Does “lacp port-priority” not work on Cisco Catalyst 3750 series

cisco-3750ieee-802.1ax

I have a Cisco WS-C3750G-12S switch physically connected with ports Gi1/0/1 and Gi1/0/11 to Cisco WS-C3750G-24TS ports Gi1/0/1 and Gi1/0/27 respectively. Ports have different LACP port-priority:

WS-C3750G-12S#sh lacp internal 
Flags:  S - Device is requesting Slow LACPDUs 
        F - Device is requesting Fast LACPDUs
        A - Device is in Active mode       P - Device is in Passive mode     

Channel group 1
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Gi1/0/1   SA      bndl      1             0x1       0x1     0x102       0x3D  
Gi1/0/11  SA      bndl      2             0x1       0x1     0x10C       0x3D  
WS-C3750G-12S#

WS-C3750G-24TS#sh lacp internal
Flags:  S - Device is requesting Slow LACPDUs
        F - Device is requesting Fast LACPDUs
        A - Device is in Active mode       P - Device is in Passive mode

Channel group 1
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Gi1/0/1   SA      bndl      1             0x1       0x1     0x102       0x3D
Gi1/0/27  SA      bndl      2             0x1       0x1     0x11C       0x3D
WS-C3750G-24TS#

As seen above, ports have different LACP priorities, but are still bundled. Same can be seen with sh etherchannel 1 summary:

WS-C3750G-12S#sh etherchannel 1 summary 
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port


Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/1(P)  Gi1/0/11(P) 

WS-C3750G-12S#


WS-C3750G-24TS#sh etherchannel 1 summary 
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port


Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/1(P)  Gi1/0/27(P) 

WS-C3750G-24TS#

Does lacp port-priority not work on Cisco Catalyst 3750 series? Or am I doing something wrong?

Best Answer

This looks normal to me. I think you may have a misunderstanding about the nature of this command.

This should help, from Cisco documentation:

The lacp port-priority interface configuration command determines which ports are bundled and which ports are put in hot-standby mode when there are more than eight ports in an LACP channel group.

On a Layer 3 switch:

Log into the switch and issue the following command (where ipaddress is the ip address of the host you are trying to locate:
```
show ip arp *ipaddress*
```

The output should look similar to below, and give you the mac-address of the device (I've highlighted the mac-address below in bold).

LYKINS-1861#show ip arp 172.20.1.100
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  172.20.1.100            0   **28cf.da1d.1b05**  ARPA   Vlan10

Now issue one of the following commands (where mac-address is the hardware address from previous step). Depending on the Cisco platform, sometimes the command is listed in either form.
```
show mac address-table address *mac-address*
show mac-address-table address *mac-address*
```

The output should look similar to below, the port you are looking for should be listed as the Destination Port:

LYKINS-1861#show mac-address-table address 28cf.da1d.1b05
Destination Address  Address Type  VLAN  Destination Port
-------------------  ------------  ----  --------------------
28cf.da1d.1b05          Dynamic      10     **FastEthernet0/1/1**

On a Layer 2 switch:

Find out what device is doing the routing for this switch (you may have to look at the network documentation). Sometimes it is a "Router on a Stick", where the Layer 2 VLANs are being trunked up to the router for the Layer 3 decisions.

Log into the routing device, and issue the following command (where ipaddress is the ip address of the host you are trying to locate:
```
show ip arp *ipaddress*
```

The output should look similar to below, and give you the mac-address of the device (listed below in bold).

LYKINS-1861#show ip arp 172.20.1.100
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  172.20.1.100            0   **28cf.da1d.1b05**  ARPA   Vlan10

Now log into the Layer 2 switch
On that device issue one of the following commands (where mac-address is the hardware address from previous step). Depending on the Cisco platform, sometimes the command is listed in either form.
```
show mac address-table address *mac-address*
show mac-address-table address *mac-address*
```

The output should look similar to below, the port you are looking for should be listed as the Destination Port:

LYKINS-1861#show mac-address-table address 28cf.da1d.1b05
Destination Address  Address Type  VLAN  Destination Port
-------------------  ------------  ----  --------------------
28cf.da1d.1b05          Dynamic      10     **FastEthernet0/1/1**

Cisco Environment – Using Channel-Group Mode On

LACP gives you a significant advantage in that it verifies connectivity in both directions over each link. You can think of it as also combining the features of UDLD with link aggregation. For example, imagine you have the links configured in "mode on". Now lets say one of the links from the 3750 to the 2960 fails but in only one direction, say the direction toward the 2960 (lets say the fiber got kinked and only one of the two strands has broken). The 3750 will still see link on both ports (because the direction from the 2960 to the 3750 is fine) and it will still try to use the failed path because it does not know that the 2960 has lost link.

If you configure "mode active" the LACP packets must flow over the link in both directions between the 3750 and the 2960. If the link fails in one direction, LACP fails to work and the 3750 stops forwarding over the failed link even though it still has light from the 2960.

So -- using LACP gives you protection against unidirectional link failure. It also gives you protection against plugging the link into the wrong port. If you accidentally plug the link into the wrong port, LACP will fail, the 3750 will not bring that link up, and you have protected yourself against a problem that is difficult to troubleshoot.

Best Answer

Related Solutions

Cisco – How to find the port a device is connected to, based on IP, on a Cisco Catalyst switch

On a Layer 3 switch:

On a Layer 2 switch:

Cisco Environment – Using Channel-Group Mode On