[nycphp-talk] multiple gateways and linux

Jim Suto jim at
Tue Jun 4 12:18:10 EDT 2002

We have 3 T1s, 2 are from Worldcom, one is from Cable & Wireless.  These are
already in place with each provider having its own CISCO router.  If we use
HSRP, we are only going to be using one single router at any time and the
other one will sit by idle, right?  It's good for redundancy but not for
maximizing usage of the available bandwidth.  I was thinking of using
roundrobing DNS for incoming connections to our server using the 3 IPs (1
from each T1 block).  The outgoing connections could be returned via source
routing perhaps?  And if a gateway is down, use another gateway even if that
will create an assymetrical route?
In your point (e) if we got two routers for each circuit we would need to
have 6 routers, do you think the cost would be justified for slightly
reduced failover risk?  I think the company could live in a pinch even on
just a single working T1 but it seems like a waste to have all that other
bandwidth just sitting there unused.  Let me know what you think.


-----Original Message-----
From: Larry Chuon [mailto:LarryC at]
Sent: Tuesday, June 04, 2002 11:55 AM
To: NYPHP Talk
Subject: RE: [nycphp-talk] multiple gateways and linux

Keep in mind that when you have multiple circuits, you're running into some
complecity with your network design.  Here is a general guideline.
1. Are you planning to subscribe the add'l T1 from the same ISP of from a
different one?
a. Easy to loadbalance (LB) your package both inbound and outbound
b. You don't require to have a heavy duty router.  This way you can accept
partial BGP route.  As a result your, hardware requirement is much less.
c. Use vendor assigned Autonymous System Number (ASN).
d. If the ISP is down, assuming both T1 are from the same c/o, you're out!
e. I recommend that you have two separate routers for each circuit.

a. You must apply for you own ASN from ARIN.
b. Tedious BGP configuration
c. Setup of LB might be difficult depends on your environment, but not
d. Complete ISP redundancy - due to two separate path in and out
e. Same as 1e above.

With the two routers as your gateway, you can configure HSRP/VSRP (depends
on the vendors).  Essentially, you have two real gateways and one virtual
one that everyone is pointing to.  When any of the real gateways is down, it
won't affect your connectivity at all.  Linux has something to that affect
as well.  I don't have experience with it nor do I know what it calls.

-----Original Message-----
From: Jim Suto [mailto:jim at]
Sent: Tuesday, June 04, 2002 11:10 AM
To: NYPHP Talk
Subject: [nycphp-talk] multiple gateways and linux

I am looking for any ideas people might have to maximize the value of having
multiple T1s from different providers and being able to provide redundancy
and possibly performance gains from having a linux server utilize these
multiple gateways.  Some ideas I was thinking about:
1) enable linux to do dead gateway detection and avoid using a gateway if it
is not up
2) enable linux to distribute traffic among the gateways to whichever one is
least busy (this might be a little bit more difficult)
3) enable linux to figure out from a list of hosts which gateway would allow
connection to a specific host with minimum lag and/or hops

I guess some shell scripts could accomplish some of these tasks unless
someone knows a better way?


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