Junos Asymmetric Routing

Joe Blanchard jbfixurpc at gmail.com
Fri May 28 01:33:02 CDT 2010

That would seem to be a good resolution (Firewall/NAT) . Aside from 
that, perhaps a load
balancer for each segment might help?
One question that comes to mind is why (if ISP2 is a backup) would valid 
be using that route?
Unless maybe your loadbalancing using a DNS round robin perhaps to hit 
the second IP space or loadbalancing
the 2 ISPs?
Another "maybe" resolve would be to multi-home the application to that 
segment, i.e. 2 nics on the
server, one on the primary network, the other on the secondary with 
appropriate Def.GWs, of course
since there is little information on the infrastructure here this may 
not be possible.
I suppose if one were to get really detailed about this, you could look 
into reverse routing using MAC, but
theoretically that would/could open a whole other set of issues.

Joe Blanchard

Jian Gu wrote:
> Wouldn't simply configure source NAT on firewall resolve the
> problem gracefully? when connection requests coming in through ISP2,
> source NAT the incoming traffic's source IP with IPs on firewall
> inside interface, that way when server replies, firewall will
> guarantee to receive the ACK because ACK traffic won't follow default
> routing to ISP1.
> On Thu, May 27, 2010 at 4:27 PM, Ken Gilmour <ken.gilmour at gmail.com> wrote:
>> Hi all,
>> I have a very peculiar situation here that i seem to have difficulty
>> explaining in such a way for people to understand. I just got off the phone
>> to the Juniper Devs after about 4 hours with no result. They understand the
>> problem but can't seem to think of a working solution (last solution led to
>> the primary firewall hard crashing and then failing over after a commit
>> (which also makes me wonder what made the primary crash and not the
>> secondary)). I am wondering if there is anyone "creative" on the list who
>> has encountered and worked around this problem before...
>> Here goes *sigh*
>> ISP1 -
>> ISP2 -
>> ISP1 is the default gateway, ISP2 is a backup provider but which is always
>> active. Client comes in on ISP1's link, traffic goes back out on ISP1s link.
>> Client comes in on ISP2's link (non default gateway) but for some reason,
>> the packets seem to be going back out through the link for ISP1.
>> So look at it this way:
>> SYN comes from client at aimed at, packet is received by the
>> firewall. Firewall sends a SYN/ACK but the firewall at sees it in
>> TCPDump, the firewall at never sees it.
>> Here's a log snippet (I can send you more if you need:
>> May 27 21:38:49 21:38:48.1509569:CID-1:RT:  route lookup: dest-ip *orig
>> ifp reth3.0* *output_ifp reth2.0* orig-zone 19 out-zone 19 vsd 3
>> You will see that the orig and out zones are the same zone, however this was
>> a last ditch effort (putting both interfaces into one zone, effectively
>> creating a swamp).
>> Our current (non-preferred) solution is to put match-all rules on our
>> Catalyst 6513s and put both providers into a swamp and the switch will then
>> intercept the packets if they are destined for the wrong interface and send
>> them out the right one based on a bunch of boolean.
>> We've tried setting up a virtual instance on the offending interface and a
>> firewall filter, but this had little to no effect (at one point it stopped
>> passing the packets to the end machine altogether). We're using small SRX
>> 650ies. Why do we want to do it this way you ask? In the event of a BGP
>> session failure we need to be able to use our statically routed IPs and rely
>> on someone else.
>> Thanks!
>> Ken

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