EtherChannel concept

EtherChannel technology
scaling link bandwidth by aggregating, or bundling, parallel links
2-8 links of Ethernet are bundled as one logical link Etherchannel,respectively.
full duplex fast ethernet,將8 port合成etherchannel,max throughput可達1600m
the load isn’t always distributed equally among the individual links

EtherChannel also provides redundancy with several bundled physical links
If one of the links within the bundle fails, traffic sent through that link automatically is moved to an adjacent link. Failover occurs in less than a few milliseconds and is transparent to the end

A switch also provides some inherent protection against bridging loops with EtherChannels
When ports are bundled into an EtherChannel, no inbound (received) broadcasts and multicasts are sent back out over any of the remaining ports in the channel. Outbound broadcast and multicast frames are load-balanced like any other: The broadcast or multicast address becomes part of the hashing calculation to choose an outbound channel link


methods distributes traffic使用hashing algorithm
1.The hash algorithm computes a binary pattern that selects a link number in the bundle to carry each frame
2.binary pattern一般會由address或port根據hashing產生
ex:The algorithm can use source IP address, destination IP address, or a combination of source and destination IP addresses, source and destination MAC addresses, or TCP/UDP port numbers

If only one address or port number is hashed:
a switch forwards each frame by using one or more low-order bits of the hash value as an index into the bundled links.
an EtherChannel consisting of two links bundled together requires a 1-bit index.
If the index is 0, link 0 is selected; if the index is 1, link 1 is used.
A two-link bundle uses a hash of the last 1 bits
A four-link bundle uses a hash of the last 2 bits
an eight-link bundle uses a hash of the last 3 bits
IP —>
dst ip第四位數46, bits is 00101110
a two-link EtherChannel,uses a hash of the last 1 bits(=0),causing Link 0 in the bundle to be used
a for-link EtherChannel,uses a hash of the last 2 bits(=10),causing Link 2 in the bundle to be used
If two addresses or port numbers are hashed:
a switch performs an XOR operation on one or more low-order bits of the addresses or TCP/UDP port numbers as an index into the bundled links.
IP —>
src ip第四位數1, bits is 00000001
dst ip第四位數46, bits is 00101110
a two-link EtherChannel, a 1-bit XOR is performed on the rightmost address bit :
1 XOR 0 = 1, causing Link 1 in the bundle to be used
A four-link EtherChannel produces a 2-bit XOR:
01 XOR 10 = 11,causing Link 3 in the bundle to be used
an eight-link EtherChannel requires a 3-bit XOR:
001 XOR 110 = 111, where Link 7 in the bundle is selected


EtherChannel Negotiation Protocols
dynamic link configuration
To provide automatic EtherChannel configuration and negotiation between switches
PAgP(Port Aggregation Protocol),Cisco-proprietary
LACP(Aggregation Control Protocol), standards based,IEEE 802.3ad

PAgP and LACP mode
on mode
 All ports channeling
 unconditionally bundles the links.
 no Negotiation Packets Sent:does not send or receive PAgP or LACP packets.
 Therefore,both ends should be set to on mode before the channel can form
active mode:desirable(PAgP),active(LACP)
 a switch actively asks a far end switch to negotiate an EtherChannel
 using Negotiation Packets Sent
 Therefore, the other end must be set to either desirable or auto mode
passive mode:auto(PAgP),passive(LACP)
 passively listen , Waits to channel until asked(far end asks for participation)
  switch negotiates an EtherChannel only if the far end initiates it
 using Negotiation Packets Sent
 Therefore, two switches in the auto or passive mode will not form an EtherChannel
on mode和passive mode無法形成etherchannel

silent submode(PAgp default)
PAgP packets Not necessarily be received if a channel will form
allows ports to be added to an EtherChannel even if the other end of the link is silent and never transmits PAgP packets
同時擁有desirable和auto modes特點
This allows a switch to form an EtherChannel with a device (such as a file server or a network analyzer) that doesn’t participate in PAgP
The silent submode listens for any PAgP packets from the far end, looking to negotiate a channel
If none is received, silent submode assumes that a channel should be built anyway,so no more PAgP packets are expected from the far end
兩switch同時使用default PAgP auto mode and silent submode.
the total delay before data can pass over the link is actually approximately 45 or 50 seconds
15sec:PAgP silent mode waiting to hear inbound PAgP messages
 Each interface waits to be asked to form a channel
 each interface waits and listens before accepting silent channel partners
30sec:the STP moving through the listening and learning stages.
Even if the two interfaces are using PAgP auto mode, the link will still eventually come up,
although not as a channel
err-disable:EtherChannel misconfiguration
假設Switch A設定FEC為on mode(該mode不會發送PAgP packet和相連的Switch B negotiation FEC, 該mode假設Switch B已設定好FEC)
若Swtich B未設定FEC,且Switch B狀態超過1分鍾,則Switch A的STP會判斷出現loop,因此會出現err-disable
解決辦法就是配置mode desirable non-silent

non-silent submode
PAgP packets must be received before a channel will form.
This requires each port to receive PAgP packets before adding them to a channel
If you expect a PAgP-capable switch to be on the far end, you should add the non-silent keyword to the desirable or auto mode
If PAgP isn’t heard on an active port, the port remains in the up state, but PAgP reports to the STP that the port is down

PAgP packets are exchanged between switches over EtherChannel-capable ports
1. Neighbors are identified and port group capabilities are learned and compared with local switch capabilities
2. Ports that have the same neighbor device ID and port group capability are bundled together as a bidirectional , point-to-point EtherChannel link.
3. PAgP 封包每30sec傳送一次
4. Dest Multicast MAC為0010:0CCC:CCCC
5. Ether Type:0X0104

PAgP forms an EtherChannel only on ports that are configured for either identical static VLANs or trunking

PAgP dynamically modifies parameters of the EtherChannel
if the configured VLAN, speed, or duplex mode of a port in an established bundle is changed, PAgP reconfigures that parameter for all ports in the bundle.

1. LACP packets are exchanged between switches over EtherChannel-capable ports
ps:As with PAgP, neighbors are identified and port group capabilities are learned and compared with local switch capabilities
2. LACP also assigns roles to the EtherChannel’s endpoints.
可設定最多16 potential links,並根據LACP switch and port roles選出最多8 active links
The other links are placed in a standby state and will be enabled in the EtherChannel if one of the active links goes down

LACP switch and port roles
LACP system priority
The switch with the lowest system priority is allowed to make decisions about what ports actively are participating in the EtherChannel at a given time
ps: system priority=2byte priority+6byte switch MAC address
LACP port priority
Ports are selected and become active according to their port priority value
a lower port priority for any interfaces that must be active
a higher priority for interfaces that might be held in the standby state
ps:port priority=2byte priority+2byte port number