Bonded T-1's to Multi-T-1's - what impact on network traffic, configuration, etc.

The main reason for bonded connections is to get more speed.  The "bonding" makes the datalink speed the equivalent to the sum of the speeds of the bonded links.

The main reason for multiple lines is redundancy/reliability.  Usually you get service from more than one provider and so if one goes down you are still (possibly) up.  You can load balance on multiple lines but the maximum speed will be the speed of the individual line that the traffic is flowing on.

What is best for you depends on what problem or situation you are trying to resolve.

If they are not bonded they can only be used to enhance your environment if you assign them roles and use clever settings on your routers and your workstations'/servers' default gateways.  For example you could have one T-1 for all mail in and out, and the other for web browsing.  Manually splitting the duties.

If I have 2 bonded T's I get a single 3 meg pipe, but 2 multi-link T's I get 2 1.5meg pipes, but load-balanced with possible redundance/failover -- is this a correct summary?

Also - there should not be any problems with bonded T's at the corporate center feeding multilink T's at a clinic - it will just throtle down to the max speed of the slowest link - correct?

For a large file transfer (say 40 to 400 Meg) will I get any improvement in transfer time with a multi-link T-1?, or is it simply that the file xfer will take one T, and all other traffic (terminal services, e-mail, telnet, etc.) takes the other?  Would this happen automatically - i.e. the router knows one is full and shunts everything to the other until finished - or is the router 'dumb' and it tries to feed new data-streams round robin with now knowledge of existing load???

We used to have four 512Kb bonded ADSL lines linked in a Cisco router.  This provided a much faster link for browsing and file sharing, etc, even exchange traffic.  

However, for somthing that is mission critical and requires a guaranteed stable connection I would not recommend it.  We had serious problems with Terminal Services clients connecting via the bonded lines, meaning we couldn't function as a business.  The bonded lines were scrapped for a more expensive but more stable leased line.  

Your summary is correct.

You should see improvement with a multi-link T1 since it will consume additional bandwidth now.  However, the file copy would most likely not use just one T1, and the other traffic would use the other. If you setup packet load balancing, it goes round-robin and will evenly balance the total load.  Using GLBP, on the other hand, is circuit based.  You get less-even balancing, but a connection tends to stay on the single T1.  As Carl Legere eluded to, you can specifically control the traffic and assign it to a T1, but I never favored that.  I just balance the total load to take advantage of my total bandwidth.

dsbahr,

Perhaps to further clarify, I don't need bonded T1s to consume 3MB bandwidth.  If I kicked off a huge copy or transfer, and I have two load balanced T1s, I could potentially saturate both T1s and use my full bandwidth.