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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 27 Configuring EtherChannels
Configur ing EtherChannels

EtherChannel Configuration Guidelines

If improperly configured, some EtherChannel interfaces are automatically disabled to avoid network
loops and ot her pr oblems. Follow these g ui delin es t o avoid c onfiguratio n p roble ms:
•Configure an Ether Channe l with up to eight Ethern et interfac es of the same type .
Note Do not configure a GigaStack GBIC port as part of an EtherChannel.
•Configure a ll int erfac e s i n an E t herC h ann el to op era t e at th e s ame sp ee ds an d d upl ex m od es.
•Enable all interfaces in an EtherChannel. An interface in an EtherChannel that is disabled by using
the shutdown interface configuration command is treated as a link failure, and its traffic is
transferred to one of the remaining interfaces in the EtherChannel.
•When a group is first cr ea ted , all ports follow the paramete rs set for the first port to be added to the
group. If you change the configuration of one of these parameters , you must also make the changes
to all ports in the group:
–
Allowed-VLAN list
–
Spanning-tree path cost for each VLAN
–
Spanning -tree po rt priori ty for eac h VLAN
–
Spanning-t ree Port Fast setting
•If you con figure Switc h Port Analyz er (SPAN) on a por t that is a me mber of th e EtherC hanne l, it
leaves the Ether Channel .
•Do not con figure a p ort tha t be long s t o an E the rChann el port group a s a sec ur e por t.
•Before e nabling 802 .1X on the port, you must first remove it from the Eth erChann el. If you tr y to
enab le 802.1X on an Ethe rChannel or on an act ive port in an E therCha nnel, a n error message
appear s, and 802 .1X is no t enabled . If you enabl e 802.1X on a not-y et active port of an
Ether Channe l, th e po rt d oe s n ot joi n t he Et herC hann el.
•Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks.
•If you con figure an Ether Channel from trunk interface s, verify tha t the trunking mode (80 2.1Q) is
the same o n al l the trunk s. Incon si stent trun k m odes on Ethe r Channe l i nte rface s ca n h ave
unexpected re sult s.
•An EtherCh annel sup ports the same allowed range of VLA Ns on all the inte rfaces in a trun king
Layer 2 EtherC hannel . If the allowed rang e of VLAN s is not the same , the inte rfaces d o not form
an EtherChannel even when PAgP is set to the auto or desirable mode.
•Interfaces with different spanning-tree path costs can form an EtherChannel if they are otherwise
compatibly configured. Setting dif ferent spanning-tree path costs does not, by itself, mak e interfa ces
incompatible for the formation of an EtherChannel.

Configuring Layer 2 EtherChannels

You co nfigure Layer 2 EtherCh annels by configurin g the Eth ernet int erfaces with the channel-group
interface configuration command, which creates the port-channel logical interface.
Note La yer 2 inter faces mus t be con ne cted a nd func ti oning fo r IO S to c reate p ort- chann el in terfac es.