IT Support, Sr. Analyst/Programmer

[jsciscovpn]

I'm not able to configure two ports on a C2950 as trunk ports, in order to connect to another similar switch.

I did what's recommended:

en

conf t

int range fa0/23 - 24

switchport mode trunk

end

Keeping the default encapsulation (802.1q)

The ports still show in the default vlan 1 and there's nothing in: sh int trunk

What am I missing?

Jerry

[Jon Marshall]

Jerry

Have you actually connected the ports to the other device ?

Until they are up/up they won't show as trunks.

Jon

[jsciscovpn]

yes, I have one cable running from sw1-p17 to sw4-p23 (I intend to connect a second one later, p18 to p24) No light on either ports, I only rebooted sw4, the newer one, but sw1 is running live, do I need to reboot it?

[Jon Marshall]

You shouldn't need to reboot either switch to do this.

If there are no lights on the ports then they aren't up ie. what does a "sh ip int brief" show on the switches ?

Until you fix that issue you will not see them as trunks.

Jon

[jsciscovpn]

they're all DOWN:

sw1
FastEthernet0/17           unassigned      YES unset  down                  down

FastEthernet0/18           unassigned      YES unset  down                  down

same as sw4

what could be keeping them down? Do you need a special pinout cable for trunks? I'm using a cable that works for the laptop, I'll check the speed, because the ports are set to 100

[Jon Marshall]

What type of cable are you using ?

Jon

[jsciscovpn]

cat 5e, just tested it, works fine for the laptop, connects at 100, I'm able to connect/browse -no prob there

[Jon Marshall]

Okay, traditionally switches needed to use crossover cables rather than straight thru which is what you use for end devices connected to clients.

However most modern switches support auto-mdix which basically means you can use a straight thru for it to work.

Not sure whether the 2950 does but what is the switch you are connecting to ?

Either way auto-mdix needs the port to be set to auto.

So first thing to try is to set the ports on both switch to auto/auto and see if that works.

Word of caution here.

If this is a new switch it could become the STP root depending on your current STP settings and the switch priroities.

If you are doing this during working hours be aware that you may get a blip in traffic.

In addition if you are going to connect up the switches with two ports you will definitely want them in an etherchannel rather than separate physical trunks.

If any of the above is not clear or new to you then you may want to take a step back before connecting all this up.

Jon

[jsciscovpn]

Thank you. The 3 switches are c2950 (sw1 to 2 and to 3) the new one is the same model, just got it, to keep them uniform.

The two pair of patch cords used (from sw1 to sw2 and from sw1 to sw3) seem to be crossover, as they're marked "CROSS" and CK002009

The STP setting is the same on all (default)

So does that mean that I should remove the trunk mode on both sides and just get a pair of crossover patches?

[Peter Paluch]

Jon, Jerry,

Pardon me for dropping in abruptly. A few comments/suggestions.

First of all, Catalyst 2950 do not have the auto-cross functionality. To connect two 2950 switches together, cross-over cable must be used. Jerry, I suggest you have a good look at both ends of the cable if possible, and not just on some label: A cross-over cable can be recognized by one end going from left with white-green/green/white-orange/blue/etc., while the other goes from left with white-orange/orange/white-green/blue/etc.

Second, do not hardcode speed and duplex. This is a bad habit carried over from past times where the autonegotiation was not reliable. Today, however, you can cause more trouble by hardcoding the speed and duplex rather than leaving switches settle these things automatically.

Third, check the output of the show interface status and make sure that the ports are not marked as err-disabled. If they are, use the show interface status err-disabled to find out why - or ideally, post the output here. If they are err-disabled then it means that the switch thought it is dangerous for some reason to keep them running, and has disabled them to prevent some nasty thing from happening. If the ports are marked as down or notconnect then it means that the other end is not connected, or that the cable is not good/appropriate.

Just to clear a misunderstanding: The operating mode of a port (access vs. trunk) has nothing to do with the cable type. In your situation, you want to run the ports as trunks but you need to use crossover cables.

Best regards,
Peter

[Jon Marshall]

Peter

Thanks for confirming that 2950s don't support auto-mdix as I wasn't sure.

Jon

[saif musa]

could you upload the running configuration of the switch here as attachment. that will helps alot

[Jon Marshall]

So does that mean that I should remove the trunk mode on both sides and just get a pair of crossover patches?

No, two different things.

The cables are a physical issue but as they are crossovers they should work. You may well want to try using auto on both ends and see if the ports come up.

Trunks simply allow multiple vlans across the same link so it is nothing to do with the actual physical cables used ie. trunks don't care.

If you are going to use two connections though between the switches then you need to make them an etherchannel trunk ie. an etherchannel treats both physical link as one logical link and you make the logical link a trunk so it can carry multiple vlans between the switches.

If you don't use an etherchannel then one of the links will be blocked by STP.

Apologies if I am telling you things you already know :-)

Jon

[jsciscovpn]

{thanks Peter}

No apology needed, I'm fairly new to this level of switch settings.

ok, how do you make the two ports 'etherchannel' ?

so I still keep the ports as 'switchport mode trunk', remove the speed 100 and duplex Full, and use crossover patches.

I'll get back to you tomorrow

Jerry

[Jon Marshall]

Jerry

See this link for configuring etherchannel -

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_22_ea11x/configuration/guide/scg/swethchl.html

you should use either LACP or PaGP your choice, doesn't really make much difference.

Jon