x0, duration=802187.786s, table=20, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=0 actions=resubmit(,21)
cookie=0x0, duration=802038.514s, table=20, n_packets=1239, n_bytes=83620, idle_age=735, hard_age=65534, priority=2,dl_vlan=1,dl_dst=fa:16:3e:7e:ab:cc actions=strip_vlan,set_tunnel:0x3e9,output:2
cookie=0x0, duration=802187.653s, table=21, n_packets=17, n_bytes=1426, idle_age=65534, hard_age=65534, priority=0 actions=drop
cookie=0x0, duration=802055.878s, table=21, n_packets=40, n_bytes=1736, idle_age=65534, hard_age=65534, dl_vlan=1 actions=strip_vlan,set_tunnel:0x3e9,output:2
這里table 10是用來學習的。table 20是learning table。如果table 20是空的，也即還沒有學到什么，則會通過priority=0的規則resubmit到table 21.
table 21是發送規則，將br-int上的vlan tag消除，然后打上gre tunnel的id。
上面的情況中，table 20不是空的，也即發送給dl_dst=fa:16:3e:7e:ab:cc的包不用走默認規則，直接通過table 20就發送出去了。
table 20的規則是通過table 10學習得到的，table 10是一個接受規則。最終output 1，發送給了br-int
NXM_OF_VLAN_TCI[0..11]是記錄vlan tag，所以學習結果中有dl_vlan=1
NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[]是將mac source address記錄，所以結果中有dl_dst=fa:16:3e:7e:ab:cc
load:0->NXM_OF_VLAN_TCI[]意思是發送出去的時候，vlan tag設為0，所以結果中有actions=strip_vlan
load:NXM_NX_TUN_ID[]->NXM_NX_TUN_ID[]意思是發出去的時候，設置tunnul id，所以結果中有set_tunnel:0x3e9
output:NXM_OF_IN_PORT[]意思是發送給哪個port，由於是從port2進來的，因而結果中有output:2
測試一：從port 1來一個vlan為20的mac為50:00:00:00:00:01的包
$ sudo ovs-appctl ofproto/trace helloworld in_port=1,vlan_tci=20,dl_src=50:00:00:00:00:01 -generate
Flow: metadata=0,in_port=1,vlan_tci=0x0014,dl_src=50:00:00:00:00:01,dl_dst=00:00:00:00:00:00,dl_type=0x0000
Rule: table=0 cookie=0 priority=0
OpenFlow actions=resubmit(,1)
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Rule: table=1 cookie=0 priority=99,in_port=1
OpenFlow actions=resubmit(,2)
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Rule: table=2 cookie=0
OpenFlow actions=learn(table=10,NXM_OF_VLAN_TCI[0..11],NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]),resubmit(,3)
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
No match
Final flow: unchanged
Relevant fields: skb_priority=0,in_port=1,vlan_tci=0x0014/0x0fff,dl_src=50:00:00:00:00:01,dl_dst=00:00:00:00:00:00/ff:ff:ff:ff:ff:f0,dl_type=0x0000,nw_frag=no
Datapath actions: drop
$ sudo ovs-ofctl dump-flows helloworld
NXST_FLOW reply (xid=0x4):
cookie=0x0, duration=90537.25s, table=0, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=0 actions=resubmit(,1)
cookie=0x0, duration=90727.209s, table=0, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, dl_src=01:00:00:00:00:00/01:00:00:00:00:00 actions=drop
cookie=0x0, duration=90662.724s, table=0, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, dl_dst=01:80:c2:00:00:00/ff:ff:ff:ff:ff:f0 actions=drop
cookie=0x0, duration=86147.941s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=2,vlan_tci=0x0000 actions=mod_vlan_vid:20,resubmit(,2)
cookie=0x0, duration=86147.941s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=4,vlan_tci=0x0000 actions=mod_vlan_vid:30,resubmit(,2)
cookie=0x0, duration=86147.941s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=3,vlan_tci=0x0000 actions=mod_vlan_vid:30,resubmit(,2)
cookie=0x0, duration=86278.986s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=1 actions=resubmit(,2)
cookie=0x0, duration=86357.407s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=0 actions=drop
cookie=0x0, duration=83587.281s, table=2, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, actions=learn(table=10,NXM_OF_VLAN_TCI[0..11],NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]),resubmit(,3)
cookie=0x0, duration=31.258s, table=10, n_packets=0, n_bytes=0, idle_age=31, vlan_tci=0x0014/0x0fff,dl_dst=50:00:00:00:00:01 actions=load:0x1->NXM_NX_REG0[0..15]
table 10多了一條，vlan為20，dl_dst為50:00:00:00:00:01，發送的時候從port 1出去。
測試二：從port 2進來，被打上了vlan 20，mac為50:00:00:00:00:02
$ sudo ovs-appctl ofproto/trace helloworld in_port=2,dl_src=50:00:00:00:00:02 -generate
Flow: metadata=0,in_port=2,vlan_tci=0x0000,dl_src=50:00:00:00:00:02,dl_dst=00:00:00:00:00:00,dl_type=0x0000
Rule: table=0 cookie=0 priority=0
OpenFlow actions=resubmit(,1)
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Rule: table=1 cookie=0 priority=99,in_port=2,vlan_tci=0x0000
OpenFlow actions=mod_vlan_vid:20,resubmit(,2)
Resubmitted flow: metadata=0,in_port=2,dl_vlan=20,dl_vlan_pcp=0,dl_src=50:00:00:00:00:02,dl_dst=00:00:00:00:00:00,dl_type=0x0000
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Rule: table=2 cookie=0
OpenFlow actions=learn(table=10,NXM_OF_VLAN_TCI[0..11],NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]),resubmit(,3)
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
No match
Final flow: unchanged
Relevant fields: skb_priority=0,in_port=2,vlan_tci=0x0000,dl_src=50:00:00:00:00:02,dl_dst=00:00:00:00:00:00/ff:ff:ff:ff:ff:f0,dl_type=0x0000,nw_frag=no
Datapath actions: drop
$ sudo ovs-ofctl dump-flows helloworld
NXST_FLOW reply (xid=0x4):
cookie=0x0, duration=90823.14s, table=0, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=0 actions=resubmit(,1)
cookie=0x0, duration=91013.099s, table=0, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, dl_src=01:00:00:00:00:00/01:00:00:00:00:00 actions=drop
cookie=0x0, duration=90948.614s, table=0, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, dl_dst=01:80:c2:00:00:00/ff:ff:ff:ff:ff:f0 actions=drop
cookie=0x0, duration=86433.831s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=2,vlan_tci=0x0000 actions=mod_vlan_vid:20,resubmit(,2)
cookie=0x0, duration=86433.831s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=4,vlan_tci=0x0000 actions=mod_vlan_vid:30,resubmit(,2)
cookie=0x0, duration=86433.831s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=3,vlan_tci=0x0000 actions=mod_vlan_vid:30,resubmit(,2)
cookie=0x0, duration=86564.876s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=99,in_port=1 actions=resubmit(,2)
cookie=0x0, duration=86643.297s, table=1, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, priority=0 actions=drop
cookie=0x0, duration=83873.171s, table=2, n_packets=0, n_bytes=0, idle_age=65534, hard_age=65534, actions=learn(table=10,NXM_OF_VLAN_TCI[0..11],NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[],load:NXM_OF_IN_PORT[]->NXM_NX_REG0[0..15]),resubmit(,3)
cookie=0x0, duration=4.472s, table=10, n_packets=0, n_bytes=0, idle_age=4, vlan_tci=0x0014/0x0fff,dl_dst=50:00:00:00:00:02 actions=load:0x2->NXM_NX_REG0[0..15]
cookie=0x0, duration=317.148s, table=10, n_packets=0, n_bytes=0, idle_age=317, vlan_tci=0x0014/0x0fff,dl_dst=50:00:00:00:00:01 actions=load:0x1->NXM_NX_REG0[0..15]
摘錄自http://www.cnblogs.com/popsuper1982/p/3800535.html