Cisco Hands On Training Podcast

VTP is the VLAN trunking protocol. It's used to disseminate uniform vlan information between switches over 802.1q or ISL trunks. It can also "prune" vlans, dynamically removing unneeded VLANs from trunks. This decreases unneeded frame flooding.VTP can eliminate outages thanks to the uniform VLAN configuration. But it can also cause outages if incorrect VLAN information is uniformly distributed.We also attempt a loopguard demonstration, but it doesn't work out well. We'll have to revisit the documentation because it's obvious loopguard is not acting as I expect.

Switches can have multiple vlans. When we connect switches together we use 802.1q trunks (or older ISL trunks) to run multiple vlans over one physical link. With either trunking protocol, a tag is added to the ethernet frame with the vlan information. ISL is an older Cisco-proprietary trunking protocol. Newer switches do not even support ISL. Newer switches use the 802.1q vendor-indepentend trunking protocol. Cisco switches also speak the dynamic trunk protocol (DTP) to dynamically negotiate whether to enable a trunk.

We cover intermediate spanning tree concepts. The importance of specifying your root bridge and backup root bridge with spanning-tree priority. Using portfast to allow host ports to start forwarding without waiting for 30 seconds. Using bpduguard to disable portfast-enabled ports where someone erroneously plugs in a switch. Using errdisable timeout to automatically reenable those ports after 15 minutes. Using rootguard to prevent improper switches from becoming your spanning-tree root. The dangers of using bpdufilter to ignore and delete BPDUs. How uplinkfast can enable access switches to failover between uplinks without delay. And using backbone fast to improve responsiveness to indirect link failures by eliminating the need for the 20 second maxage timeout.

So far we've talked about how IPv4 encodes data into a packet, and how routers learn which direction to forward those IPv4 packets based on the destination IP address and the route table.
But in the end, routers and hosts need to encode the IPv4 packet onto a physical medium. Examples of physical mediums include fiber, twisted pair, coax, radio waves, lasers, and microwaves. Each encoding rate and medium requires a specification or protocol definition.
Ethernet is a family of similar encoding specifications which is dominant on the Internet today. Ethernet uses a 48-bit address known as a MAC address or hardware address. There are ethernet specifications for speeds ranging from 10 megabit to 10 gigabit and for various copper and fiber physical media.
Using ethernet requires the ability to map an IPv4 address into a MAC address. That is accomplished using the address resolution protocol (ARP).
In this episode, we introduce the ethernet frame format, ethernet address, and how ARP works.

In BGP, MED stands for Multi Exit Discriminator. It is a well-known optional attribute which allows one autonomous system to inject it's IGP route metrics into its BGP advertisements to another BGP autonomous system. This allows the second autonomous system to make intelligent routing decisions regarding which of multiple paths to take to send traffic to a particular destination in the first autonomous system.
Because different AS's use different IGP's and can calculate metrics in different ways, by default MEDs are only compared when multiple paths exist between the same two autonomous systems.
BGP MEDs are fairly late in the BGP route selection process, coming after local-preference and AS-PATH length.
In this episode we show how to inject MEDs into BGP advertisements, and how they are used to influence routing decisions.

We filter BGP routes in 4 different ways.

We put together what we learned about eBGP, iBGP, and OSPF.

An iBGP example with 1 autonomous system with 3 routers.

An eBGP example with 3 autonomous systems with 1 router each.

An introduction to BGP theory.