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What is an UPLINK port?

A colleague recently encountered some problems with keepalives on switch ports. He wrote a post about it. Keepalives are, quoted from his blog post:

By default Cisco routers and switches periodically test their (Fast) Ethernet links by sending out Loopback frames (ethertype 0×9000) addressed to themselves. Call it a “L2 self-ping” if you will. In a switched environment it can be used to test the functionality of the switch and/or keep the router’s MAC address in the switch’s address table. Another thing what this Loopback frames do, is to check for a loop. If there is a loop in the network, the resulting Loopback frame will be seen by the sending switch and the port will be err-disabled.

Cisco says that starting in 12.2SE based releases, keepalives are NO longer sent by default on fiber and uplink interfaces.

The big question, where I am struggling with is:

When is a port an UPLINK port?

Looking at a Cisco Catalyst 2960-24PC-L I get the following information about the keepalives information on the interfaces.

SW01#sh int | i 0/23|0/24|GigabitE|Keep

FastEthernet0/23 is down, line protocol is down (notconnect)
  Keepalive set (10 sec)
FastEthernet0/24 is down, line protocol is down (notconnect)
  Keepalive set (10 sec)
GigabitEthernet0/1 is up, line protocol is up (connected)
  Keepalive not set
GigabitEthernet0/2 is up, line protocol is up (connected)
  Keepalive not set

The output above shows that keepalives are enabled on the FastEthernet connections by default and disabled on the GigabitEthernet connections. The GigabitEthernet connections are dual-purpose, but I don’t use the fiber connection. But because it is a dual-purpose port I can imagine that keepalives are disabled by default, because of the fiber properties.

So I looked at at Cisco Catalyst 2960-48TT-L switch with Software Version 12.2(44)SE. This switch doesn’t have any fiber connections, but only copper. From this switch I get the following keepalives information.

SW01#sh int | i 0/47|0/48|GigabitE|Keepa

FastEthernet0/47 is down, line protocol is down (notconnect)
  Keepalive set (10 sec)
FastEthernet0/48 is down, line protocol is down (notconnect)
  Keepalive set (10 sec)
GigabitEthernet0/1 is down, line protocol is down (notconnect)
  Keepalive set (10 sec)
GigabitEthernet0/2 is up, line protocol is up (connected)
  Keepalive set (10 sec)

The output tells me that keepalives are enabled on all switch ports. GigabitEthernet0/2 is configured as a trunk port, so now I am very confused.

I draw the following conclusions:

  1. Keepalives are disabled on uplink and fiber ports starting in 12.2SE releases. GigabitEthernet0/2 from the 48TT-L switch is configured as a trunk port. So a trunk port doesn’t mean the same as an uplink port;
  2. The “special” ports on the right side of a switch are no uplink ports, because else GigabitEthernet0/2 from the 48TT-L switch would have keepalives disabled;

After drawing these conclusion I still don’t know the exact definition of an UPLINK port :-(. Please let me know if you have any suitable definition for an UPLINK port………

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René Jorissen

Co-owner and Solution Specialist at 4IP Solutions
René Jorissen works as Solution Specialist for 4IP in the Netherlands. Network Infrastructures are the primary focus. René works with equipment of multiple vendors, like Cisco, Aruba Networks, FortiNet, HP Networking, Juniper Networks, RSA SecurID, AeroHive, Microsoft and many more. René is Aruba Certified Edge Expert (ACEX #26), Aruba Certified Mobility Expert (ACMX #438), Aruba Certified ClearPass Expert (ACCX #725), Aruba Certified Design Expert (ACDX #760), CCNP R&S, FCNSP and Certified Ethical Hacker (CEF) certified. You can follow René on Twitter and LinkedIn.

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