[c-nsp] Multilink PPP (MLPPP) Asymmetrical Throughput Problem NxT1

Fri May 25 22:15:52 EDT 2007

I'm encountering an odd problem bonding T1 circuits with Multilink PPP
(Cisco 7206 down to a 2651XM).  In this particular case, I've got three (3)
T-1 lines bonded together and, taking into account overhead, I'm getting
(via direct Ethernet -> laptop connection - no other traffic):

4.2 to 4.3 mbps downstream, but .
Only around 2.0 mbps upstream (tried two different "check my speed" sites).

I was able to initiate multiple large packet repeating pings between the two
devices and the show interface statistics showed in the neighborhood of 4.0
mbps in both directions.  This put the CPU in the 60 to 70 percent
utilization range on the remote 2651XM device (with it originating about 50%
of the pings but not otherwise routing any traffic, Ethernet interface was
disconnected) 7206 Utilization (handling traffic for numerous locations) was
well under 10%.  No ACLs or Policy-Maps at either end of the links. 

Any suggestions or insight on the problem are greatly appreciated.  I've not
observed this kind of difference on various other 2xT1 (3 mbps) connections
that I've got in place but this is the first 3xT1.  Thanks a bunch!

HOST END
Cisco 7206 (non VXR)

interface Multilink2
 description Multilink (ch16-18)
 ip unnumbered Loopback2
 ppp multilink
 no ppp multilink fragmentation
 multilink-group 2

interface Serial3/0:16
 description MLPPP #1 
 mtu 1524
 no ip address
 no ip redirects
 encapsulation ppp
 no fair-queue
 no ppp lcp fast-start
 ppp multilink
 multilink-group 2
!
interface Serial3/0:17
 description MLPPP #2
 mtu 1524
 no ip address
 no ip redirects
 encapsulation ppp
 no fair-queue
 ppp multilink
 multilink-group 2
!
interface Serial3/0:18
 description MLPPP #3
 mtu 1524
 no ip address
 no ip redirects
 encapsulation ppp
 no fair-queue
 ppp multilink
 multilink-group 2

REMOTE END
CISCO 2651XM w/WIC-DSU-T1 Cards
IOS C2600 Version 12.3(6a)

Router> show run (relevant sections)

network-clock-participate slot 1
no network-clock-participate wic 0
no aaa new-model
ip subnet-zero
ip cef
!
!
!
interface Multilink1
 description MultiLink PPP 3xT1
 ip address xx.xx.xx.xx xx.xx.xx.xx
 no ip mroute-cache
 load-interval 30
 ppp multilink
 ppp multilink fragment disable
 ppp multilink group 1
! 
interface FastEthernet0/0
 description LAN Interface
 ip address xx.xx.xx.xx 255.255.255.248
 speed auto
 full-duplex
 no cdp enable
!
interface Serial0/0
 description MLPPP T1 1
 mtu 1524
 no ip address
 encapsulation ppp
 load-interval 120
 no fair-queue
 ppp multilink
 ppp multilink group 1
 ppp multilink endpoint none
!
interface Serial0/1
 description MLPPP T1 2
 mtu 1524
 no ip address
 encapsulation ppp
 load-interval 120
 no fair-queue
 ppp multilink
 ppp multilink group 1
 ppp multilink endpoint none
!
interface Serial1/0
 description MLPPP T1 3
 mtu 1524
 no ip address
 encapsulation ppp
 load-interval 120
 no fair-queue
 ppp multilink
 ppp multilink group 1
 ppp multilink endpoint none
!

ROUTER> show interface

Serial0/0 is up, line protocol is up
  Hardware is PQUICC with Fractional T1 CSU/DSU
  Description: MLPPP T1 1
  MTU 1524 bytes, BW 1544 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Open, multilink Open, loopback not set
  Last input 00:00:00, output 00:00:01, output hang never
  Last clearing of "show interface" counters 05:13:27
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  2 minute input rate 0 bits/sec, 0 packets/sec
  2 minute output rate 0 bits/sec, 0 packets/sec
     179815 packets input, 246334673 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     1 input errors, 0 CRC, 1 frame, 0 overrun, 0 ignored, 0 abort
     175062 packets output, 233229743 bytes, 0 underruns
     0 output errors, 0 collisions, 2 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions
     DCD=up  DSR=up  DTR=up  RTS=up  CTS=up

Serial0/1 is up, line protocol is up
  Hardware is PQUICC with Fractional T1 CSU/DSU
  Description: MLPPP T1 2
  MTU 1524 bytes, BW 1544 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Open, multilink Open, loopback not set
  Last input 00:00:04, output 00:00:04, output hang never
  Last clearing of "show interface" counters 05:13:35
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  2 minute input rate 0 bits/sec, 0 packets/sec
  2 minute output rate 0 bits/sec, 0 packets/sec
     179809 packets input, 246361009 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     2 input errors, 1 CRC, 1 frame, 0 overrun, 0 ignored, 0 abort
     175057 packets output, 233616175 bytes, 0 underruns
     0 output errors, 0 collisions, 2 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions
     DCD=up  DSR=up  DTR=up  RTS=up  CTS=up

Serial1/0 is up, line protocol is up
  Hardware is DSCC4 with integrated T1 CSU/DSU
  Description: MLPPP T1 3
  MTU 1524 bytes, BW 1544 Kbit, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation PPP, LCP Open, multilink Open, loopback not set
  Last input 00:00:05, output 00:00:05, output hang never
  Last clearing of "show interface" counters 05:13:40
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  2 minute input rate 0 bits/sec, 0 packets/sec
  2 minute output rate 0 bits/sec, 0 packets/sec
     179807 packets input, 246218180 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     5 input errors, 0 CRC, 5 frame, 0 overrun, 0 ignored, 0 abort
     175054 packets output, 233403514 bytes, 0 underruns
     0 output errors, 0 collisions, 2 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions
     DCD=up  DSR=up  DTR=up  RTS=up  CTS=up