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Traffic Forwarding Is Unbalanced as a Result of the Transport Address Configuration.

Publication Date:  2013-01-07 Views:  22 Downloads:  0
Issue Description

Router B is added to the link between Router A and Router C. Router B and Router C are connected through POS 1/0/0 and POS 2/0/0. Router B and Router C are configured to load balance traffic destined for each other over the two POS links. The networking diagram is as follows.

After the preceding configurations are complete, Router C-to-Router B traffic fails to be load balanced while Router B-to-Router C traffic is load balanced as expected. Information about the traffic on the interfaces of Router C is shown as follows.

<HUAWEI> display interface brief

Interface                   PHY   Protocol InUti OutUti   inErrors  outErrors

Pos11/0/0                   up    up         41%     1%          0          0

Pos12/0/0                   up    up         40%    83%          0          0

Handling Process

1.         Since the load imbalance occurs when traffic is forwarded from Router C to Router B and Router C originally exists on the link, traffic does not change whether Router B is added or not. Therefore, the load balancing problem is not caused by the traffic model.

2.         The on-site Huawei technical support personnel check whether load balancing is performed across the routes using the display ip routing-table command.

The command output shows that the routes destined for POS 1/0/0 and POS 2/0/0 participate in load balancing.

<HUAWEI> display ip routing-table

Route Flags: R - relay, D - download to fib

------------------------------------------------------------------------------Routing Tables: Public

Destinations : 18409    Routes : 21796

Destination/Mask    Proto  Pre  Cost     Flags NextHop         Interface

        0.0.0.0/0   O_ASE  150  1           D  112.100.5.45    Pos2/0/0

                      O_ASE  150  1           D  112.100.5.49    Pos1/0/0

      1.192.0.0/13  BGP    255  100        RD  112.100.7.2     Pos2/0/0

                       BGP    255  100        RD  112.100.7.2     Pos1/0/0

    1.193.128.0/18  BGP    255  100        RD  112.100.7.2     Pos2/0/0

                       BGP    255  100        RD  112.100.7.2     Pos1/0/0

The preceding command output shows only part of the segments as evidence. To check whether the traffic is forwarded in route load balancing mode, check the entire routing table.

IP routes can participate in load balancing, but the traffic is not load balanced over the two POS links. Therefore, the traffic is not forwarded in the IP route load balancing mode. LSP load balancing may be performed during the forwarding process.

1.         The on-site Huawei technical support personnel check whether load balancing is performed across the LSPs.

They check the LSP information about load balancing on POS 1/0/0 and POS 2/0/0 respectively using the following commands. The command output shows that only on LSP that is established on POS 2/0/0 exists. This result matches the fact that no traffic reaches POS 1/0/0.

<HUAWEI> display mpls lsp outgoing-interface Pos 1/0/0 // No information is displayed.

<HUAWEI> display mpls lsp outgoing-interface Pos 2/0/0

-----------------------------------------------------------------------------

                 LSP Information: LDP LSP

-----------------------------------------------------------------------------

FEC                In/Out Label  In/Out IF                      Vrf Name

112.100.5.16/30    NULL/3        -/Pos2/0/0

112.100.5.16/30    1041/3        -/Pos2/0/0

112.100.7.1/32     1043/3        -/Pos2/0/0

112.100.7.1/32     NULL/3        -/Pos2/0/0

After the display mpls lsp outgoing-interface Pos 1/0/0 command is run, no command output is displayed, which indicates that no LSP is established on POS 1/0/0.

2.         The on-site Huawei technical support personnel analyze the cause why no LSP is established on POS 1/0/0.

To locate the cause, they check the LDP session establishment using the display mpls ldp peer command. The following command output shows that Router C's LDP peer is established on POS 2/0/0. This may be caused by a configuration problem.

<HUAWEI> display mpls ldp peer

         LDP Peer Information in Public network

 ----------------------------------------------------------------------------

 Peer-ID                Transport-Address  Discovery-Source

 ----------------------------------------------------------------------------

 The configuration information is omitted here.

 222.171.5.7:0          222.171.116.1      GigabitEthernet14/0/0

 112.100.7.1:0          112.100.5.5        Pos2/0/0   ====No LDP peer is established on POS 1/0/0.

 ----------------------------------------------------------------------------

TOTAL: 10 Peer(s) Found.

3.         The on-site Huawei technical support personnel check the configurations on the POS interfaces and find that the transport-address command is run on both POS 1/0/0 and POS 2/0/0 to configure the current interface addresses to be transport addresses.

LDP sessions over multiple links between two LSRs must be established based on the same transport address. Therefore, to establish an LDP session over multiple links between Router B and Router C and implement load balancing, cancel the preceding configuration.

<HUAWEI> display current-configuration

......

interface Pos1/0/0

 link-protocol ppp

 undo shutdown

 ip address 112.100.5.6 255.255.255.252

 isis enable 99

 isis circuit-level level-2

 mpls

 mpls ldp

 mpls ldp transport-address interface

#

interface Pos2/0/0

 link-protocol ppp

 ip address 112.100.5.2 255.255.255.252

 isis enable 99

 isis circuit-level level-2

 mpls

 mpls ldp

 mpls ldp transport-address interface

Root Cause

Traffic load unbalancing in the following situations:

1.         The traffic model is special.

2.         Load balancing is not implemented across routes.

3.         Load balancing is not implemented across LSPs.

Solution

Delete the transport address configurations on both POS 1/0/0 and POS 2/0/0. Run the display interface brief command on both interfaces to view information about load balancing. If the command output shows that traffic is load balanced on the two POS interfaces, the problem is resolved. An example command output is shown as follows.

<HUAWEI> display interface brief

Interface                   PHY   Protocol InUti OutUti   inErrors  outErrors

Pos1/0/0                   up    up         44%    50%          0          0

Pos2/0/0                   up    up         44%    50%          0          0

END