## EtherChannel

### Usages
Bundles several links between two devices and Use them simultaneously as a single (logical) link.

### Restrictions
- **Interface types** cannot be mixed.
- Ports must be **compatibly-configured** on both devices.
- e.g. both sides configured as trunks with the same native VLAN
- Up to **eight links** bundled in an EtherChannel.
- Up to **six EhterChannels** for a Cisco IOS switches.
- One-to-one connection
- two switches or a switch and an EtherChannel-enabled server
- Normally Layer 2 ports only
- But, layer 3 EtherChannel possible for multilayer switches

### Portocols

1. Port Aggregation Protocol (PAgP)
- Cisco-proprietary protocol
2. Link Aggregation Control Protocol (LACP)
- IEEE Standard


## First Hop Redundancy Protocols (FHRPs)

### Usage
- Avoid failure of default gateway.

### Structure

- Two or more routers gives an illusion of a Virtual Router.
- Virtual router serves as default gateway.
- Forwarding Router assumes IP and MAC f Virtual router.

### Protocols

1. Hot Standby Router Protocol (HSRP)
- Cisco's initial, proprietary standard
2. Virtual Router Redundancy Protocol (VRRP)
- An open standard protocol
3. Gateway Load Balancing Protocol (GLBP)
- A more recent proprietary standard from Cisco
- Permits **load balancing** as well as **redundancy**

### Hot Standby Router Protocol (HSRP)

#### Structure
- A set of routers works in concert to present the illusion of a single virtual router
- One Active router
- One Standby router
- One or more Listening router

#### Behaviors
- Only Active Router forwards packets sent to Virtual Router.
- If Active Router fails, Standby router assumes the role of Active Router.
- MAC address is generated automatically by HSRP.
- Hello messages exchanged every **3** seconeds
- Hold Down Timer
Elapsed time Standby Router will wait for Hello message (before assuming Active Router is down) ‒ Default value is **10 seconds** (roughly 3 times of elapsed time of Hello message.)
- Priority: router having higher priority will become the active router ‒ Default 100, but changeable
- (Standby) Preempt
- a state allows a Standby Router (whose priority has become higher) to take overthe role of active router
- A Standby Router not at Standby Preempt state will not preempt Active Router even if the Standby Router has a higher priority.
- Often combined with Standby Track
- Standby Track: Active Router tracks an interface and decreases priority when the interface goes down (make priority lower than the Standby Router priority)