Thursday, August 2, 2012

Alternatives to VXLAN


NVGRE (Network Virtualization Generic Routing Encapsulation) achieves the same functional goal as VXLAN on the network, however the encapsulation of VLAN traffic is done on the virtualised server instead.   In essence the network VLAN limitations are overcome by application software.

The server builds a gre tunnel and the virtual subnet id is included in the GRE header.  Each virtual machine is mapped to a host PA (physical address).  End to end communication is completed over existing networks that are essentially unaware of the encapsulated payloads and server vlan mappings.

IP Rewrite

An alternative approach to GRE tunnelling is ‘IP rewrite’. This solution requires each virtual server has its own IP address. This ip address is rewritten to a physical ip address for transport across the network.  This mapping is reversed on reaching the destination.  

OTV or Overlay Transport Virtualisation is Cisco's proprietary solution to extending VLANs over IP networks. It encapsulates L2 within IP allowing VLANs to be tunneled over a routed IP network. It is currently enabled on the NEXUS OS.

Transparent Interconnection of Lots Of Links or TRILL, enables extension of L2 domains, through use of RBridges or (Routing Bridges).  TRILL RBridges communicate with a link state protocol, however this  executes within the L2 domain i.e. no ip addresses are required.  RBridges have knowledge of the topology consisting of all the RBridges and all the links between RBridges.

When an RBridge receives an Ethernet frame from an end node it encapsulates the frame in a TRILL header, addressing the packet to the RBridge with the destination MAC attached to it.  The destination RBridge performs the decapsulation before sending the packet onwards.

TRILL like the other technoloigies described enables a large L2 cloud to be created with a common subnet. Therefore hosts within the L2 cloud do NOT need  to change their ip address if they relocate. TRILL is an ietf standard.

to be continued .....

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