Troubleshoot Nexus 5000 Port-channel Loadbalancing

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Updated:December 5, 2016
Document ID:200762

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Introduction

This document describes how to troubleshoot port-channel load balancing in Nexus 5000 switches with multicast traffic.

Contributed by Sivakumar Sukumar, Cisco TAC Engineer.

Prerequisites

Requirments

Cisco recommends that you have knowledge of these topics:

  • Cisco Nexus 5672UP and router e.g ASR supports multicast       
  • Basic undesrstanding of Virtual Port-channel (vPC) , fapric path(FP) and Multicat(MC) technology

Components Used

  

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Problem


Multicast Traffic is not equally distributed between Port-Channels as well as links within Port-Channel. 

  

Network Diagram

Troubleshoot

Scenario 1: Multicast traffic forwarding when FP enabled on vPC peer-link

When fabricpath running only between vPC link, the multicast traffic from host traverses over Peer-link to upstream router 

If fabricpath disabled (on vPC PL), then MC Traffic is distributed over the Port-channels to the L3 GW's (ASR) and does not transverse the vPC PL.

  

Scenario 2: Multicast loadbalancing within port-channel

Traffic not evenly load-balanced and always uses one link inside a port-channel.

Port-channel 15 bundles eth 1/1 and eth 1/8

394(config-if)# sh int port-c 15 | i pps
  input rate 248 bps, 0 pps; output rate 301.67 Mbps, 377.54 Kpps
  input rate 248 bps, 0 pps; output rate 301.67 Mbps, 377.54 Kpps
394(config-if)# sh int eth 1/8 | i pps
  input rate 168 bps, 0 pps; output rate 280.01 Mbps, 145.79 Kpps
394(config-if)# sh int eth 1/1 | i pps
  input rate 80 bps, 0 pps; output rate 10.08 Mbps, 231.76 Kpps

Solution

When MC traffic hitting SVI on Nexus is on a FP enabled VLAN, switch forwards via one of the MC Forwarding Tag (FTag). Refer the link below to know more about FTag. Since there is only one FP interface on the switch i.e the vPC peer-link, the FTag tree prefers peer-link interface as soon as enters the switch. The traffic cannot go to upstream interfaces (to the router) because they are not FP enabled interfaces 

show fabricpath isis topology summary
 FabricPath IS-IS Topology Summary
 Fabricpath IS-IS domain: default
 MT-0
 Configured interfaces: port-channel99
 Max number of trees: 2 Number of trees supported: 2
 Tree id: 1, ftag: 1, root system: 002a.6ab9.20c1, 3941
 Tree id: 2, ftag: 2 [transit-traffic-only], root system: 002a.6ab6.9ac1, 3940
 Ftag Proxy Root: 002a.6ab9.20c1
show fabricpath switch-id
 Total Switch-ids: 4
 =============================================================================
 SWITCH-ID  SYSTEM-ID      FLAGS     STATE   STATIC EMULATED/ANYCAST
 --------------+----------------+------------+-----------+--------------------
 [E] 394    002a.6ab6.9ac1 Primary Confirmed No    Yes
394        002a.6ab9.20c1 Primary Confirmed No    Yes
 * 3940     002a.6ab6.9ac1 Primary Confirmed Yes   No
 3941       002a.6ab9.20c1 Primary Confirmed Yes   No
show fabricpath isis database detail | egrep "Hostname|Affinity|Numgraphs"
 Hostname : 394 Length : 14
 Affinity :
 Nickname : 394 Numgraphs: 1 Graph-id: 1
 Hostname : 395 Length : 14
 Affinity :
 Nickname : 394 Numgraphs: 1 Graph-id: 2

Verify if the MC traffic comes with different src/dest ip/mac/port to make an optimal hash using loadbalancing algorithm configured on the switch. Run commands above to check if there is any problem with port-channel load balancing.

394(config-if)# show mac address-table 
Legend: 
        * - primary entry, G - Gateway MAC, (R) - Routed MAC, O - Overlay MAC
        age - seconds since last seen,+ - primary entry using vPC Peer-Link
   VLAN     MAC Address      Type      age     Secure NTFY   Ports/SWID.SSID.LID
---------+-----------------+--------+---------+------+----+------------------
+ 925      0000.0000.0a01    dynamic   0          F    F  3339.0.0
+ 925      0000.0000.0a4f    dynamic   0          F    F  3339.0.0
+ 925      0000.0000.0b11    dynamic   0          F    F  3339.0.0
+ 925      0000.0037.4e8d    dynamic   0          F    F  3339.0.0
* 925      002a.6a31.5f41    static    0          F    F  3339.0.0

394(config-if)# show int port-c 15 | i pps
    input rate 248 bps, 0 pps; output rate 301.67 Mbps, 377.54 Kpps
394(config-if)# show int eth 1/8 | i pps
    input rate 168 bps, 0 pps; output rate 175.60 Mbps, 145.79 Kpps
394(config-if)# sh int eth 1/1 | i pps
    input rate 80 bps, 0 pps; output rate 126.08 Mbps, 231.76 Kpps

394(config-if)# show port-channel load-balance forwarding-path interface port-channel 15 vlan 925 src-ip 10.1.1.1 dst-ip 231.1.1.1 dst-mac 0100.5e01.0101 src-mac 0000.0037.4e8d
Missing params will be substituted by 0's.
Load-balance Algorithm on switch: source-dest-ip
crc_hash: 231 Polynomial: CRC10b        Outgoing port id  Ethernet1/8
Param(s) used to calculate load-balance:
        seed: 0xe
        vlan: 0x39d
        dst-ip:   231.1.1.1
        src-ip:   10.1.1.1
        dst-mac:  0100.5e01.0101
394(config-if)# show port-channel load-balance forwarding-path interface port-channel 15 vlan 925 src-ip 10.1.1.2 dst-ip 231.1.1.2 dst-mac 0100.5e01.0102 src-mac 0000.0000.0a01
Missing params will be substituted by 0's.
Load-balance Algorithm on switch: source-dest-ip
crc_hash: 250 Polynomial: CRC10b Outgoing port id Ethernet1/1
Param(s) used to calculate load-balance:
        seed: 0xe
        vlan: 0x39d
        dst-ip:   231.1.1.2
        src-ip:   10.1.1.2
        dst-mac:  0100.5e01.0102
        src-mac:  0000.0000.0a01

Usefeul Commands

  • show port-channel load-balance forwarding-path interface port-channel <num> vlan src-ip dst-ip dst-mac src-mac
  • show fabricpath isis database detail | egrep "Hostname|Affinity|Numgraphs"
  • show system internal rtm sdb ftag multicast
  • show fabricpath isis trees multidestination 1
  • show fabricpath route switchid
  • show fabricpath isis topology summary

Related Information

Known defects

Cisco Bug ID CSCvb13924     vPC+ multicast flooded on peer-link irrespective of affinity
Cisco Bug ID CSCts77757      L3 PO load-balance displays incorrect interface

TAC Authored

Contributed by Cisco Engineers

  • Sivakumar Sukumar
    Cisco TAC Engineer

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