help: routing multiple interfaces

help: routing multiple interfaces

Post by Joseph Car » Fri, 05 Nov 1999 04:00:00

Here is the problem:
3 interfaces:  (of course IP's have been changed to protect the innocent)
            local IP        router on other side
eth0       internal LAN

ppp0 is a dialup link to the net
eth1 is another link to the net (higher bandwidth)
eth0 is the internal network

basic routing table after all interfaces have come up:
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface UH    0      0        0 ppp0   U     0      0       55 eth0       U     0      0       29 lo         UG    0      0       29 ppp0

  I would like the default gateway for traffic on eth0 and eth1
to be eth1.  But, I would like traffic comming in on ppp0 to be
sent out on ppp0 too.  So, ppp0 should listen and respond to all
it's traffic (from the outside) separate from the rest of the
internal network.
  I am able to setup eth1 as the default gateway, but then ppp0
doesn't work correctly.

Please help with route commands and maybe what the table should
look like.



1. Multiple default routes on multiple interfaces

I have two DSL lines from the same provider on two different static IPs
and two interfaces for my linux box.  I am not particularly worried
about load-balancing here.  I just want failover between the two lines,
with next-hop detection if possible.  The idea is that I don't need
heavy-weight applications like OSPF or BGP (since it's the same
provider), nor any fancy routers; just something that determines if
line 1 is down, switch the default route to line 2 (automatically:  I
know I can do it by hand, or even write a shell script).

For those who will ask, I did try a switch to bring everything to a
single interface and single IP on the linux box.  But when line 1 is
down, the default route and IP for line 1 won't work over line 2.
Initially, I thought it was an arp issue, but it wouldn't resolve after
several minutes, nor after arp -d.  The IPs are contiguous /29 networks
(which I have successfully combined into a /28), but it does not
failover properly.  In the "single interface" scenario, I would have to
assign a new default route *and* IP (and even source routing,

I understand that iproute2 will/should allow multiple default routes,
but then it will round-robin based on destination (this is how I used
to setup Cisco behaviour and how I believe Solaris handles it).  But
what is the failure detection?  Link loss?  Next-hop down?  Anyone have
an easy solution to what should be relatively common?

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