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  1. ### Introduction
  2.  
  3. Link aggregation is the ability to create one logical link using multiple physical links between two devices. This allows load sharing among the physical links, rather than having STP block one or more of the links. EtherChannel is a form of link aggregation used in switched networks.
  4.  
  5. #### Advantages of EtherChannel
  6.  
  7. EtherChannel technology has many advantages:
  8.  
  9. - Most configuration tasks can be done on the EtherChannel interface instead of on each individual port, ensuring configuration consistency throughout the links.
  10. - EtherChannel relies on existing switch ports. There is no need to upgrade the link to a faster and more expensive connection to have more bandwidth.
  11. - Load balancing takes place between links that are part of the same EtherChannel. Depending on the hardware platform, one or more load-balancing methods can be implemented. These methods include source MAC to destination MAC load balancing, or source IP to destination IP load balancing, across the physical links.
  12. - EtherChannel creates an aggregation that is seen as one logical link. When several EtherChannel bundles exist between two switches, STP may block one of the bundles to prevent switching loops. When STP blocks one of the redundant links, it blocks the entire EtherChannel. This blocks all the ports belonging to that EtherChannel link. Where there is only one EtherChannel link, all physical links in the EtherChannel are active because STP sees only one (logical) link.
  13. - EtherChannel provides redundancy because the overall link is seen as one logical connection. Additionally, the loss of one physical link within the channel does not create a change in the topology; therefore a spanning tree recalculation is not required. Assuming at least one physical link is present; the EtherChannel remains functional, even if its overall throughput decreases because of a lost link within the EtherChannel.
  14.  
  15. #### Implementation Restrictions
  16.  
  17. Interface types cannot be mixed; for example, Fast Ethernet and Gigabit Ethernet cannot be mixed within a single EtherChannel.
  18.  
  19. Currently each EtherChannel can consist of up to **eight** compatibly-configured Ethernet ports. (up to 800 Mb/s (Fast Etherchannel) or 8Gb/s (Gigabit Etherchannel))
  20.  
  21. An EtherChannel link can be created between two switches or an EtherChannel link can be created between an EtherChannel-enabled server and a switch.
  22.  
  23. ### Port Aggregation Protocol(PAgP)
  24.  
  25. EtherChannels can be formed through negotiation using one of two protocols, **PAgP** or **LACP**. These protocols allow ports with similar characteristics to form a channel through dynamic negotiation with adjoining switches.
  26.  
  27. **PAgP** (pronounced “Pag – P”) is a Cisco-proprietary protocol that aids in the automatic creation of EtherChannel links. When an EtherChannel link is configured using PAgP, PAgP packets are sent between EtherChannel-capable ports to negotiate the forming of a channel. When PAgP identifies matched Ethernet links, it groups the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single port.
  28.  
  29. Modes for PAgP:
  30.  
  31. - **On** - This mode forces the interface to channel without PAgP. Interfaces configured in the on mode do not exchange PAgP packets.
  32. - **PAgP desirable** - This PAgP mode places an interface in an active negotiating state in which the interface initiates negotiations with other interfaces by sending PAgP packets.
  33. - **PAgP auto** - This PAgP mode places an interface in a passive negotiating state in which the interface responds to the PAgP packets that it receives, but does not initiate PAgP negotiation.
  34.  
  35. | S1 | S2 | Channel Establishment |
  36. | -- | -- | -- |
  37. | On | On | Yes |
  38. | Auto/Desirable | Desirable | Yes |
  39. | On/Auto/Desirable | Not Configured | No |
  40. | On | Desirable | No |
  41. | Auto/On | Auto | No |
  42.  
  43. ### Link Aggregation Control Protocol
  44.  
  45. LACP is part of an IEEE specification (802.3ad) that allows several physical ports to be bundled to form a single logical channel. LACP allows a switch to negotiate an automatic bundle by sending LACP packets to the peer. It performs a function similar to PAgP with Cisco EtherChannel. Because LACP is an IEEE standard, it can be used to facilitate EtherChannels in multivendor environments. On Cisco devices, both protocols are supported.
  46.  
  47. LACP was originally defined as IEEE 802.3ad. However, LACP is now defined in the newer IEEE 802.1AX standard for local and metropolitan area networks.
  48.  
  49. Modes for LACP:
  50.  
  51. - **On** - This mode forces the interface to channel without LACP. Interfaces configured in the on mode do not exchange LACP packets.
  52. - **LACP active** - This LACP mode places a port in an active negotiating state. In this state, the port initiates negotiations with other ports by sending LACP packets.
  53. - **LACP passive** - This LACP mode places a port in a passive negotiating state. In this state, the port responds to the LACP packets that it receives, but does not initiate LACP packet negotiation.
  54.  
  55. LACP allows for **eight active links**, and **also eight standby links** (summary 16 ports). A standby link will become active should one of the current active links fail.
  56.  
  57. | S1 | S2 | Channel Establishment |
  58. | -- | -- | -- |
  59. | On | On | Yes |
  60. | Active/Passive | Active | Yes |
  61. | On/Active/Passive | Not Configured | No |
  62. | On | Active | No |
  63. | Passive/On | Passive | No |
  64.  
  65. ### Configuring EtherChannel
  66.  
  67. The following guidelines and restrictions are useful for configuring EtherChannel:
  68.  
  69. - **EtherChannel support** - All Ethernet interfaces on all modules must support EtherChannel with no requirement that interfaces be physically contiguous, or on the same module.
  70. - **Speed and duplex** - Configure all interfaces in an EtherChannel to operate at the same speed and in the same duplex mode.
  71. - **VLAN match** - All interfaces in the EtherChannel bundle must be assigned to the same VLAN, or be configured as a trunk (also shown in the figure).
  72. - **Range of VLANs** - 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 EtherChannel, even when set to auto or desirable mode.
  73.  
  74. ```
  75. interface range f0/1-24, g0/1-2
  76. sh
  77.  
  78. S1(config)# int range fa0/1-8
  79. S1(config-if-range)# channel-group 1 mode active
  80.  
  81. S1(config)# int port-channel 1
  82.  
  83. S1(config)# int range fa0/1-8
  84. S1(config-if-range)# no sh
  85.  
  86. S1# show int br | in channel
  87. S1# show interfaces port-channel 1
  88. S1# show etherchannel summary
  89. S1# show etherchannel porthannel
  90. S1# show intefaces f0/1 etherchannel
  91. ```
  92.  
  93. ### Troubleshooting EtherChannel
  94.  
  95. All interfaces within an EtherChannel must have the same configuration of speed and duplex mode, native and allowed VLANs on trunks, and access VLAN on access ports:
  96.  
  97. - Assign all ports in the EtherChannel to the same VLAN, or configure them as trunks. Ports with different native VLANs cannot form an EtherChannel.
  98. - When configuring a trunk on an EtherChannel, verify the trunking mode on the EtherChannel. It is not recommended that you configure trunking mode on individual ports that make up the EtherChannel. But if it is done, verify that the trunking configuration is the same on all interfaces.
  99. - An EtherChannel supports the same allowed range of VLANs on all the ports. If the allowed range of VLANs is not the same, the ports do not form an EtherChannel even when PAgP is set to the auto or desirable mode.
  100. - The dynamic negotiation options for PAgP and LACP must be compatibly configured on both ends of the EtherChannel.
  101.  
  102.  
  103. ## First Hop Redundancy Protocol
  104.  
  105. One way to prevent a single point of failure at the default gateway, is to implement a virtual router. To implement this type of router redundancy, multiple routers are configured to work together to present the illusion of a single router to the hosts on the LAN. By sharing an IP address and a MAC address, two or more routers can act as a single virtual router.
  106.  
  107. The ability of a network to dynamically recover from the failure of a device acting as a default gateway is known as first-hop redundancy.
  108.  
  109. The following list defines the options available for First Hop Redundancy Protocols (FHRPs).
  110.  
  111. - **Hot Standby Router Protocol (HSRP)** - A Cisco-proprietary FHRP designed to allow for transparent failover of a first-hop IPv4 device. HSRP provides high network availability by providing first-hop routing redundancy for IPv4 hosts on networks configured with an IPv4 default gateway address. HSRP is used in a group of routers for selecting an active device and a standby device. In a group of device interfaces, the active device is the device that is used for routing packets; the standby device is the device that takes over when the active device fails, or when pre-set conditions are met. The function of the HSRP standby router is to monitor the operational status of the HSRP group and to quickly assume packet-forwarding responsibility if the active router fails.
  112. - **HSRP for IPv6** - Cisco-proprietary FHRP providing the same functionality of HSRP, but in an IPv6 environment. An HSRP IPv6 group has a virtual MAC address derived from the HSRP group number and a virtual IPv6 link-local address derived from the HSRP virtual MAC address. Periodic router advertisements (RAs) are sent for the HSRP virtual IPv6 link-local address when the HSRP group is active. When the group becomes inactive these RAs stop after a final RA is sent.
  113. - **Virtual Router Redundancy Protocol version 2 (VRRPv2)** - A non-proprietary election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on an IPv4 LAN. This allows several routers on a multiaccess link to use the same virtual IPv4 address. A VRRP router is configured to run the VRRP protocol in conjunction with one or more other routers attached to a LAN. In a VRRP configuration, one router is elected as the virtual router master, with the other routers acting as backups, in case the virtual router master fails.
  114. - **VRRPv3** - Provides the capability to support IPv4 and IPv6 addresses. VRRPv3 works in multi-vendor environments and is more scalable than VRRPv2.
  115. - **Gateway Load Balancing Protocol (GLBP)** - Cisco-proprietary FHRP that protects data traffic from a failed router or circuit, like HSRP and VRRP, while also allowing load balancing (also called load sharing) between a group of redundant routers.
  116. - **GLBP for IPv6** - Cisco-proprietary FHRP providing the same functionality of GLBP, but in an IPv6 environment. GLBP for IPv6 provides automatic router backup for IPv6 hosts configured with a single default gateway on a LAN. Multiple first-hop routers on the LAN combine to offer a single virtual first-hop IPv6 router while sharing the IPv6 packet forwarding load.
  117. - **ICMP Router Discovery Protocol (IRDP)** - Specified in RFC 1256, is a legacy FHRP solution. IRDP allows IPv4 hosts to locate routers that provide IPv4 connectivity to other (nonlocal) IP networks.
  118.  
  119. #### HSRP Overview
  120.  
  121. Hot Standby Router Protocol (HSRP) was designed by Cisco to allow for gateway redundancy without any additional configuration on end devices. Routers configured with HSRP work together to present themselves as a single virtual default gateway (router) to end devices, as shown in the figure. One of the routers is selected by HSRP to be the active router. The active router will act as the default gateway for end devices. The other router will become the standby router. If the active router fails, the standby router will automatically assume the role of the active router. It will assume the role of default gateway for end devices. This does not require any configuration changes on the end devices.
  122.  
  123. Hosts are configured with a single default gateway address that is recognizable by both the active and standby routers. The default gateway address is a virtual IPv4 address along with a virtual MAC address that is shared amongst both HSRP routers. End devices use this virtual IPv4 address as their default gateway address. The HSRP virtual IPv4 address is configured by the network administrator. The virtual MAC address is created automatically. Regardless of which physical router is used, the virtual IPv4 and MAC addresses provide consistent default gateway addressing for the end devices.
  124.  
  125. Only the active router will receive and forward traffic sent to the default gateway. If the active router fails, or if communication to the active router fails, the standby router will assume the role of the active router.
  126.  
  127. #### HSRP Versions
  128.  
  129.  
  130.  
  131. The default HSRP version for Cisco IOS 15 is version 1. HSRP version 2 provides the following enhancements:
  132.  
  133. - **HSRPv2** expands the number of supported groups. HSRP version 1 supports group numbers from 0 to 255. HSRP version 2 supports group numbers from 0 to 4095.
  134. - **HSRPv1** uses the multicast address of 224.0.0.2. HSRP version 2 uses the IPv4 multicast address 224.0.0.102 or the IPv6 multicast address FF02::66 to send hello packets.
  135. - **HSRPv1** uses the virtual MAC address range 0000.0C07.AC00 to 0000.0C07.ACFF, where the last two hexadecimal digits indicate the HSRP group number. HSRPv2 uses the MAC address range from 0000.0C9F.F000 to 0000.0C9F.FFFF for IPv4 and 0005.73A0.0000 through 0005.73A0.0FFF for IPv6 addresses. For both IPv4 and IPv6, the last three hexadecimal digits in the MAC address indicate the HSRP group number.
  136. - **HSRPv2** adds support for MD5 authentication, which is beyond the scope of this course.
  137.  
  138. **Note**: Group numbers are used for more advanced HSRP configurations that are beyond the scope of this course. For our purposes, we will use group number 1.
  139.  
  140. The role of the **active and standby** routers is determined during the HSRP election process. By default, the router with the numerically **highest IPv4** address **is elected as the active router**. However, it is always better to control how your network will operate under normal conditions rather than leaving it to chance.
  141.  
  142. The router with the **highest HSRP priority** will become the **active router**. **By default**, the HSRP priority is **100**. If the priorities are equal, the router with the numerically highest IPv4 address is elected as the active router. The range of the HSRP priority is 0 to 255.
  143.  
  144. The active and standby HSRP routers send **hello packets** to the HSRP group multicast address **every 3 seconds**, **by default**. The standby router will become active **if it does not receive a hello message** from the active router **after 10 seconds**. You can lower these timer settings to speed up the failover or preemption. However, to avoid increased CPU usage and unnecessary standby state changes, **do not set the hello timer below 1 second or the hold timer below 4 seconds**.
  145.  
  146. #### HSRP States
  147.  
  148. - **Initial** - This state is entered through a configuration change or when an interface first becomes available.
  149. - **Learn** - The router has not determined the virtual IP address and has not yet seen a hello massage from the active router. In this state, the router waits to hear from the active router.
  150. - **Listen** - The router knows the virtual IP address, but the router is neither the active router nor the standby router. It listens for hello messages from those routers.
  151. - **Speak** - The router sends periodic hello messages and actively participates in the election of the active and/or standby router.
  152. - **Standby** - The router is a candidate to become the next active router and sends periodc hello massages.
  153. - **Active** - The router forwards packets that are sent to te group virtual MAC address. The router sends periodic hello messages.
  154.  
  155. ### HSRP Configuration
  156.  
  157. ```
  158. ! configure version
  159. R1(config-if)# standby version 2
  160.  
  161. ! virtual ip
  162. R1(config-if)# standby 1 192.168.100.1
  163.  
  164. ! priority
  165. R1(config-if)# standby 1 priority 150
  166.  
  167. ! preempt
  168. R1(config-if)# standby 1 preempt
  169.  
  170. R1# show standby
  171. R1# show standby brief
  172.  
  173. R1(config-if)# standby 1 name ccna
  174.  
  175.  
  176. R1# debug standby packets
  177. R1# debug standby terse
  178. ```
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