Distributing Cartage in EtherChannel
Traffic in an EtherChannel is statistically load-balanced beyond the alone links bundled
together. However, the amount is not necessarily counterbalanced appropriately beyond all of the links. Instead,
frames are forwarded on a specific articulation as a action of the addresses present in the frame. Some
combination of antecedent and destination addresses (either MAC or IP addresses) is acclimated to form
a bifold arrangement acclimated to baddest a articulation cardinal in the bundle.
Switches accomplish an exclusive-OR (XOR) operation on one or added low-order $.25 of the
addresses to actuate what articulation to use. For example, an EtherChannel consisting of two links
bundled calm requires the XOR of the aftermost bit of the addresses in the frame. A four-link
bundle uses the XOR of the aftermost two bits. Likewise, an eight-link array uses the XOR of the
last three bits. The aftereffect of the XOR operation selects the outbound articulation of the
EtherChannel. Table 5-2 shows the after-effects of an XOR on a two-link bundle.
The XOR operation is performed apart on anniversary bit position in the abode value. If the
two abode ethics accept the aforementioned bit value, the XOR aftereffect is 0. If the two abode $.25 differ,
the XOR aftereffect is 1. In this way, frames can be statistically broadcast amid the links with the
assumption that MAC or IP addresses are statistically broadcast throughout the network. In a
four-link EtherChannel, the XOR is performed on the lower two $.25 of the abode values
resulting in a atomic XOR amount (each bit is computed separately) or a articulation cardinal from 0 to 3.
A chat amid two accessories will consistently be beatific through the aforementioned EtherChannel link
because the two endpoint addresses break the same. However, back a accessory talks to several
Table 5-2 Anatomy Administration on a Two-Link EtherChannel
Binary Addresses Two-Link EtherChannel XOR and Articulation Number
Addr1: ... xxxxxxx0
Addr2: ... xxxxxxx0 ... xxxxxxx0: Articulation 0
Addr1: ... xxxxxxx0
Addr2: ... xxxxxxx1 ... xxxxxxx1: Articulation 1
Addr1: ... xxxxxxx1
Addr2: ... xxxxxxx0 ... xxxxxxx1: Articulation 1
Addr1: ... xxxxxxx1
Addr2: ... xxxxxxx1 ... xxxxxxx0: Articulation 0
152 Chapter 5: Redundant About-face Links
other devices, affairs are that the destination addresses are appropriately broadcast with zeros and
ones in the aftermost bit (even and odd abode values). This causes the frames to be broadcast across
the EtherChannel links. Note that a chat amid two end accessories to actualize a load
imbalance is accessible application one of the links in a array because all cartage amid a brace of
stations will use the aforementioned link.
Switches with an Ethernet Bundling Controller (EBC) are bound to distributing frames based
on antecedent and destination MAC addresses only. For anniversary frame, the antecedent MAC abode is
XOR’d with the destination MAC address. Because this is the alone choice, no switch
configuration is necessary.
Switches such as the IOS-based Catalyst 2900 and 3500XL administer frames according to a
different criteria. By default, EtherChannel frames are broadcast by the low-order $.25 of their
source MAC addresses. The ambassador can baddest either antecedent or destination addresses as
the administration adjustment by application the afterward command (the anchorage accumulation is authentic in the next
section):
Switch (config-if)# anchorage accumulation group-number [distribution {source | destination}]
Other switches, such as the Catalyst 6000, action added adaptability in accretion frame
distribution. The XOR operation can be performed on either MAC or IP addresses and can be
based alone on antecedent or destination addresses or both. Use the afterward command to
configure anatomy administration for all EtherChannel about-face links:
Switch> (enable) set anchorage approach all administration {ip | mac} [source | destination
| both]
The absence agreement is to use IP addresses, both antecedent and destination. Normally, this
action should aftereffect in a statistical administration of frames. However, you should actuate if the
EtherChannel is imbalanced according to the cartage patterns present. For example, if a single
server is accepting best of the cartage on an EtherChannel, the antecedent IP addresses of the stations
talking to the server can account one articulation to be overused. In the case of a four-link EtherChannel,
perhaps two of the four links are overused. Configuring the use of MAC addresses or alone the
source IP addresses ability account the administration to be added counterbalanced beyond all the bundled
links.
In applications involving switches like the Catalyst 6000, some EtherChannel cartage may
consist of protocols added than IP. For example, IPX or SNA frames may be switched forth with
IP. Non-IP protocols would charge to be broadcast according to MAC addresses because IP
addresses are not applicable. Here, the about-face should be configured to use MAC addresses
instead of the IP default.
Switch Anchorage Aggregation with EtherChannel 153
NOTE A appropriate case after-effects back a router is affiliated to an EtherChannel because the router will
use its own MAC abode in all frames that it assiduously to abounding end stations. For the EBC-based
switch, this agency that the destination MAC abode is consistently the aforementioned for frames destined
through the router. Usually this won’t present a botheration because the antecedent MAC addresses are
all different. Back two routers are forwarding frames to anniversary other, however, both antecedent and
destination MAC addresses will abide connected and alone one articulation of the EtherChannel will be
used. The adaptability in the Catalyst 6000 about-face allows the ambassador to baddest absolutely which
criteria frames will be distributed. If the MAC addresses are actual constant, you should
choose IP addresses instead.