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Uneven Load Sharing Because LDP Sessions Were Established Based on the Same Transport Address

Publication Date:  2013-09-26 Views:  73 Downloads:  0
Issue Description

Router B was added between Router A and Router C. Two POS links were configured between Router B and Router C to share traffic.

            -----Pos1/0/0------
RA
----
RB                    RC
            -----Pos2/0/0------

Traffic from Router B to Router C was evenly shared but traffic from Router C to Router B was unevenly shared. The following is information about traffic on the involved Router C interface:

<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

Huawei performed the following operations to address the problem:

1. Excluded that the problem was caused by the traffic model.

This was because Router C was a legacy device and traffic would not change due to the addition of Router B.

2. Checked whether load sharing was available between routes. 

Loading share was available between routes to POS1/0/0 and POS2/0/0.

<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 is only about some key network segments. The entire routing table needs to be checked to determine whether load sharing was available between routes.

Load sharing was available between routes, but traffic was not distributed over the two links, indicating that traffic was not distributed in IP load sharing mode. Traffic might be distributed in LSP load sharing mode.

3. Checked whether load sharing was available between LSPs.

Checked for LSP load sharing information about the two interfaces. Only an LSP had been created on interface POS2/0/0 but no LSP had been created on interface POS1/0/0.

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

<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

The system did not return any information after the display mpls lsp outgoing-interface Pos 1/0/0 command was run, indicating that no LSP had been created on interface POS1/0/0. 

4. Analyzed why no LSP had been created on interface POS1/0/0.

According to LDP session creation information, Router C had created an LDP peer only on interface POS2/0/0.

<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 POS1/0/0 LDP PEER

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

TOTAL: 10 Peer(s) Found.

5. Checked the configurations on the two interfaces.
The transport-address command had been applied to both interfaces to specify the current interface to be a transport address.

With this command, LDP sessions over multiple links between two LSRs must be established based on the same transport address. Therefore, only after this command is deleted, can LDP sessions be established on multiple links to implement load sharing.

<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

Possible causes for uneven load distribution include:

1. The traffic model is incorrect.

2. Load sharing is unavailable between routes.

3. Load sharing is unavailable between LSPs.
Solution

After transport-address command was deleted from the interfaces, the problem was resolved.

<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
None

END