Wrap-up and resources
Key Takeaways
- 1. Backup links without loops: STP and EtherChannel: Redundancy at layer 2 makes a loop, and STP breaks it by blocking a port. Choose the root bridge yourself, run RSTP, put PortFast and BPDU Guard on device ports, and bundle parallel links into an EtherChannel so they all carry traffic instead of half being blocked.
- 2. A gateway that never dies: HSRP, VRRP and DHCP relay: Give devices a virtual gateway address held by two routers, and make the active one the same switch that is STP root for that network. Relay DHCP through the router with ip helper-address, and guard it with DHCP snooping.
- 3. IPv6 in practice: the plan, automatic addresses and dual stack: Plan IPv6 by boundaries, not by counting: always a /64 per network, and a /48 or /56 per site. Devices build their addresses from the router's advert and neighbours meet over ICMPv6, so guard the adverts with RA Guard and never block all of ICMPv6.
- 4. BGP: how the world's networks agree on the road: BGP is a policy protocol: it picks the road you decide (Local Preference for outbound, AS_PATH and MED for inbound), not by link speed. Announce only your own addresses, filter what you receive, and sign your announcements with RPKI.
Glossary
| Term | Definition | Part |
|---|---|---|
| Broadcast storm | A broadcast circling a loop and multiplying until it chokes the network. | 1 |
| Root bridge | The switch with the lowest Bridge ID, the centre of the STP tree. | 1 |
| Root port | A switch's lowest-cost port towards the root. | 1 |
| FHRP | Protocols that make the gateway a virtual address held by more than one router. | 2 |
| HSRP | Cisco's protocol: hello every 3 s, preemption off by default. | 2 |
| VRRP | The standard alternative (RFC 5798), preemption on by default. | 2 |
| Link-local | An fe80::/10 address on every interface, used by routers as the next hop. | 3 |
| SLAAC | The device builds its address from the RA prefix with no server. | 3 |
| NDP | Neighbour and router discovery over ICMPv6, ARP's replacement. | 3 |
| AS | A network with one administration, one routing policy and a unique number. | 4 |
| eBGP and iBGP | Between different systems, and inside one system. | 4 |
| Local Preference | An internal attribute; highest wins; steers the exit. | 4 |
Resources for this lesson
Grouped by type. Start with the official docs for precision, or an article or video for the explanation.
Articles2
reference5
RFC 5798: VRRP Version 3The official standard for the VRRP backup gateway protocol.rfc-editor.orgRFC 2131: DHCPThe standard for the four DHCP messages, the relay agent and giaddr.rfc-editor.orgRFC 4861: Neighbor Discovery for IPv6The standard that explains NS, NA, RS and RA.rfc-editor.orgRFC 4862: IPv6 Stateless Address AutoconfigurationHow a device builds its address with SLAAC.rfc-editor.org
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