Configuring Layer 2 and Layer 3 Ether Channel

EtherChannel is yet another form of trunk line you can configure. The common way to configure E) bs Laner 2 borween two switches. Usua lly ISL om 802.1Q is also ronOig ured. On the Ca talyst 3550 w boftwa-e installed, yo u can also configure Layer 3 EtherChannel.

EtherChannel has some limitations you should be aware of when configuring it. Some of these limit hardware speciei c; there fore, it is a go od idea to loo k up tee spec i fic E therChann el limitations for tl you are co nfiguring.

The following lis t a pulles to the configurat ion g mCeM neb ox -he Catalyst 3550 Ethernet switch:

• Each EtherChannel can have up to eight compatibly configured Fast Ethernet interfaces and u Gigabit Ethernet i nterfuce s.

• Do not conegure a GlgaStack GBIC port a s part oof an etherChannel.

• Configure all interfaces in an EtheuCharnei to operate at ehe same speeds and duplex modus.

• Enable all interfaces in an EtherChannel. An interface in an EtherChannel disabled by using tk interfane c onfig rratlon comma nd is t -ea-ed as a link fa mure, and its tra CHc is trrnshefred to o remainrng interfaces in she EtherChannel.

t When a grou p is first create do all porfs follo w the parameters set for the first port to be added re you change the configncation of one of1 these parameters, you mest also male the ciiynges

• An EtherChannel does not form if one of the interfaces is a Switch Port Analyzer (SPAN) destii You may use the EtherChannel group as a source of SPAN to monitor the entire group.

• A port that belongs to an EtherChannel port group cannot be configured as a secure port.

• Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks. Inte different native VLANs cannot form an EtherChannel.

• If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (ISL or same on all the trunks.

t ho group.

• An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not form an Et even when PAgP is set to the auto or desirable mode.

• Before enabling 802.1X on the port, you must first remove it from the EtherChannel. If you ei on a not-yet-active port of an EtherChannel, the port does not join the EtherChannel.

• Interfaces woth different STP path costs can form an EtherChannel as long they are otherwise configured. Setting different STP path costs does not, by itself, make interfaces incompatible ■ formation mf an EtherChannel.

• For Layer 3 EtherChannels, assign the Layer 3 address to the port-channel logical interface, n physical i nte rfaces in the channel.

• By default, PAgP will have no Layer 2 or Layer 3 Channel groups assigned or defined. The Eth config Noation for PAgP is auto and silent; the interface will respond to PAgP packets but will n negotiation. PAgP is configured as an aggregate-port learner with a PAgP priority of 128 on al

Configuring Layer 2 EtherChannel ISL/802.1Q trunks on the Catalyst 3550 is a three-step process. the state of the port prior to configuration, you may have to disable autonegotiation mode. By defc set to negotiate the encapsulation and to be in the dynamic and desirable mode.

Step 1. Configure the trunk encapsulation as ISL or 802.1Q. Step 2. Co ntigure th e port as a trunk. Step 3. Configure EtherChannel port groups.

These steps are accomplished with the following commands from the interface configuration mode:

Switch#(config-if)#switchport trunk encapsulation [isl | dotlq / negotiate] Switch#(config-if)#switchport mode [trunk | dynamic {auto | desirable}] Switch#(config-if)#channel-group [1-64]mode {auto [non-silent] | desirable [non-si

The first two commands are identical to configuring a normal ISL or 802.1Q trunk. The channel-gi command creates a virtual interface called interface port channel x, where x is the channel group n virtual interface lists all the common properties that must be associated with any link joining the po virtual interface is also where you assign an IP address for Layer 3 EtherChannel. The channel groi range from 1 to 64. The mode keyword enables or disables PAgP. PAgP works rather reliably, but when you choose the mode in which to run PAgP that you use the same mode on all interfaces in th group. The mode keyword has the following parameters:

auto— Enables PAgP only if another PAgP device is detected. It places the interface into a pas negotiating state, in which the interface responds to PAgP frames it receives but does not stai negotiation.

• desirable— Unconditionally enables PAgP. This keyword places the interface into an active n< state, in which the interface starts negotiations with other interfaces by sending PAgP frames.

• on— Forces the interface to channel without PAgP. With the on mode, a usable EtherChannel when aT interfaoe group in the on mode is connected to another interface group in the on mo>

• active(LACP)— Sets the interface into an active negotiating state, in which the interface start with other interfaces by sending LACP packets.

• passive (LACP)— Sets the interface into a passive negotiating state. In this mode, the interfai LACP packets that it receives but does not start LACP packet negotiation. This setting uses mi packets.

An EtherCh annel can also be assigned to a specific VLAN, although this is less common. To accomp assign all interfaces as static-access ports in the same VLAN.

When configuring EtherChannel, you will notice the link drops and initializes quite a bit—once for a encapsulation change, and at least one other time when it joins the channel group. To prevent this happening, just shut down the link before configuring any trunking or EtherChannel parameters. T interface from the EtherChannel group, use the no channel-group interface configuration commai

Figure 1- 16 represents a commo n network. In this LAN, the core switches are connected to each ot another edge switch. The core switches are going to use Gigabit EtherChannel to trunk the two swi together. 802.1Q will be the VLAN trunking protocol and will eventually allow VLAN 192 full connec

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