gistfile1.md

1. mininet: mn --controller=remote,ip=127.0.0.1 --topo topo --custom topo.py
2. h1 ping h4
3. open other terminal
4. type "ovs-ofctl dump-flows s2" to make sure the flow is pass through s2.
5. type "ifocnfig s1-eth1" to shutdown the link between s1 and s2
6. The result in the mininet promt:

64 bytes from 10.0.0.4: icmp_req=71 ttl=64 time=0.028 ms
64 bytes from 10.0.0.4: icmp_req=72 ttl=64 time=195 ms
64 bytes from 10.0.0.4: icmp_req=73 ttl=64 time=0.157 ms
64 bytes from 10.0.0.4: icmp_req=74 ttl=64 time=0.023 ms
64 bytes from 10.0.0.4: icmp_req=75 ttl=64 time=0.025 ms
64 bytes from 10.0.0.4: icmp_req=76 ttl=64 time=0.023 ms
64 bytes from 10.0.0.4: icmp_req=77 ttl=64 time=0.041 ms
64 bytes from 10.0.0.4: icmp_req=78 ttl=64 time=10.2 ms
64 bytes from 10.0.0.4: icmp_req=79 ttl=64 time=0.320 ms
64 bytes from 10.0.0.4: icmp_req=84 ttl=64 time=118 ms
64 bytes from 10.0.0.4: icmp_req=85 ttl=64 time=0.290 ms
64 bytes from 10.0.0.4: icmp_req=86 ttl=64 time=0.038 ms
64 bytes from 10.0.0.4: icmp_req=87 ttl=64 time=0.031 ms
64 bytes from 10.0.0.4: icmp_req=88 ttl=64 time=0.032 ms
64 bytes from 10.0.0.4: icmp_req=89 ttl=64 time=0.033 ms

According to the above result, you can see i shutdown the interface on icmp_req=79, and the ping will work again after icmp_req=85. 7. type "ovs-ofctl dump-flows s3" to make sure the flow is pass through s3 now.

topo.py

from mininet.topo import Topo
class MininetTopo(Topo):
  def __init__(self,**opts):
    Topo.__init__(self, **opts)
    host1 = self.addHost('h1')
    host2 = self.addHost('h2')
    host3 = self.addHost('h3')
    host4 = self.addHost('h4')
    self.switch = {}
    for s in range(1,5):
        self.switch[s-1] = self.addSwitch('s%s' %(s))
    self.addLink(self.switch[0], self.switch[1])
    self.addLink(self.switch[0], self.switch[2])
    self.addLink(self.switch[1], self.switch[2])
    self.addLink(self.switch[1], self.switch[3])
    self.addLink(self.switch[2], self.switch[3])
    self.addLink(self.switch[0], host1)
    self.addLink(self.switch[1], host2)
    self.addLink(self.switch[3], host3)
    self.addLink(self.switch[3], host4)
topos = {'topo':(lambda:MininetTopo())}