EtherChannel Questions

If you are not sure about EtherChannel, please read our EtherChannel tutorial.

Question 1

Explanation

The way EtherChannel load balancing works is that the switch assigns a hash result from 0-7 based on the configured hash method ( load balancing algorithm ) for the type of traffic. This hash result is commonly called as Result Bundle Hash (RBH).

Now we need to convert Load value from Hexadecimal to Binary numbers. Therefore:
+ Gi1/1: 36 (Hex) = ‭00110110‬ (Bin) -> Bits 3, 4, 6, 7 are chosen
+ Gi1/2: 84 (Hex) = ‭10000100‬ (Bin) -> Bits 1, 6 are chosen
+ Gi1/3: 16 (Hex) = ‭00010110‬ (Bin) -> Bits 4, 6, 7 are chosen

Therefore if the RBH is 3, it will choose Gi1/1. If RBH is 4, it will choose Gi1/1 and Gi1/3 interfaces. If RBH is 6 it will choose all three above interfaces. And the bit sharing ratio is 3:3:2 (from “No of bits” column) hence two links has higher probability of getting utilized as compared to the third link.

Question 2

Explanation

EtherChannel misconfiguration occurs when the channel parameters do not match on both sides of the EtherChannel, resulting in the following message:

%PM-SP-4-ERR_DISABLE: channel-misconfig error detected on Po3, putting E1/3 in err-disable state

Therefore from the output above we can see that when miconfigured, the physical (member) interface is put into err-disable state.

But this question asks above “the status of port-channel” (not the physical member interface) so answer “Disabled” is a better choice.

Question 3

Explanation

All interfaces in an EtherChannel must be configured identically to form an EtherChannel. Specific settings that must be identical include:
+ Speed settings
+ Duplex settings
+ STP settings
+ VLAN membership (for access ports)
+ Native VLAN (for trunk ports)
+ Allowed VLANs (for trunk ports)
+ Trunking Encapsulation (ISL or 802.1Q, for trunk ports)

Question 4

Explanation

LACP is the IEEE Standard (IEEE 802.3ad) and is the most common dynamic Etherchannel protocol, whereas PAgP is a Cisco proprietary protocol and works only between supported vendors and Cisco devices.

Question 5

Explanation

There are two PAgP modes:

Auto	Responds to PAgP messages but does not aggressively negotiate a PAgP EtherChannel. A channel is formed only if the port on the other end is set to Desirable. This is the default mode.
Desirable	Port actively negotiates channeling status with the interface on the other end of the link. A channel is formed if the other side is Auto or Desirable.

The table below lists if an EtherChannel will be formed or not for PAgP:

PAgP	Desirable	Auto
Desirable	Yes	Yes
Auto	Yes	No

Question 6

Explanation

All interfaces in an EtherChannel must be configured identically to form an EtherChannel. Specific settings that must be identical include:
+ Speed settings
+ Duplex settings
+ STP settings
+ VLAN membership (for access ports)
+ Native VLAN (for trunk ports)
+ Allowed VLANs (for trunk ports)
+ Trunking Encapsulation (ISL or 802.1Q, for trunk ports)

In the output of the “show interface fa0/1” commands we see the speed of interface Fa0/1 of SW1 is “100Mb/s” while that of SW2 is “10Mb/s” so the speed is mismatched here -> an Etherchannel will not be formed.