Two routers on one LAN - possible, if so how?

Is it sensible/possible to have two routers on the same LAN (and same
subnet)?

I have two POTS telephone lines and am thinking of subscribing to a
second ISP to get ADSL on the second line to provide a bit more
bandwidth and some backup.

If I use Zyxel routers (which I presently have) I can use the "WAN
Backup" facility to route traffic to the other router if/when the
'main' ADSL service fails.

However I'm not at all sure what happens about DHCP. DHCP has to be
turned on in the router to allow one to use it for DNS, thus DHCP must
be turned on in both routers, what happens then if something asks for
an IP address?

--
Chris Green

(E-Mail Removed) wrote:

> Is it sensible/possible to have two routers on the same LAN (and same
> subnet)?

Yes, it's easy.

> I have two POTS telephone lines and am thinking of subscribing to a
> second ISP to get ADSL on the second line to provide a bit more
> bandwidth and some backup.

Within MS-DOS one can use the 'route' command to determine which IP
to use for the traffic to/from the PC. Not sure how easy with Linux.

> If I use Zyxel routers (which I presently have) I can use the "WAN
> Backup" facility to route traffic to the other router if/when the
> 'main' ADSL service fails.

Pass... past posts have included information about a device which
could handle two WAN connections from your LAN. Try Google...

> However I'm not at all sure what happens about DHCP. DHCP has to be
> turned on in the router to allow one to use it for DNS, thus DHCP must
> be turned on in both routers, what happens then if something asks for
> an IP address?

I don't use DHCP. I have fixed IPs for PCs, and set the DNS entries in
each PC manually, using IP addresses from more than one ISP (remember,
you have two to choose from and other IP addresses can be used, too).
Recently bought some new routers and found they were set to use DNS
from two different ISPs [neither of which do I use for ADSL services
though I also don't know if those ISPs have given permission !!])

Peter M <us-(E-Mail Removed)> wrote:
> (E-Mail Removed) wrote:
>
> > Is it sensible/possible to have two routers on the same LAN (and same
> > subnet)?
>
> Yes, it's easy.
>
Well that's a good start! :-)


> > I have two POTS telephone lines and am thinking of subscribing to a
> > second ISP to get ADSL on the second line to provide a bit more
> > bandwidth and some backup.
>
> Within MS-DOS one can use the 'route' command to determine which IP
> to use for the traffic to/from the PC. Not sure how easy with Linux.
>
Linux has a similar route command, however I want things to happen
automagically, not to have to manually change the routing all the
time.

> > However I'm not at all sure what happens about DHCP. DHCP has to be
> > turned on in the router to allow one to use it for DNS, thus DHCP must
> > be turned on in both routers, what happens then if something asks for
> > an IP address?
>
> I don't use DHCP. I have fixed IPs for PCs, and set the DNS entries in
> each PC manually, using IP addresses from more than one ISP (remember,
> you have two to choose from and other IP addresses can be used, too).
> Recently bought some new routers and found they were set to use DNS
> from two different ISPs [neither of which do I use for ADSL services
> though I also don't know if those ISPs have given permission !!])
>
I don't use DHCP either but it's not at all clear if I don't whether
the router(s) will forward DNS requests to the right place, that's the
issue.

It is actually best to use your own ISPs DNS (usually) simply because
it's quicker to get to that DNS.

--
Chris Green

<(E-Mail Removed)> wrote in message news:drbesn$5gn$(E-Mail Removed)...
> Is it sensible/possible to have two routers on the same LAN (and same
> subnet)?
>
> I have two POTS telephone lines and am thinking of subscribing to a
> second ISP to get ADSL on the second line to provide a bit more
> bandwidth and some backup.

The fundamental difficulty with this is that each connection will have a
different IP address. You would need cooperation from the ISPs (and flexible
routers) to do what I think you ideally want to do.

Probably your best bet is to have one network consisting of the two routers
plus a PC (preferably running Linux or *BSD), with that PC also attached to
a second network with the other PCs. Then you do all the backup/balancing
stuff (such as you can) on the PC in the middle, and the other PCs don't see
a difference.

Alex

<(E-Mail Removed)> wrote in message news:drbesn$5gn$(E-Mail Removed)...
> Is it sensible/possible to have two routers on the same LAN (and same
> subnet)?
>
> I have two POTS telephone lines and am thinking of subscribing to a
> second ISP to get ADSL on the second line to provide a bit more
> bandwidth and some backup.
>
> If I use Zyxel routers (which I presently have) I can use the "WAN
> Backup" facility to route traffic to the other router if/when the
> 'main' ADSL service fails.
>
> However I'm not at all sure what happens about DHCP. DHCP has to be
> turned on in the router to allow one to use it for DNS, thus DHCP must
> be turned on in both routers, what happens then if something asks for
> an IP address?
>

I don't know the Zyxel router but if I have understood what you are trying
to do you should make ONE of the routers supply DCHP and turn it off in the
other. If possible you should add the second router as an alternate default
gateway to the DHCP configuration. In this case the network would
automatically change over to the other network if your primary router
failed. DNS requests would find their way to the configured DNS via IP
routing on the Internet.

It sounds as if your router can failover in which case all the traffic on
your network will be passed to the primary router as the default gateway and
it will forward traffic to the other network if the ADSL line fails. This
does not achieve load balancing between the networks however unless the
router has some capability in this area.

It should not matter what DNS server you use. It does not have to the one at
your ISP. It can, in principle, be anywhere on the Internet provided its IP
address is known.

(E-Mail Removed) wrote:

> It is actually best to use your own ISPs DNS (usually) simply because
> it's quicker to get to that DNS.

It's usually quicker, but that server might have had proven problems
over an extended period. Anyway, with two ISPs, and route choice,
it's quite easy enough to use your backup for DNS requests, if
only to be 100% sure you're giving it _some_ traffic and know
that it is working, come the day you need to use it.

As for 'automagical' switchover I checked using Google groups as
there was a device costing around 90 quid. Mentioned by someone
(perhaps this group, or u.t.broadband)... Possibly second of :-

a) Edimax BR-6541
http://www.edimax.com/html/english/products/BR-6541.htm

b) Edimax BR-6524
http://www.edimax.com/html/english/products/BR-6524.htm

c) Hawking H2WR54G
http://www.hawkingtech.com/products/...ID=19&FamID=13

d) NetEyes Cyclone 200
http://www.xrio.co.uk/cyclone200.html

There may be more options... Peter M.

John Steele <(E-Mail Removed)> wrote:
>
> <(E-Mail Removed)> wrote in message news:drbesn$5gn$(E-Mail Removed)...
> > Is it sensible/possible to have two routers on the same LAN (and same
> > subnet)?
> >
> > I have two POTS telephone lines and am thinking of subscribing to a
> > second ISP to get ADSL on the second line to provide a bit more
> > bandwidth and some backup.
> >
> > If I use Zyxel routers (which I presently have) I can use the "WAN
> > Backup" facility to route traffic to the other router if/when the
> > 'main' ADSL service fails.
> >
> > However I'm not at all sure what happens about DHCP. DHCP has to be
> > turned on in the router to allow one to use it for DNS, thus DHCP must
> > be turned on in both routers, what happens then if something asks for
> > an IP address?
> >
>
> I don't know the Zyxel router but if I have understood what you are trying
> to do you should make ONE of the routers supply DCHP and turn it off in the
> other. If possible you should add the second router as an alternate default
> gateway to the DHCP configuration. In this case the network would
> automatically change over to the other network if your primary router
> failed. DNS requests would find their way to the configured DNS via IP
> routing on the Internet.
>
Yes, OK, that's the conclusion I reached regarding DHCP - turn one
off. However what I'm not clear about on these Zyxel routers is
whether they will still forward DNS requests to the right place if
DHCP is turned off, the manuals are a bit unclear/ambiguous. Maybe I
should ask Zyxel support, they're usually quite helpful.


> It sounds as if your router can failover in which case all the traffic on
> your network will be passed to the primary router as the default gateway and
> it will forward traffic to the other network if the ADSL line fails. This
> does not achieve load balancing between the networks however unless the
> router has some capability in this area.
>
Yes, it can failover if/when the ADSL line fails. As you say it
doesn't achieve load balancing but what I might do is set one PC (the
one that runs BitTorrent) to use the backup ADSL as its default route
and leave all the others on the main/default router.


> It should not matter what DNS server you use. It does not have to the one at
> your ISP. It can, in principle, be anywhere on the Internet provided its IP
> address is known.
>
Yes, I know this, but it's surely best to use a DNS server which is
'close' in internet terms and is thus fast to access. There is a
specific protocol for obtaining the DNS server address, IPSPI or
something like that it's called. I'd prefer to use this so that the
addresses are loaded automatically into the router in the same way
that the router's host address is loaded, it seems the obvious way to
do it.

--
Chris Green

<(E-Mail Removed)> wrote in message news:drco55$hto$(E-Mail Removed)...
[snip]
> However what I'm not clear about on these Zyxel routers is whether they
> will still forward DNS requests to the right place if DHCP is turned off,
> the manuals are a bit unclear/ambiguous. Maybe I should ask Zyxel
> support, they're usually quite helpful.

I don't see why the state of the router's DHCP server should have any effect
on the router's DNS forwarding, but you should be able find out whether it
does or not by experimenting. This would probably be quicker than asking
Zyxel, even if they are usually quite helpful.

However, DNS forwarding is almost guaranteed to be non-functional if the WAN
link is down, which is another potential source of problems. If you can
specify the DNS server addresses to be provided via DHCP (and put both
routers LAN addresses in) it should work, but how well will depend on the
ability of DHCP client machines to recognise that one of the DNS servers
isn't working and try the other one first in future.

I don't know what features the DHCP servers provide but from routers I have
seen it wouldn't surprise me if using DHCP was entirely out of the
question - ie you just can't make it do what you need.

Alex

Alex Fraser <(E-Mail Removed)> wrote:
> <(E-Mail Removed)> wrote in message news:drco55$hto$(E-Mail Removed)...
> [snip]
> > However what I'm not clear about on these Zyxel routers is whether they
> > will still forward DNS requests to the right place if DHCP is turned off,
> > the manuals are a bit unclear/ambiguous. Maybe I should ask Zyxel
> > support, they're usually quite helpful.
>
> I don't see why the state of the router's DHCP server should have any effect
> on the router's DNS forwarding, but you should be able find out whether it
> does or not by experimenting. This would probably be quicker than asking
> Zyxel, even if they are usually quite helpful.
>
Neither do I but the way that it's arranged in the configuration
suggests that there is a connection. The DHCP setup is part of the
LAN setup where you specify the router's IP address etc. You can set
the DHCP to 'Server', 'Relay' or 'None' and under this setting you
enter your DHCP start address and pool size *and* DNS addresses and
the remote DHCP server address. If you set DHCP to 'None' then you
cannot enter the DNS Server addresses, they are greyed out.

I'll experiment! I have asked Zyxel support and they have answered to
the effect that DNS will work with DHCP turned off but I'm not totally
convinced that I believe them.

--
Chris Green

In MsgID<43d94226$0$1470$(E-Mail Removed)> within
uk.comp.home-networking, 'Peter M' wrote:

>(E-Mail Removed) wrote:
>
>> Is it sensible/possible to have two routers on the same LAN (and same
>> subnet)?
>
>Yes, it's easy.

I think you mean one router as a router and one router only using its
switch section.

A router can only route traffic between different subnets.

If that's what was meant, and he only meant the second router to be taking
it's feed from the first subnet then I apologise for misunderstanding.

Dave J