[ovs-discuss] How to use tcpdump with OVS

Ben Pfaff blp at nicira.com
Tue Jul 30 16:31:10 UTC 2013


On Tue, Jul 30, 2013 at 05:55:27PM +0300, Max Mazur wrote:
> As you can see, I have 2 ports in br-int:
> * qvo22eb488a-9a
> * qvo6715963b-e7
> These ports are attached to VMs with ip addresses 192.168.1.2 and
> 192.168.1.4
> 
> ip link show dev qvo22eb488a-9a
> 35: qvo22eb488a-9a: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500
> qdisc pfifo_fast state UP qlen 1000
>     link/ether 3a:01:46:ad:33:cc brd ff:ff:ff:ff:ff:ff
> [root at compute1 ~]# ip link show dev qvo6715963b-e7
> 27: qvo6715963b-e7: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500
> qdisc pfifo_fast state UP qlen 1000
>     link/ether ee:da:16:bd:93:ac brd ff:ff:ff:ff:ff:ff
> 
> 
> 
> I'm trying to understand traffic flow with tcpdump  and got unexpected (for
> me) results (for one VM. other have same results):
> 
> 
> tcpdump  -n -i qvo22eb488a-9a
> listening on qvo22eb488a-9a, link-type EN10MB (Ethernet), capture size
> 65535 bytes
> 17:20:35.139298 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
> Request from fa:16:3e:27:07:47, length 280
> 17:20:36.905414 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP,
> Request from fa:16:3e:5f:4f:28, length 280
> 17:20:36.906830 IP 192.168.1.3.bootps > 192.168.1.4.bootpc: BOOTP/DHCP,
> Reply, length 321
> 17:20:36.906961 IP 192.168.1.4 > 192.168.1.3: ICMP 192.168.1.4 udp port
> bootpc unreachable, length 357
> 17:20:43.755827 IP 192.168.1.3 > 192.168.1.4: ICMP echo request, id 11567,
> seq 2, length 64
> 
> So I see traffic.
> 
> BUT I can't see it on OVS ifaces:
> 
> 
> tcpdump  -n -i br-int

OVS is a switch, not a hub, so it's not going to forward every packet
to every port.  Presumably no unicast traffic is going to or from
br-int, because you have not added any IP addresses there.



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