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Multilink and Serial Interface Selection

rmeans
Level 3
Level 3

I have a 2951 routers with 3 T1's connected to controller cards/serial interfaces.  How does the router determine which serial interface to use?  

5 Replies 5

Joseph W. Doherty
Hall of Fame
Hall of Fame

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The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

By examination of your route table, i.e. which interface, or interfaces, is best next hop toward packet's destination.

rmeans
Level 3
Level 3

More information

I am thinking CEF will be involved.  I but I haven't seen how CEF is balancing the load.

interface Multilink1
 bandwidth 4608
 ip address 1.1.1.21 255.255.255.252
 ip flow ingress
 ip flow egress
 no peer neighbor-route
 ppp chap hostname *
 ppp multilink
 ppp multilink interleave
 ppp multilink group 1
 no cdp enable
 service-policy output qos_pm
 hold-queue 256 in
 hold-queue 4096 out

interface Serial0/0/0:0
 no ip address
 encapsulation ppp
 ppp chap hostname *
 ppp multilink
 ppp multilink group 1
 hold-queue 256 in
 hold-queue 4096 out

interface Serial0/0/1:1
 no ip address
 encapsulation ppp
 ppp chap hostname *
 ppp multilink
 ppp multilink group 1
 hold-queue 256 in
 hold-queue 4096 out

interface Serial0/1/0:2
 no ip address
 encapsulation ppp
 ppp chap hostname *
 ppp multilink
 ppp multilink group 1
 hold-queue 256 in
 hold-queue 4096 out


Multilink1 is up (if_number 43)
  Corresponding hwidb fast_if_number 43
  Corresponding hwidb firstsw->if_number 43
  Internet address is 1.1.1.21/30
  ICMP redirects are never sent
  Per packet load-sharing is disabled
  IP unicast RPF check is disabled
  Input features: Ingress-NetFlow
  Output features: CCE Post NAT Classification, QoS Marking, Post-Ingress-NetFlow, Egress-Netflow
  IP policy routing is disabled
  BGP based policy accounting on input is disabled
  BGP based policy accounting on output is disabled
  Interface is marked as point to point interface
  Hardware idb is Multilink1
  Fast switching type 7, interface type 105
  IP CEF switching enabled
  IP CEF switching turbo vector
  IP Null turbo vector
  IP prefix lookup IPv4 mtrie 8-8-8-8 optimized
  Input fast flags 0x0, Output fast flags 0x10004000
  ifindex 41(41)
  Slot  Slot unit 1 VC -1
  IP MTU 1500

 

IPv4 CEF is enabled and running
VRF Default
 491 prefixes (490/1 fwd/non-fwd)
 Table id 0x0
 Database epoch:        0 (491 entries at this epoch)

0.0.0.0/0, epoch 0, flags cover dependents, rib only nolabel, rib defined all labels, default route, RIB[B], refcount 7, per-destination sharing
  sources: RIB, DRH
  feature space:
   IPRM: 0x00018000
   NetFlow: Origin AS 0, Peer AS 0, Mask Bits 0
  subblocks:
   Covered dependent prefixes: 1
     notify cover updated: 1
  ifnums:
   Multilink1(43)
  path 1BCB1E0C, path list 1BBB5A38, share 1/1, type recursive, for IPv4
  recursive via 1.1.1.22[IPv4:Default], fib 1BBC2038, 1 terminal fib, v4:Default:1.1.1.22/32
    path 1BCB2FFC, path list 1BBB5CB8, share 1/1, type recursive, for IPv4, flags doesnt-source-via, cef-internal
    recursive via 1.1.1.20/30<nh:1.1.1.22>[IPv4:Default], fib 1BBC70A8, 1 terminal fib, v4:Default:1.1.1.20/30
      path 1BCB0CFC, path list 1BBB4688, share 1/1, type connected prefix, for IPv4
      connected to Multilink1, adjacency IP adj out of Multilink1 1BB465A0
  output chain:
    loadinfo 0233303C, per-session, 1 choice, flags 0183, 260 locks
    flags: Per-session, for-rx-IPv4, 2buckets, indirection
    1 hash bucket
      < 0 > IP adj out of Multilink1 1BB465A0
    Subblocks:
     None

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Ah, your three T1 appear to be configured as MLPPP.  From a routing perspective, there's only one link.

I believe MLPPP load balances by sending packets to the next available physical interface.

Any show commands you can suggest?

 

show ppp multilink - the weight of each serial interface (equal).

 

Ultimately, I am trying to figure why my 4.6 Mbps (3 T1's) are not fully utilized during afterhours system backups.  Some offices use the maximum bandwidth, other do not.

 

Multilink1
  Bundle name: 5B3EA11C45316A0B294597F30000002800000065
  Remote Endpoint Discriminator: [4] 5B3EA11C45316A0B294597F30000002800000065
  Local Endpoint Discriminator: [1] 204.64.155.21cpa
  Bundle up for 8w0d, total bandwidth 4608, load 17/255
  Receive buffer limit 36000 bytes, frag timeout 1000 ms
  Interleaving enabled
    0/0 fragments/bytes in reassembly list
    1190 lost fragments, 87647793 reordered
    0/0 discarded fragments/bytes, 0 lost received
    0x4E59EB received sequence, 0x4BA5B0 sent sequence
  Member links: 3 active, 0 inactive (max 255, min not set)
    Se0/0/0:0, since 8w0d, 5760 weight, 1496 frag size
    Se0/0/1:1, since 3w3d, 5760 weight, 1496 frag size
    Se0/1/0:2, since 01:45:23, 5760 weight, 1496 frag size
No inactive multilink interfaces

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If TCP, that might be caused by some clients' stacks not supporting your BDP.

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