в вашем случае лучше использовать etherchannel

C технологией Etherchannel тоже все очень интересно, делаем как написано на cisco.com:

c2950c-24-1#configure terminal
c2950c-24-1(config)#interface port-channel 1
c2950c-24-1(config-if)#switchport mode trunk
c2950c-24-1(config-if)#switchport trunk allowed vlan 1,6,500,501
c2950c-24-1(config-if)#exit
c2950c-24-1(config)#interface range fastethernet 0/25 - 26
c2950c-24-1(config-if)#switchport mode trunk
c2950c-24-1(config-if)#switchport trunk allowed vlan 1,6,500,501
c2950c-24-1(config-if)#channel-group 1 mode desirable
c2950c-24-1(config-if)#exit
c2950c-24-1(config)#exit
c2950c-24-1#rcommand 1
c2950c-24-2#configure terminal
c2950c-24-2(config)#interface port-channel 1
c2950c-24-2(config-if)#switchport mode trunk
c2950c-24-2(config-if)#switchport trunk allowed vlan 1,6,500,501
c2950c-24-2(config-if)#exit
c2950c-24-2(config)#interface range fastethernet 0/25 - 26
c2950c-24-2(config-if)#switchport mode trunk
c2950c-24-2(config-if)#switchport trunk allowed vlan 1,6,500,501
c2950c-24-2(config-if)#channel-group 1 mode desirable
c2950c-24-2(config-if)#exit
c2950c-24-2(config)#exit
c2950c-24-2#exit
c2950c-24-1#show etherchannel port
                Channel-group listing:
                -----------------------

Group: 1
----------
                Ports in the group:
                -------------------
Port: Fa0/25
------------

Port state    = Up Mstr In-Bndl
Channel group = 1           Mode = Desirable-Sl     Gcchange = 0
Port-channel  = Po1         GC   = 0x00010001          Pseudo port-channel = Po1
Port index    = 0           Load = 0x00        Protocol =   PAgP

Flags:  S - Device is sending Slow hello.  C - Device is in Consistent state.
        A - Device is in Auto mode.        P - Device learns on physical port.
        d - PAgP is down.
Timers: H - Hello timer is running.        Q - Quit timer is running.
        S - Switching timer is running.    I - Interface timer is running.

Local information:
                                Hello    Partner  PAgP     Learning  Group
Port      Flags State   Timers  Interval Count   Priority   Method  Ifindex
Fa0/25    SC    U6/S7   H       30s      1        128        Any      30

Partner's information:

          Partner              Partner          Partner         Partner Group
Port      Name                 Device ID        Port       Age  Flags   Cap.
Fa0/25    c2950c-24-2.inner.tu 0007.84ff.9fc0               20s SAC     10001

Age of the port in the current state: 00d:00h:23m:43s

Port: Fa0/26
------------

Port state    = Up Sngl-port-Bndl Mstr Not-in-Bndl
Channel group = 1           Mode = Desirable-Sl     Gcchange = 0
Port-channel  = null        GC   = 0x00010001          Pseudo port-channel = Po1
Port index    = 0           Load = 0x00        Protocol =   PAgP

Flags:  S - Device is sending Slow hello.  C - Device is in Consistent state.
        A - Device is in Auto mode.        P - Device learns on physical port.
        d - PAgP is down.
Timers: H - Hello timer is running.        Q - Quit timer is running.
        S - Switching timer is running.    I - Interface timer is running.

Local information:
                                Hello    Partner  PAgP     Learning  Group
Port      Flags State   Timers  Interval Count   Priority   Method  Ifindex
Fa0/26          U4/S4   H       30s      0        128        Any      0

Age of the port in the current state: 00d:00h:00m:02s

c2950c-24-1#show etherchannel summary
Flags:  D - down        P - in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        u - unsuitable for bundling
        U - in use      f - failed to allocate aggregator

        d - default port
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         PAgP      Fa0/25(Pd) Fa0/26(I)
--- END
То есть, каким-то образом получается, что порт fa0/26 на обоих свичах ну никак не желает становиться участником Etherchannel'а...

Может подскажет кто, где трабл?