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How to bond two T1's into a single 3Mb connection point to point with VOIP and Normal Data Traffic QoS

Posted on 2008-11-06
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Last Modified: 2012-06-27
Hello, thank you for your assistance.

We have two T1's from Location A to B, and a Cisco 1841 Router with a Dual T1 WIC at each location. Our requirement is to have the two T1's bonded into a 3Mbps pipe. If one T1 shall go down, the link should still stay up on the other T1 line.

Since our phones which are VOIP based, are going to be chatting over this connection I need to make sure that call quality is exceptional and stable. Do I need to use QoS? Should I use MLPPP?

What is the best way to accomplish this? I am not allowed to change the routers, I must keep the 1841's one at each location.  I would really appreciate a concrete example or cisco config as I have not done this before.

Thank you very much!
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Question by:inetdee
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packetguy earned 2000 total points
ID: 22902780
The easiest way to do this is with MLPPP (Multilink PPP), which creates a virtual "multilink" interface out of the individual T1s, to which you can apply a single QoS policy. I've attached sample config. The "ip cef" is important for performance reasons, as it prevents packets from begin process-switched. This can be a problem with MLPPP on low-end routers, as MLPPP is somewhat processor intensive. However, you shouldn't have any problem on an 1841 with just two T1s. I've used MLPPP on a 2621XM running four T1s and the CPU on that slower router was only at about 50% with max load. The "hold-queue 300 out" setting is something you have to play with to get best performance. If it's too high or low, you'll see the dropped packet counter in "show interface" increase consistently under load. Tinker with the number to find the best balance for available memory and your specific packet size mix.

There are many references out there for tagging VoIP traffic for priority. You'll have to evaluate them to see which is best for your situation. A very good description of one method that works well with PPP links is Cisco's "VoIP over PPP Links with Quality of Service" technical paper:

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094660.shtml




ip cef
!
interface Multilink1
 description T1 Multilink PPP bundle 
 bandwidth 3000
 ip address 1.1.1.1 255.255.255.252
 ip route-cache flow
 load-interval 30
 no cdp enable
 ppp multilink
 multilink-group 1
 hold-queue 300 out
!
interface Serial0/0
 bandwidth 1544
 no ip address
 encapsulation ppp
 no ip route-cache cef
 load-interval 30
 no fair-queue
 down-when-looped
 serial restart-delay 0
 service-module t1 lbo -15db
 ppp multilink
 multilink-group 1
!
interface Serial0/1
 bandwidth 1544
 no ip address
 encapsulation ppp
 load-interval 30
 no fair-queue
 down-when-looped
 serial restart-delay 0
 service-module t1 lbo -15db
 ppp multilink
 multilink-group 1
!
interface FastEthernet0/0
 ip address 1.2.3.4 255.255.255.0
 no ip proxy-arp
 ip route-cache flow
 load-interval 30
 random-detect

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Author Closing Comment

by:inetdee
ID: 31514241
Thank you so much, I appreciate your help. I tried to close the question a while ago but I must have done it wrong, thank you again and your solution worked excellently. One day hopefully I wlil gain enough knowledge to help others as well. Keep up the good work!
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