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do I need to enable ELRP on all the VLANs where physical ports are identical?

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Titledo I need to enable ELRP on all the VLANs where physical ports are identical?
Question
do I need to enable ELRP on all the VLANs where physical ports are identical?
Environment
  • Summit and Black Diamond platforms
  • EXOS All
Answer
Generally, the ports are identical in Voice and Data VLANs that support end-users at the edge. The configuration is such that PCs and phones are connected to the same physical port on the switch in typical setup. 

Since ELRP detects physical loops, it will be sufficient to enable ELRP on one VLAN as compared to enabling ELRP on all the VLANs which will contribute to CPU over-head, especially, on an eight node stack.

Below are the lab results from three VLANs (data, voice, wlan) configured to have ELRP with identical number of ports.
 
* X460-24p.29 # sh vlan
-----------------------------------------------------------------------------------------------
Name            VID  Protocol Addr       Flags                         Proto  Ports  Virtual
                                                                              Active router
                                                                              /Total
-----------------------------------------------------------------------------------------------
data            100  10.155.167.250 /22  ----------------------------- ANY    1 /34  VR-Default
Default         1    ------------------------------------------------- ANY    0 /0   VR-Default
Mgmt            4095 ------------------------------------------------- ANY    1 /1   VR-Mgmt
voice           200  10.155.159.250 /22  ----------------------------- ANY    1 /34  VR-Default
wlan            900  10.155.155.250 /22  ----------------------------- ANY    1 /34  VR-Default
 
enable elrp-client
configure elrp-client periodic data ports all interval 1 log-and-trap disable-port ingress permanent
configure elrp-client periodic voice ports all interval 1 log-and-trap disable-port ingress permanent
configure elrp-client periodic wlan ports all interval 1 log-and-trap disable-port ingress permanent 
  • Loop ports 5 and 15 to test the ELRP behavior.
  • Loop is detected in all three VLANs – port 15 admin state is toggled and both ports are disabled.
10/02/2017 10:29:54.21 <Info:vlan.msgs.portLinkStateDown> Port 5 link down
10/02/2017 10:29:53.42 <Info:vlan.msgs.portLinkStateDown> Port 15 link down
10/02/2017 10:29:53.39 <Warn:ELRP.Report.Message> [CLI:wlan:3] LOOP DETECTED : 661 transmitted, 4 received, ingress slot:port (5) egress slot:port (15)
10/02/2017 10:29:53.39 <Warn:ELRP.Report.Message> [CLI:wlan:3] LOOP DETECTED : 661 transmitted, 3 received, ingress slot:port (15) egress slot:port (5)
10/02/2017 10:29:53.38 <Warn:ELRP.Report.Message> [CLI:voice:2] LOOP DETECTED : 682 transmitted, 4 received, ingress slot:port (5) egress slot:port (15)
10/02/2017 10:29:53.38 <Info:vlan.dbg.info> Toggling AdminState on Port 15 with pif 0x5cb558
10/02/2017 10:29:53.38 <Warn:ELRP.Report.Message> [CLI:voice:2] LOOP DETECTED : 682 transmitted, 3 received, ingress slot:port (15) egress slot:port (5)
10/02/2017 10:29:53.38 <Warn:ELRP.Report.Message> [CLI:data:1] LOOP DETECTED : 707 transmitted, 4 received, ingress slot:port (5) egress slot:port (15)
10/02/2017 10:29:53.38 <Warn:ELRP.DsblPortLoopDtect> Disabling port 15. Permanent
10/02/2017 10:29:53.38 <Warn:ELRP.Report.Message> [CLI:data:1] LOOP DETECTED : 707 transmitted, 3 received, ingress slot:port (15) egress slot:port (5)
10/02/2017 10:29:52.56 <Info:vlan.msgs.portLinkStateUp> Port 5 link UP at speed 1 Gbps and full-duplex
10/02/2017 10:29:52.49 <Info:vlan.msgs.portLinkStateUp> Port 15 link UP at speed 1 Gbps and full-duplex
  • Remove ELRP config from wlan < unconfig elrp-client wlan> and enable the port.
  • Loop gets detected under voice and data VLANs and ports are disabled.
10/02/2017 10:35:23.95 <Info:vlan.msgs.portLinkStateDown> Port 5 link down
10/02/2017 10:35:23.41 <Info:vlan.msgs.portLinkStateDown> Port 15 link down
10/02/2017 10:35:23.38 <Warn:ELRP.Report.Message> [CLI:voice:2] LOOP DETECTED : 1012 transmitted, 6 received, ingress slot:port (5) egress slot:port (15)
10/02/2017 10:35:23.38 <Warn:ELRP.Report.Message> [CLI:voice:2] LOOP DETECTED : 1012 transmitted, 5 received, ingress slot:port (15) egress slot:port (5)
10/02/2017 10:35:23.38 <Info:vlan.dbg.info> Toggling AdminState on Port 15 with pif 0x5cb558
10/02/2017 10:35:23.38 <Warn:ELRP.Report.Message> [CLI:data:1] LOOP DETECTED : 1037 transmitted, 6 received, ingress slot:port (5) egress slot:port (15)
10/02/2017 10:35:23.38 <Warn:ELRP.DsblPortLoopDtect> Disabling port 15. Permanent
10/02/2017 10:35:23.38 <Warn:ELRP.Report.Message> [CLI:data:1] LOOP DETECTED : 1037 transmitted, 5 received, ingress slot:port (15) egress slot:port (5)
10/02/2017 10:35:22.47 <Info:vlan.msgs.portLinkStateUp> Port 15 link UP at speed 1 Gbps and full-duplex
10/02/2017 10:35:22.45 <Info:vlan.msgs.portLinkStateUp> Port 5 link UP at speed 1 Gbps and full-duplex 
  • Remove ELRP config from data <config elrp-client data> and enable the port again.
  • Loop gets detected under voice VLAN and ports are disabled as expected.
10/02/2017 11:03:42.09 <Info:vlan.msgs.portLinkStateDown> Port 5 link down
10/02/2017 11:03:41.41 <Info:vlan.msgs.portLinkStateDown> Port 15 link down
10/02/2017 11:03:41.39 <Info:vlan.dbg.info> Toggling AdminState on Port 15 with pif 0x5cb558
10/02/2017 11:03:41.39 <Warn:ELRP.Report.Message> [CLI:voice:2] LOOP DETECTED : 2710 transmitted, 12 received, ingress slot:port (5) egress slot:port (15)
10/02/2017 11:03:41.39 <Warn:ELRP.DsblPortLoopDtect> Disabling port 15. Permanent
10/02/2017 11:03:41.39 <Warn:ELRP.Report.Message> [CLI:voice:2] LOOP DETECTED : 2710 transmitted, 11 received, ingress slot:port (15) egress slot:port (5)
10/02/2017 11:03:41.33 <Info:vlan.msgs.portLinkStateUp> Port 15 link UP at speed 1 Gbps and full-duplex
10/02/2017 11:03:41.31 <Info:vlan.msgs.portLinkStateUp> Port 5 link UP at speed 1 Gbps and full-duplex
 
In short, a physical loop is detected and actions are taken by ELRP regardless of the VLANs configuration as long as the same ports are in all the VLANs.
 
If you have ELRP configured across all the VLANs, the ‘Show ELRP’ output will tell you how many packets are generated and processed by CPU.
 
* X460-24p.8 # sh elrp
 ELRP Standalone Client:       Enabled
 Number of ELRP sessions:          3
Number of ELRP pkts transmitted:  5138
Number of ELRP pkts received:     22
                                                        Pkts    Pkts        Disable
Client  VLAN        Ports   Interval Count Cyclic      Xmit    Rcvd Action Port (sec)
-------------------------------------------------------------------------------------
CLI     data        All            1     0 Yes         2979      12 LTI    Perm
CLI     voice       All            1     0 Yes         2979      12 LTI    Perm   
CLI     wlan        All            1     0 Yes         2979      12 LTI    Perm
-------------------------------------------------------------------------------------
Action : (P) Print, (L) Log, (T) Trap, (C) Callback, (E) Egress, (I) Ingress
 
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