There are ways to perform some of the important features of the layer 2 network that managed switches provide when working with a routed (layer 3) network in GNS3. In this post, I will show how a Cisco c3725 with an NM-16ESW module can be configured to perform specific layer 2 functions such as Spanning Tree Protocol (IEEE [802.1d], but not Rapid STP [802.1w] or Multipe STP [802.1s]), VLANs, VLAN Trunking, VLAN Trunk Protocol (VTP). In all of the examples, I will be working with the following topology using the Advanced Enterprise Services version of 12.4(15)T7,
The connections are configured this way...
|S0 - FastEthernet 1/0||S1 - FastEthernet 1/0|
|S0 - FastEthernet 1/1||S1 - FastEthernet 1/1|
|S0 - FastEthernet 1/2||S2 - FastEthernet 1/0|
|S0 - FastEthernet 1/3||S2 - FastEthernet 1/1|
|S1 - FastEthernet 1/2||S2 - FastEthernet 1/2|
|S1 - FastEthernet 1/3||S2 - FastEthernet 1/3|
Let's get started with the basics of the layer 2 network.
Configuring the NM-16ESW for Switch-mode OperationUse the switchport command to make the ports switched (rather than routed) ports,
S0(config)#interface range fastethernet 1/0 - 15 S0(config-if-range)#switchport
Virtual Local Area Networks (VLANs)
VLANs allow broadcast domains to be isolated to specific ports across a number of switches in a layer 2 topology. Multiple VLANs travel across a single link using a trunking protocol such as the industry standard 802.1Q or the Cisco-proprietary Inter-Switch link (ISL). The familiar commands from the global configuration mode of the router are available to configure VLANs and VLAN Trunking Protocol (VTP) settings. In this example I configure 2 VLANS named MikesBlog (10) and Internet (20) on all of the routers. Based on many industry recommendations, I configure the switches to be in VTP transparent mode (effectively disabling VTP on the particular switch, but allowing VTP frames to be forwarded through the switch for switches that support VTP version 2).
; ;Configuration on S0, this is the same on S1 and S2 ; S0(config)#vtp mode transparent Setting device to VTP TRANSPARENT mode. S0(config)#vlan 10 S0(config-vlan)#name MikesBlog S0(config-vlan)#vlan 20 S0(config-vlan)#name InternetUnlike a switch, there is no show vlan command on the router IOS. The show vlan-switch command is used instead
S0#show vlan-switch VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Fa1/1, Fa1/2, Fa1/3, Fa1/4 Fa1/5, Fa1/6, Fa1/7, Fa1/8 Fa1/9, Fa1/10, Fa1/11, Fa1/12 Fa1/13, Fa1/14, Fa1/15 10 MikesBlog active Fa1/0 20 Internet active 1002 fddi-default act/unsup 1003 token-ring-default act/unsup 1004 fddinet-default act/unsup 1005 trnet-default act/unsup VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2 ---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------ 1 enet 100001 1500 - - - - - 1002 1003 10 enet 100010 1500 - - - - - 0 0 20 enet 100020 1500 - - - - - 0 0 1002 fddi 101002 1500 - - - - - 1 1003 1003 tr 101003 1500 1005 0 - - srb 1 1002 1004 fdnet 101004 1500 - - 1 ibm - 0 0 1005 trnet 101005 1500 - - 1 ibm - 0 0Trunking is different on the router IOS. Dynamic Trunking Protocol is not supported, so there is no switchport mode dynamic desirable or switchport mode dynamic auto command.
S0(config-if)#switchport mode ? access Set trunking mode to ACCESS unconditionally trunk Set trunking mode to TRUNK unconditionally
Spanning Tree ProtocolSpanning Tree Protocol was developed to help prevent layer 2 loops within a network that has redundant links. For the c3725 image that I used, the spanning tree mode command was not available, so the only spanning tree mode that is available is 802.1d (known in Cisco IOS as Per-VLAN Spanning Tree, PVST+).
Ultimately, GNS3 and Dynamips are good tools for building and troubleshooting router configurations, but switch configuration testing requires actual Cisco switches. It is possible to connect to a Cisco switch using a host system interface and the cloud object in GNS3. This is a similar technique to this post.
Connecting GNS3 and Hyper-V
Router on a Stick with GNS3
The Road to the CCIE