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maximum number of T1 serial interfaces via MLPPP?

blackladyJR
Level 1
Level 1

Hello,

Do you know if I have a 2800 series and a 3800 series router, do you know what is the maximum number of T1 serial interface I can bundle together to form one logical multilink Port?

I usually have 4xT1 (6Mbps) and done via two vwic2-2MFT-T1/E1 interface cards. But I wonder if it is okay to do 8xT1 or even more to form a single logical Port?

thanks.

1 Accepted Solution

Accepted Solutions

Here is a relatively recent document (Updated: Jan 29, 2008) that suggests the use of 8 T1/E1 is possible at least in high end products, and warns about CPU usage even in this case:

http://www.cisco.com/en/US/tech/tk713/tk628/technologies_configuration_example09186a00800a5efa.shtml

The above document refers to a white paper:

http://www.cisco.com/en/US/products/hw/modules/ps2033/products_white_paper09186a0080091d4b.shtml

Table 1 in this white paper suggests that you can expect use of MLPPP with any serial PA (probably because, as this same table says, MLPPP is a software only solution) and you can have 2-8 T1/E1's per bundle in general.

Still, to be on the safe side, you might consider asking cisco about this as Joseph suggested.

View solution in original post

24 Replies 24

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Joyce,

it should be possible but probably a single DS3 or E3 link can be cheaper then 8 T1 (this depends from the offers)

I would move to a T3 or E3 link if possible both C2800 and C3800 support network module for single T3 or E3.

It should be easier for the router to handle a single T3.

Hope to help

Giuseppe

paolo bevilacqua
Hall of Fame
Hall of Fame

8 T1s is OK, probably even 12.

As Giuseppe recommends, beyond few circuits, look for a DS3, it's much less trouble.

Hello,

Yes I know DS3 is better but I have the issue in the location with DS3 delivery and the price is significant higher than 8T1. So I need to know if Cisco has any offical documentation to speak to the maxiumum number of MLPPP bundle the router can handle via multi-port serial in either 2800 ro 3800.

In the old days when we just do load sharing, there are limitation on eigrp of 4 or 6 maximum..etc. So that's why I wonder if there is any document for MLPPP to show the max number of T1 interfaces. I can't find any reference in cisco website so wonder if anyone may have the information. for example, will the maxiumum number varies of which type of interface card to use on which router platform. So say if I am using 3800 and using 4 port HWIC serial, so how many of these 4 port card can I bundle together? Or if i have just the vwic-2mft cards instead, so 2 per card, how many of those can I bundle together?

thanks,

Joyce

Hello,

First of all, I agree with the previous posts. Also, the general tendency when bandwidth demands grow up to this point, is to grow fast to another level, so be prepared. I cannot answer all your questions in full. Still, I can say what we were doing in a similar situation that we were using 8 E1's until we were able to get an E3, just in case this can be of any help, although admittedly limited.

We were using 7200 routers at the endpoints between 8 E1's. We had 2 multilinks with 4 E1's each and we were load balancing traffic over the 2 multilinks. I don't know the reason that lead to the grouping per-four because those were already in place when I started working and bandwidth demands grew fast, so the multilinks quickly died. Note that each multilink is one entity to the eyes of the routing protocol. In our case, OSPF was load balancing between 2 equal cost links. Even if you do run into some type of limitation, the worst case scenario (besides not being able to bundle at all), is to be able to bundle only 2 interfaces in one multilink. Even in this case, you can still make it with the routing, provided that you have enough available slots on your routers of course.

Kind Regards,

Maria

p.s. Note also that those routers were dedicated to handle just those 2 multilinks. They were not latest NPE, but you can get an idea of the overhead. When we removed those multilinks and put the E3 in place, the CPU usage graphs took a dive.

I don't recall seeing a documented logical bundle limit for MLPPP. It's possible you'll first run into a limit for the number of supported T1 interfaces on the platform, plus you'll likely exceed the Cisco recommendation for T1 interfaces on the various ISRs (which isn't too high for the 2800 series). This might be a question best answered by TAC or Cisco sales support.

Joseph is probably right. We might have done the grouping per-four just because we had available 4 port adapters or to keep at least one multilink somewhat stable in case we had a failure in one of the E1's (flapping E1 going up and down all the time) or something like that (e.g. adapter failure). Those failure scenarios are probably factors to take into account.

Here is a relatively recent document (Updated: Jan 29, 2008) that suggests the use of 8 T1/E1 is possible at least in high end products, and warns about CPU usage even in this case:

http://www.cisco.com/en/US/tech/tk713/tk628/technologies_configuration_example09186a00800a5efa.shtml

The above document refers to a white paper:

http://www.cisco.com/en/US/products/hw/modules/ps2033/products_white_paper09186a0080091d4b.shtml

Table 1 in this white paper suggests that you can expect use of MLPPP with any serial PA (probably because, as this same table says, MLPPP is a software only solution) and you can have 2-8 T1/E1's per bundle in general.

Still, to be on the safe side, you might consider asking cisco about this as Joseph suggested.

Hi Maria,

Thanks for finding the URLs, from the white paper, it seems like you said in Table 1 that it says 2-8 and didn't say it has to be PA card for 7x00 series router. So probably lower end 3800 is fine for 8T1.

Interesting in Table 2 that puzzles me, it says "Supports IOS Quality of Service

" NO. What does it mean? I certainly have policy-map applied to the mulitlink interface today in many routers so that feature in table 2 saying "no" is very odd or unless it means some different QoS :)

Yes, to be safe, I will open a tac case to give them the exact 2800 and 3800 that I plan to use to see if they see any issue with it.

I won't go higher than 8 anyway as 8 T1 will be close enough to the cost of DS3. Right now I have 4 T1 and planning to increase upto 8 and it still saves money to stay with 8T1 vs DS3. Customer is fully aware of the benefit to go with DS3 with a frac-Port speed with the benefit to grow easily.

thanks for all your help.

Joyce

Joyce,

I did not comment on how recent the white paper is, because it doesn't say. I thought that even if it is old, those would probably be the worst news you could hear. I also noticed earlier that at the last line of the table it mentions even 2500/3600 CPE support, so this is probably encouraging. The fun with this white paper is at the beginning, where it describes all the difficulties that those solutions try to overcome. The description is very realistic.

I am glad I could be of some help. It is always a pleasure to try to answer your tough questions :-)

Kind Regards,

Maria

"I did not comment on how recent the white paper is, because it doesn't say."

The PDF version shows a copyright of 1998.

Joseph, thanks for pointing that out. I found myself puzzled at various occasions about the date missing at the bottom of some cisco web documents. Now I learned the trick :-)

Perhaps another option, if you don't intend to use MLPPP fragmentation is usage of multiple routed links. (I suspect MLPPP fragmentation would be the major CPU consumer.) I realize routers didn't used to support more than 6 maximum-paths, but this has been increased on some later IOSs. At the moment I'm looking at a 12.4 2811 OSPF that notes it supports 16. CEF does a fairly decent job of flow balancing, quickly.

The first document above says about the CPU usage: "The trade-off for the increased functionality is that Multilink PPP requires greater CPU processing than load balancing solutions. Packet reordering, fragment reassembly, and the Multilink PPP protocol itself increases the CPU load." So, many factors contribute to the end result of the actual CPU usage of the MLPPP solution, but I suppose Joseph is right in the sense that MLPPP fragmentation is something you might consider avoiding, in order to cut some of the overhead (unless you are not satisfied with the traffic distribution, your router can handle more load and you think you could push things further to balance the links). Still, the document suggests at the very beginning to "Disable Multilink PPP fragmentation whenever possible". We were not using MLPPP fragmentation and we still had high CPU load on the 7200's.

I have seen per-destination load balancing with CEF work well only in links of higher speed, such us STM-1. For such low speed links the CEF load balancing would have to be per-packet to avoid some links being underutilized while others are congested (which can happen very easily in networks with unpredictable traffic patterns). With per-packet load balancing you may have packet re-ordering. MLPPP preserves packet order. Still, we were using 2 multilinks, with per-packet load balancing between the 2 multilinks and we had no issues. However, this will depend on the services one runs. We were not passing voice traffic over those multilinks, but only internet traffic.

p.s. Now that I think about it more, it seems the fellow engineer who must have done our setup actually balanced the 2 solutions. MLPPP with CEF flavor. Is she brilliant or what? :-)

I have a few 4xT1 MLPPP sites with 2851 and it seems the CPU load is very low. I have other 2801 with 2xT1 with per-destination load sharing via "maximum-path 2" under BGP (I have BGP between CE and PE on the 2xT1 WAN links).

For the site that I need upto 8xT1, I will use minimum 3825 instead of 2800 series.

But like Joe said, usually the load sharing normally is 6xT1/E1, so that's why I post the original question wondering what may be the maximum for MLPPP if I want 8xT1 in one logical port. I know MLPPP is a different animal as layer 3 load sharing, but since I can't find the document in MLPPP showing the maximum, that's why I want to find out to be safe before telling customer to order 8xT1 and then if it doesn't work well, then it will be bad.

thanks,

Joyce

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