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2511 dial on demand to ISP and round robin load sharing


I try to get an solution with a Cisco 2511 router running.

My IOS I am using is 12.1 (Enterprise version).

I got stocked with my IOS configuration.


I do have ONE 115 kbit leased line and SIX analog modems,

each connected one ISP with different user and password names for

each connection.

I would like to have and dial on demand and round robin setup.

The ISP sadly does not support PPP multilink, so I assume I am bound

to have round robin setup.


Where could I find IOS configuration examples for such a setup?

Does round robin work with diffent speed lines?

How do I configure dial on demand for all 6 analog modems?

Can I meter the bandwidth usage on analog modems before letting the

next modem dial into the ISP?

Do I have to use dialing rotary groups?

How does a NAT implementation works for such a set?

where do I have to set the default route to?

Thanks a lot for your help. Very appreciated!



Re: 2511 dial on demand to ISP and round robin load sharing

Yes, round robin is your best choice if MPPP is not supported. Round Robin works at layer3 (IP), so the actual speed of the link(s) is not taken into consideration.

Since each interface will use a different username and password, you need to configure each individually. So something like this:

If you want to add more connections based on usage, then you would NOT be able to use different usernames for each connection (since that would mean that each connection is a different user, and not additional BW for a given user). At any rate if you did want to do this, the config would look something like this:

To this you would want to add the command to determine when an additional link would be added.

Ini general, you would use a rotary-group when dialout out to the same site with the same username.

NAT works with DDR like it would for any other connection; the ISP interface would be outside with a legal address, the LAN interface would be inside with a private address.

The default route would be to either the rotary-group interface, or to each of the async interfaces. With "no ip route-cache" on the async interfaces, we will load balance per packet. With "ip route-cache" on the async interfaces, we will load balance per destination. "no ip route-cache" is likely the best approach for your needs.

New Member

Re: 2511 dial on demand to ISP and round robin load sharing


Thanks a lot for your respond!

Just got a bit confused about bandwidth on demand?! You are saying, it is not possible to assign more bandwidth to a single user connection. That makes sense for me.


In my scenario, I use a W2k terminal server. About 20 thin clients are accessing this server and mainly surf the web. So, logically, there is just ONE client (terminalserver). These users are surfing different web pages and request different web sources.


In that case, is it possible to assign bandwidth on demand to the terminal server? I am not interested in assigning more bandwidth to a single session, like somebody downloads a file. The other thin clients should be able to use in this case just an other modem (on demand)?! Hope this makes sense to you....

Thanks a lot!



Re: 2511 dial on demand to ISP and round robin load sharing

No, you can assign more BW to a particular user; but in your case, it sounded like each interface would be presenting itself to the ISP as a unique user. This means that you can't add BW to an interface, if each interface will act as a unique user.

Yes, whoever is connecting to the ISP is considered to be the user; if that is the cisco (the end device generating the traffic is irrelevant), then you have one user connecting to the ISP.

Keep in mind the relevance here is the connection to the ISP and the BW available. So if you have something like this (keeping it simple):


Then the cisco is dialing the ISP, and additional links can be dialed if the load exceeds some configured value. It's (mostly) irrelevant who the source or destination address is. Have a look at the URL's I provided earlier, that may help out too.

New Member

Re: 2511 dial on demand to ISP and round robin load sharing

Now it gets a bit too difficult. What you were talking about was correct. Each Async interface is connecting with a different userID to the ISP. I understood, that I can not assign more bandwidth to an single connection. For your better understanding a simple draw:


user----- ----ISPuser1

| |

user--------W2K terminal server ----2511 ----ISPuser2

| |

ect------ ----ect

So, if async ISPuser1 is 80% load with FTP downloading, can I let ISPuser2 async modem dialup and handle all other sessions to other destinations. I do NOT want to assign more bandwidht to the FTP download. Its more that I would like to have a sharing of all sessions (on demand) over several ISPuser async interfaces.

Thanks a lot!



Re: 2511 dial on demand to ISP and round robin load sharing

The problem is that (if I understand this correctly) you are connecting to the same ISP, but you want to connect AS a different user. If you were dialing to different sites, you could use Dialer Profiles, have a profile for each user, and control the BW additions.,/univercd/cc/td/doc/product/software/ios122/122cgcr/fdial_c/fnsprt5/dcdiprof.htm

However, if you are dialing the same site then this may only be feasible if you use policy based routing, and specify the source address. Then you could point that to a dialer profile, and this should work. The difficulty is that, by default, we don't care about the source address (or user). But this seems to be your fundamental requirement.

In the end, you need to determine if you want to control each user (in which case policy based routing with Dialer Profiles should work), or if you want to control the load and BW allocation (in which case straight DDR would be fine). To want to try a mixture of both isn't possible I think.

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