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David Kondicz
Beginner

Catalyst down-when-looped

Hello,

i want to know how is this function working on Catalist 2960 Lan Base. I want to use this function instead STP.

Networks structure:

Core SW (trunking vlan 1,550)-------------pop switch (acces vlan 550)--------------Client`s 2960(Vlan 1,550)

                    |                                                                                                                        |

                     ---------------------------------------backup VPN (Vlan 550)-------------------   (Applied down-whn-looped on this interface)         

Will it works?  How fast will be port statust change if main link stops work. for eg no electricity on pop switch.

Thanx

Dav

3 REPLIES 3
dixho
Frequent Contributor

look at the Command Reference, I do not see donw-when-looped listed in the command reference:

http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst2960/software/release/12.2_55_se/command/reference/cli1.html

From the description of down-when-looped, what you are looking for is Flexlink:

http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst2960/software/release/12.2_55_se/configuration/guide/swflink.html

Flexlink works fairly well. Once the 2960 detects the primary interface down, it switches to backup interface right away.

Phillip Remaker
Cisco Employee

The "down-when-looped" command is a layer 3 interface function, specifically intended for WAN interfaces.  A looped WAN interface is normally considered "up" since it is receiving a carrier detect signal and can process keepalives. A dynamic routing protocol will discover and exclude a looped interface, but a static route will not.  The "down-when-looped" command is a configuration option which causes an interface detecting a loop (hearing its own keepalives) to declare itself 'down' to the routing processes so that appropriate routing decisions can be made.

http://www.cisco.com/en/US/docs/ios/12_1/interface/command/reference/irddelay.html#wp1017522

As Dixon has noted, this is not an appropriate command for a LAN interface.

SlaadorDD
Beginner

Hey there,

"Down-when-looped" on Ethernet interfaces is just a basic loopback detection (you know, when your cable's tx pair loops back to its rx pair). It is explained, for instance, here:

http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_22ea/SCG/swint.html#wp1194052

For your needs, you should probably, with extreme care, use flexlinks.