[lvs-users] LVS-DR + 2 pools in 2 networks = hair pulling

Thomas tom at calixo.net
Thu Apr 19 10:49:49 BST 2012

Hey guys,

I would like to have to following flow with A and B being two different networks (one public and one private range) on the same load balancer. 


A client hits a serverA in pool A via VIPA which serverA with its RIP (RIPA1 or RIPA2) will hits VIPB and will receive an answer from a server in pool B. Everything works until RIPA1 or RIPA2 tries to connect to VIPB. The packets arrives from RIPA1 or RIPA2 on the load balancer (lb1) then nothing, it's like the packet disappears.

lb1 is both networks A _and_ B and only use one gateway, gateway from A.
lb1 eth0= - gw is
lb1 eth1= - no gw

A servers are only in network A and use their respective gateway for this network
 VIPA= (on eth0 so gw is

B servers are only in network B and use their respective gateway for this network
 VIPB= (on eth1 so gw is still


lb1# ipvsadm -Ln
  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
TCP rr
  ->          Route   1      0          0         
  ->          Route   1      0          0         
TCP rr
  ->            Route   1      0          0         
  ->            Route   1      0          0   

lb1# netstat -nr
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface   U         0 0          0 eth0   U         0 0          0 eth1         UG        0 0          0 eth0

lb1# tcpdump -i eth1 ip dst -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
19:18:50.969769 IP > Flags [S], seq 1331246417, win 5840, options [mss 1460,nop,nop,TS val 1603281160 ecr 0,nop,wscale 7], length 0

lb1#  arp -a
RIPA1( at 00:18:51:28:aa:d3 [ether] on eth0
RIPB1 ( at 00:18:51:5f:cd:11 [ether] on eth1
RIPA2 ( at 00:18:51:e0:c6:e3 [ether] on eth0
gwA ( at 00:18:19:9e:cf:ef [ether] on eth0
RIPB2 ( at 00:18:51:9f:88:bd [ether] on eth1

*What I see*
Everything is load balanced properly in pool A from ANY networks.
Everything is load balanced properly in pool B from this network ONLY. So any client in will be load balanced and will hit RIPB1 or RIPB2.
But when lets say RIPA1 ( sends a packet to VIPB ( I see the packet coming in via eth1 on the loadbalancer lb1 and then nothing. No ARP rewrite nothing. Weirdly enough, desperate, I changed the gw on lb1 to be, gateway of B. So it's not anymore. And guess what then it works but I lose a working pool A. It's like I cannot have both. So why should I have a gateway to make it work ? The packet is coming on the right interface (eth1) therefore the good network, so an ARP rewrite should happen and bob's your uncle... except it's not ;)

*What I CANNOT see*
A packet coming on one of the B server (RIPB1 or RIPB2)

1. So is it possible to use one load balancer to load balance 2 differents network __AND__ let the servers in the pools to hit each others. Or is it wrong by design ?
2. Where is the packet going on the load balancer ? How can I track it ? (tcpdump is not enough or I am doing it wrong):

- Centos 6.2 - kernel: 2.6.32-220.4.2.el6.x86_64
- keepalived
- VMs on VMWare with VMXNET3 NICs

Thanks for reading so far. Any kind of hints will be greatly appreciated. I am really curious to understand why this happens.  


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