Friday, February 13, 2015

Connecting a switch to GNS3

Connecting a switch to GNS3
MAC Table overflow

Working setup:

1 x 3560 L3 Switch
Arch Linux x86_64
GNS 3 v. 1.2.3 w/c7200-adventerprisek9-mz.151-4.M4.image

You need to set the options in dynamips executable to allow normal users alterations to network interfaces

sudo setcap cap_net_raw,cap_net_admin+eip /usr/bin/dynamips

Otherwise you may receive "206-unable to create Linux raw or generic Ethernet" or something similar.
More info here

- plug cable into the switch
- make sure you can reach the switch I have:
SW-A# sh int vlan 1
  Internet address is
- on the NIC you can issue: "ip addr add dev enp4s2" where enp4s2 is whatever NIC you have
- enable ssh and configure a user
mig@jenkins ~ % ssh x@


On R1:
- set the interface to "duplex full" otherwise you'll receive this on SW-A
: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/1 (not half duplex), with R1 FastEthernet0/0 (half duplex).

SW-A#sh cdp nei
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R1               Fas 0/1           105            R       7206VXR   Fas 0/0


Prepare interface on switch:

interface FastEthernet0/1
 switchport trunk encapsulation dot1q
 switchport mode trunk

DTP packets start arriving

Interface is now trunk so it does not show on this output

SW-A#sh vlan br

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------

1    default                          active    Fa0/2, Fa0/3, Fa0/4, Fa0/5

To see trunk interfaces:

SW-A#sh int trunk 

Port        Mode             Encapsulation  Status        Native vlan
Fa0/1       on               802.1q         trunking      1

Configure a SVI

SW-A(config)#vlan 100
00:51:25: %LINK-5-CHANGED: Interface Vlan100, changed state to administratively down
00:51:26: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan100, changed state to down
SW-A(config)#int vlan 100       
00:51:33: %LINK-3-UPDOWN: Interface Vlan100, changed state to up
00:51:34: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan100, changed state to up
SW-A(config-if)#ip addr 

On the PC side:
- Create the VLAN device
ip link add link enp4s2 name enp4s2.100 type vlan id 100
- add an ip 
ip addr add brd dev enp4s2.100
- bring it up 
ip link set dev enp4s2.100 up

You can check the status by:

 ~ % ip -d link show enp4s2    
3: enp4s2: mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 00:22:15:63:cf:0d brd ff:ff:ff:ff:ff:ff promiscuity 1 

 ~ % ip -d link show enp4s2.100
5: enp4s2.100@enp4s2: mtu 1500 qdisc noqueue state LOWERLAYERDOWN mode DEFAULT group default 
    link/ether 00:22:15:63:cf:0d brd ff:ff:ff:ff:ff:ff promiscuity 0 

    vlan protocol 802.1Q id 100  

A virtual network device called enp4s2.100 would be created, bound to VLAN ID 100 of the physical interface enp4s2
  • Inbound 802.1Q-encapsulated frames arriving on enp4s2 with a VLAN ID of 100 are detagged, then re-presented to the network stack as inbound frames arriving on enp4s2.100
  • Outbound frames sent to enp4s2.100 are tagged with a VLAN ID of 100, then passed to enp4s2 for transmission.

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