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Have a few remote sites that require backup to their existing DS3 facilities. In some cases multiple T1 links (3 from some sites & 5 from others) should be sufficient for backup purposes at this time. The number of T1s and sites may increase.

I'm considering 3745s at the remotes with T1 mods and possibly an aggregation CT3 card in one of the 7500s here. From a CPU perspective, if using MLPPP for up to 8 T1s from some locations should both the 3745s and 7500 be able to handle something like this. Or would IMA be a safer bet since this is essentially done in hardware?



As you said, ATM can give you enhanced performance since it is being done in hardware. But, the investment cost will definitely be more if you are not using ATM already. Also, since one of them works at layer-2 and other at layer-1 you may have to take into account the amount of byte overhead in each case. Depending on the type of performance you need and these factors, you must be able to decide the best solution for you.

New Member


I don't know the distance between your sites, but if we take in consideration that most TELCOs charge T1s per mile and ATM by PVC (no matter the distance between sites) ATM should be more suitable for large distances and high capacity. Also ATM gives you more flexibility, allowing you to contract jus the BW you need. In this case my recommendation is to go with a DS3 ATM port at the main site (8 T1 IMA doesn't give you much to grow in future) and IMA on the remote sites. If you are planning this network as Backup of an existing one you may consider UBR PVCs (CIR 0)

New Member


A few things to consider...

MLPPP is pretty inexpensive for metropolitan connections provided you use a CT3 at the hub site for many remote site connections. I assume that your DS3's are ATM, but not necessarily. If they are ATM, you achieve diversity by not having your backup WAN links on the same service as your primary. We tend to limit our MLPPP config to four T1's due to processing overhead(two routers with groups of four T1's is common). If this is your backup connection, overhead may not be as important of an issue in a break fix situation as long as it is functional. It depends what service level the remote site expects when the primary link is down.

IMA is very good for long haul connections and the hub site is very easy to manage with a DS3 connection. If a site approaches 8 T1 IMA solution, the cost is approaching a DS3 and would make sense to provision just what you need of the DS3 port.

We use a mixture of both, depending on the site requirements.

MLPPP - small to medium sites with short haul metro WAN connections

IMA - small to medium sites with long haul WAN connections

DS3 - large sites with either long or metro connections

The 7500 will need a VIP module and with enough memory should not have a problem. You should check your complete 7500 hardware for compatibility. Have not worked with the 3745's so no opinion on that.


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