Link Aggregation Protocols
For achievement reasons, it is sometimes appropriate to bind several alongside links into a single
aggregated bundle. The absorbed is to accept a articulation with added bandwidth. Figure 11-5 shows
such a bundling area two links are acclimated amid about-face A and about-face B. If the links were
1 Gbps links, the aggregated bandwidth would be 2 Gbps. In Cisco switches, this
mechanism is alleged EtherChannel.
Figure 11-5 Aggregating Multiple Links
The EtherChannel (aggregated link) behaves like a articulation per itself. Spanning Tree Protocol
(STP) runs on the aggregated articulation and not on the concrete ports themselves. The aggregated
link additionally has its own MAC abode (which is about identical to one of the concrete ports).
The switches use specific load-balancing mechanisms to advance the cartage amount amid all
physical ports.
Two protocols abide to accomplish such an aggregation:
• Anchorage Aggregation Agreement (PAgP). Cisco proprietary protocol
• Articulation Aggregation Control Agreement (LACP). Standardized by the IEEE 802.3ad6
Figure 11-6 shows the packet anatomy for Cisco PAgP. The packets are beatific to the Cisco
Ethernet multicast abode of 0100.0CCC.CCCC (the aforementioned as CDP and VTP) with SNAP
encapsulation with the agreement identifier 01-04. The packet contains advice about the
local anchorage and the accomplice anchorage (MAC address, anchorage identifier—Cisco accessories use the SNMP
ifindex capabilities). Additional advice about arrangement name and anchorage name are added.
There is neither an affidavit apparatus or an candor one.
Switch A
Switch B
Aggregated Link
Port 1
Port 2 Anchorage 1
Port 2
Port 4
Port 5
Port 3
Port 5
Si
Si
Link Aggregation Protocols 175
Figure 11-6 Content of PAgP Packet
Figure 11-7 shows the IEEE 802.3ad LACP agreement abstracts assemblage (PDU). LACP is allotment of the
IEEE apathetic protocols—that is, protocols with a low throughput. The packets are beatific to the
Ethernet multicast abode 0180.C200.0002 application the Ethertype of 88-09. It is alone a
series of TLV-encoded fields about the amateur (the bounded switch) and the partner. Just like
PAgP, no aegis apparatus is congenital into LACP.
Figure 11-7 Articulation Aggregation Control PDU Format
Field Content
Header Version and Flags
Local Device
Partner Device
TLV
Device ID
Learn Capability
Hot Standby Priority
Port Ifindex
Group Capability
Group Ifindex
Count
Device Name
Port Name
Reserved
Device ID
Learn Capability
Hot Standby Priority
Port Ifindex
Group Capability
Group Ifindex
32 Bits
Subtype= Version= Amateur Advice TLV ...
... Accomplice Advice TLV ...
... Collector Advice TLV ...
Terminator TLV
LACP 0x01 0x01
176 Chapter 11: Advice Leaks with Cisco Ancillary Protocols
Because there is little aberration amid PAgP and LACP from a aegis perspective, the
next sections call the accident assay and the accident acknowledgment for both protocols. Both
protocols are about enabled by absence on all block ports. Chapter 4, “Are VLANs
Safe?,” describes how an antagonist ability accredit trunking on a anchorage with the advice of Dynamic
Trunking Agreement (DTP).