# The Switch's Domain

> Markdown source of <https://docs.calebsargeant.com/networking/cisco/switching/the-switches-domain/>.

## Understanding Loopguard

- Stops loops caused by unidirection link failures
- Example:
  - A is root bridge
  - Link between B and C is blocked (at B)
  - Link between B and C goes unidirectional
  - B releases the block, one-way loop occurs
- Loopguard should be enabled (per-port) on all switch uplinks

``` text
Switch(config-if)#spanning-tree guard loop
Switch(Config)#spanning-tree loopguard default
```

## Understanding UDLD

- Different from loopguard, but from the same goal
- Designed specifically for fiber
- Sends a "Layer 2 ping" between neighbors
- Supports normal and aggressive (preferred) mode
- Cisco recommends use both!

``` text
switch(config)#udld <enable/aggressive>
switch(config-if)#udld port <aggressive>
switch#udld reset
```

## Supervisor Redundancy Mechanisms

- Stateful Switchover (SSO)
  - Lightning quick failover between supervisor engines
  - Sync's startup, running, L2 (MAC) and L3 (FIB) tables, ACLs
- Cisco Non-Stop Forwarding (NSF)
  - Forwards traffic with CEF
  - Supports with EIGRP, OSPF, IS-IS, and BGP

## Designing the Network

- Use local VLANs
- Set up HSRP intuitively for active VLANs
- Match STP root to active HSRP gateway