HP vs Cisco Vlan trunking
May 19, 2014
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Whenever I talk to networking people about integrating or replacing Cisco switches with HP they always talk about how difficult it was to get them to pass vlan info. This is because the terminology is totally different between the two.
In the Cisco world a “Trunk” port will pass all vlan tags by default. What does this mean? It means that the port can accept/pass IP packets with different Vlan tags. Basically the trunk port is a member of every vlan, and therefore will pass traffic from every vlan its a member of. In HP the word Trunk means link aggregation example is LACP. Basically taking 2 or more links and combining them. HP uses the phrase “tagged”.
So what is a VLAN tag?
The “Vlan ID” is a 12-bit value in the IP header of the Ethernet frame that specifies which vlan that frame belongs, and thus virtually separates traffic.
So to configure a port as a trunk in Cisco the command is “switchport mode trunk”. This will allow the port to access every vlan , and understand/pass the frames with any tag.
In HP world you would “tag” multiple vlans on a single port. The “tag” command basically means that the port can pass/understand frames in those “tagged” vlans on that port. So lets say we have 2 vlans, and we want a switch to pass frames on both vlans to another port, then we would configure the port as tagged in both Vlans. Commands would be:
tagged vlan 120,140
Now that port will accept tags from both vlans and pass them along.
Access ports or Untagged
An access port in Cisco is a port who’s traffic will be tagged by the switch for the vlan the port is set to access. For example if the command on the port is “Switchport access vlan 120” then all untagged (default) traffic will be tagged for vlan 120 by the switch. This is done because the host itself is not tagging frames, but sending everything untagged.
In HP this access port would be “untagged”. This means that the traffic leaving the port will come in untagged, but will be tagged with vlan X by the switch. So the command – “untagged vlan 120” means all traffic will be in vlan 120.
int gig 1/0/24
switchport mode trunk
Access ports (this port is in this vlan)
int gig 1/0/24
switchport mode access
switchport access vlan 120