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Uneven Load Balancing Due to the Incorrect LDP Transport Address Configuration

Publication Date:  2013-09-03 Views:  22 Downloads:  0
Issue Description

After RouterB is deployed between RouterA and RouterC, two POS links are set up between RouterB and RouterC for load balancing.
            -----Pos1/0/0------
RA----RB                    RC
            -----Pos2/0/0------

After the configuration, traffic from RouterC to RouterB is not evenly load balanced, while traffic from RouterB to RouterC is evenly load balanced. Check brief information about traffic on RouterC interface connected to RouterB.
<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. Traffic from the RouterC does not change even after RouterB is added to the network. Therefore, uneven load balancing is not caused by the traffic model.
2. Check whether IP load balancing is formed on routes.
The following command output shows that routes between POS1/0/0 and POS2/0/0 are load balanced.
<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
Note:

The preceding information shows only routes on main network segments. To determine route load balancing, check the entire routing table.
IP routes are load balanced but traffic is not load balanced on the two links. This indicates that IP load balancing is not used but LSP load balancing may be used.
3. Check whether LSP load balancing is formed on routes.
Check LSP information about load balancing on POS1/0/0 and POS2/0/0. Only POS2/0/0 has LSPs created and no LSP is generated on POS1/0/0. This matches the result of no traffic on POS1/0/0.
<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 executed, no information is displayed, which indicates that POS1/0/0 has no LSP created.
4. Check the reason why POS1/0/0 has no LSP created.
First check the created LDP sessions. The following command output shows that RouterC has created an LDP peer relationship with RouterB only on POS2/0/0. This may be caused by the configuration.
<HUAWEI> display mpls ldp peer
         LDP Peer Information in Public network
 ----------------------------------------------------------------------------
 Peer-ID                Transport-Address  Discovery-Source
 ---------------------------------------------------------------------------- 
 ............. 
 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 relationship exists on POS1/0/0.
 ----------------------------------------------------------------------------
TOTAL: 10 Peer(s) Found.

5. Check the interface configuration. The mpls ldp transport-address interface command is executed on both POS1/0/0 and POS2/0/0 to specify the local interface address as the transport address.
After the mpls ldp transport-address interface command is executed, LDP sessions over multiple links between two LSRs must be established based on the same transport address. Therefore, to establish LDP sessions over multiple links and implement even traffic load balancing, delete the configuration of the mpls ldp transport-address interface command.
<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
The mpls ldp transport-address interface command is executed on both POS1/0/0 and POS2/0/0 to specify the local interface address as the transport address.
After the mpls ldp transport-address interface command is executed, LDP sessions over multiple links between two LSRs must be established based on the same transport address.
Solution
Delete the configuration of the mpls ldp transport-address interface command on POS1/0/0 and POS2/0/0. Check the traffic load balancing status on the two interfaces. Traffic in two directions is evenly load balanced, indicating that the problem is solved.
<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
Suggestions
Traffic is unevenly load balanced in the following situations:
1. The traffic model is special.
2. Traffic is not load balanced according to a route.
3. Traffic is not load balanced on an LSP.

END