Cisco 1841 Low speed serial interface

Unanswered Question
Jul 31st, 2008

I have a Cisco 1841 in my Network which is acting as a Gateway. It is connected to the outside world on a low-speed serial interface (The Router has 2 low-speed serial interfaces, 1 ethernet and 1 VPN port). I always get a slow connection over the network to it. I can see the tracert response times are always large on that hop. Is there a way to upgrade that port to a higher speed or is that even the reason for the slow connection? Can it be configured differently to increase the response times?

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Richard Burts Thu, 07/31/2008 - 13:37


If we had some more details about your environment we could give better answers to your questions. Can you give us some more detail about these serial connections? what kind of serial card are they? What kind of connection is it? What speed are they running? What kind of load are they carrying?

It would be helpful if in addition to the questions above you could post the output of show interface for all of the interfaces.

If you have some volume of traffic arriving on the faster interfaces (Ethernet and VPN) and being forwarded out the low speed serial, then the router serial interfaces could be a bottle neck and could certainly account for the slow response times.

If this is the case then perhaps you can look into the possibility of increasing the speed of the serial interfaces. This will probably require the provider of the serial circuits to make some changes to provide higher speeds.



uzmausmani Mon, 08/11/2008 - 15:27

The Cisco 1841 is acting as the Gateway to almost 40 devices (routers, Pixs, switches) which are all housed in a Lab environment. Users access the devices through a Cisco 7206-Cisco 1841 Serial link. when I do a traceroute to any device in that subnet...I see huge (800-1000ms) latency on the serial link..which in effect effects access to all the devices on that subnet since they are all connected on the ethernet interface of the Cisco 1841.

I can see there are output drops on the Cisco 1841 serial port and input errors on the Cisco 7206 serial port (which is a high speed WAN port). I will post the outputs that you requested tomorrow when i have access to the device...Thanks for any help. The Cisco 1841 is proving to be a major bottleneck. I need help urgently.

Richard Burts Tue, 08/12/2008 - 04:54


Based on this description I suspect that the bottleneck is not the 1841 but is the low speed serial link. It sounds like a lot of devices and a lot of traffic for a single low speed connection. I suspect that you will find that it is overloaded, congested, and the real source of your difficulty. And probably the solution will be either to upgrade the speed of the link or to provide another connection from the 1841 to the 7206 and to do load sharing.





uzmausmani Thu, 08/14/2008 - 07:04

Can I aggregate the Bandwidth (connect two serial links back to back) between the two routers using the Multilink PPP? Is that going to help or is there any other way to do load sharing? I'm thinking of using The Virtual Template configuration for Multilink PPP to aggregate the bandwidth.

Richard Burts Thu, 08/14/2008 - 07:12


Using multilink PPP is a good way to aggregate the bandwidth of two serial links between two routers. I believe that this would help.

There are other ways such as giving each serial link its own addresses and subnet and to route over both links to accomplish load sharing. But multilink PPP would be a good alternative.



uzmausmani Thu, 08/14/2008 - 13:23

I configured the multilink PPP but now the latency has become even worse...:( I dont know whats going on...How can it get has two links to transport data using two separate interfaces then why isnt it working? Please help..

uzmausmani Thu, 08/14/2008 - 13:25

I want to mention that The interface on the 7206 are both fast speed serial and on 1841..they are both low speed serial. and there are only static routes being used.

Richard Burts Thu, 08/14/2008 - 13:30


Perhaps it would help us understand the issue if you would post the output of show interface for the serial interfaces of both routers.



uzmausmani Fri, 08/15/2008 - 11:02

I've tried changing WAN cables..changing interfaces..adjusting hold queues...etc but nothing seems to work. one thing I have noticed is that when I tried changing the ethernet port duplex status..the latency reduced considerably for like 5 minutes and then went back to 1000ms again. I've reverted back to my original config..have removed the multilink ppp now.

Giuseppe Larosa Tue, 08/19/2008 - 12:40

Hello Uzma,

even with the two links in PPP multi they are still congested

r7206-1-test#sh ppp multilink

Bundle Sugarland-1841, 2 members, Master link is Virtual-Access1

3052 lost fragments, 317862 reordered, 1 unassigned, sequence 0xA32D7/0x375EF


2242 discarded, 3027 lost received, 242/255 load

Member Links: 2 (max not set, min not set)




You should try to use QoS and to shape at 240 kbps the output traffic on the 1841 side.

or to shape at 120 kbps at each link if used as parallel L3 links

Hope to help



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