Blog posts

Layer 3 etherchannels

Submitted by rayc on Sat, 11/13/2021 - 06:44

In my last few articles, I've talked about etherchannels and what they are and how they work as far as being a layer 2 etherchannels goes. But what if we want to load balance across layer 3 interfaces and route traffic? Well, Cisco routers and switches (MLS switches at least) support both Layer 2 and Layer 3 etherchannels. 


Submitted by rayc on Mon, 10/25/2021 - 10:51

PAgP or Port Aggregation Protocol, is a Cisco proprietary link aggregation protocol. This protocol works in a similar way to LACP in that it uses PAgP messages to negotiate and establish the etherchannel bundle. Like LACP, PAgP has two modes for configuration:


Submitted by rayc on Mon, 10/25/2021 - 10:51

In my previous article, I talked about the basic concept of etherchannels and how they work to provide additional redundancy and bandwidth in our networks. I also talked about how to configure an always on etherchanel. In this article, I will talk about how to dynamically establish an etherchannel using LACP (Link Aggregation Control Protocol)

VTP v3

Submitted by rayc on Mon, 10/25/2021 - 09:52

The main functions of VTP V3 are exactly the same as with VTPv1 and V2, however VTPv3 has some additional features and configuration requirements. 

  • VTPv3 is backward compatible with switches running V1 and V2. 
  • Allows on the VTP Primary Server to make VLAN changes.
  • Supports all 4094 VLANs. 
  • Supports propagating MST configuration changes
  • Provides feature specific primary server configuration.
  • Allows the use of PVLANs (Private VLANs)
  • Per Port VTP configuration is supported


VTP v1 and v2

Submitted by rayc on Mon, 10/25/2021 - 09:52

VTP or Virtual Trunking Protocol, is a Cisco proprietary protocol that is used to share VLAN information between switches. In a small topology such as our little 3 switch topology or it a small office network where you might have 3 or 4 switches, configuring VLANs manually on each switch might not be such a big deal. But think about a large scale campus network or Datacenter network where you have hundreds or thousands of VLANs and having to manually configure those VLANs or make changes on each device. This is why Cisco invented VTP.

STP Compatability

Submitted by rayc on Mon, 10/25/2021 - 09:38

Imagine if you will, your boss comes to you and says "Great news, we're getting all new equipment so now we can replace our old Catalyst 3500XL series core switch and 2900XL series access switches from the 1990's to some brand new Catalyst 9300 series switches" You might think all your Christmas's have come at once if you walk into a job using equipment that old and get some new stuff, either that or you could be clinically insane for taking a job like that?

Multiple Spanning Tree (MST)

Submitted by rayc on Mon, 10/25/2021 - 09:16

MSTP or 802.1s, is an industry standard version of STP that incorporates RSTP timing with the ability to load balance VLANs across instances of spanning tree. If you recall, 802.1D runs a single instance of spanning tree for all VLANs within a network called the Common Spanning Tree or CST. MST takes this concept and expands on it by allowing you to assign VLANs to an Instance of spanning tree and allowing multiple instances to be configured, hence the name Multiple Spanning Tree.  This also means that you can have a root switch for each instance of MST as well.

Additional STP Features

Submitted by rayc on Mon, 10/25/2021 - 09:15

Spanning tree protects our layer 2 network from broadcast storms and helps to create a loop free layer 2 topology. STP takes advantage of additional features that have been added over the years help prevent unnecessary changes to the network or to help prevent unintentional or sometimes malicious activities from occurring on our networks. Some of these features include

  • Portfast
  • Rootguard
  • BPDUGuard
  • loopguard
  • BPDUFilter
  • UDLD (Unidirectional Link Detection)
  • Uplinkfast
  • Backbonefast