Impact of Server Virtualization on Networking - 5

Port extension Technology

 VEPA raised some issues which are being tackled by port extension technologies. There are two standards corresponding to port extension technologies – IEEE 802.1qbh and IEEE 802.1 BR. Among these, IEEE 802.1 qbh has been withdrawn by IEEE on September 10^th, 2011 while IEEE 802.1 BR is active.

    Some years ago, Data center Networking got a new concept introduced by CISCO called – “Fabric Extenders”. Cisco used the term ‘fabric extender’ while IEEE uses terms ‘port extender’. Honestly, being marketing friendly – I like the term ‘fabric extender’. 

    

   Typically port extender technology connects Servers to Controlling switch(Edge switch) as shown follows:

Cisco’s proprietary technology used in its FEX products became the basis for 802.1Qbh, an IEEE draft that is supposed to standardize the port extender architecture.

The core ideas behind 802.1Qbh are very simple:

• After power-up, the port extender finds its controlling bridge (connected to theupstream port)
• Port extender tells the controlling bridge how many ports it has;
• The controlling bridge creates a logical interface for each port extender port and associates a tag value with it;
• Port extender tags all packets received trough its ports with tags assigned by the controlling bridge;

Here the concept of tags comes in order to segregate each logical interface.

The external network switch connects to an external port extender using logical E-channels .These logical channels appear as virtual ports in the external network switch. Because the port extender has limited functionality, the external network switch manages all the virtual ports and their associated traffic.

Port extenders either use existing proprietary Cisco technology with VN-tags or will use the upcoming E-tag from the draft IEEE 802.1 BR Port Extension specification. The E-tag is longer than the VN-tag. It has different field definitions and different field locations but serves the same purpose. 

Port extenders use the information in VN-tags or 802.1 BR E-tags to:

• Map the physical ports on the port extenders as virtual ports on the upstream switches

• Control how they forward frames to or from upstream switches

• Control how they replicate broadcast or multicast traffic

Here is a pic depicting both CISCO VN-Tag and E-Tag(802.1BR) 

So, How did Port Extender solve Network Management Visibility problem on VM traffic?

All this funda of Port extender started because with VEPA etc problem of management visibility into VM traffic came up. Introduction of Port Extension technology solves this problem by by reflecting all network traffic onto a central controlling bridge. This gives network administrators full access and control but at the cost of bandwidth and latency.

Hmmm...But.. There are problems with Port Extension Technologies

1. Port extension technology adds one or more extra hops to the typical three-tier architecture and can magnify congestion problems
2. As data centers support more clustered, virtualized, and cloud-based applications requiring high performance across hundreds or thousands of physical and virtual servers, port extension technology just seems to add cost and complexity.
3. Remember that the pre-standard VN-tags and the IEEE 802.1-BR standard E-tags use different formats. If you adopt VN-tag solutions in your data center, you will have to develop transition strategies when future hardware changes to the IEEE 802.1-BR E-tag format.

Ok.. Now the conclusion

In past 5 posts, we discussed several aspects surrounding the impact of server virtualization on Networking. We Started with the fact that Servers with the virtue of Virtualization have hypervisor software inside them. This Hypervisor is adding another layer of software called Virtual Switch/Virtual Ethernet Bridge. This VSwitch also adds on complexities in terms of Network Management and VM mobility. Then we discussed further on different kinds of VEBs - Software VEBs and Hardware VEB(SR-IOV). Issues associated with vSwitches/VEBs are targeted to be solved through IEEE 802.1 qbg by the introduction of Edge Virtual Bridging through VEPA(Virtual Ethernet Port Aggregator) Technology and S-Channel(Multi-channel VEPA) technology. While IEEE 802.1qbg solved some problems of VSwitch, it did raise some issues which are tackled by IEEE 802.1qbh and IEEE 802.1br by introducing Port Extension Technology. While IEEE 802.1qbh was withdrawn last year, IEEE 802.1BR is active and it did solve some problems while introduced some other. So, it all comes to using these solutions effectively  as per use case. It also depends on how much IT budget we have and IT needs in terms of Server requirements.

Personally, I agree with what many experts in this area say -  Virtual switches won’t be going away anytime soon, but the configuration and management of these virtual network devices shouldn’t reside with the server team merely by virtue of their ownership of the underlying VM management platform. Until the technology allows virtual port management to be pulled into a comprehensive management tool, it means the network and server teams will have to share authority for the VM platform..

That ends my series of posts on "Impact of Server Virtualization on Networking" .

[I am thinking of topic for my next blog posts. Most probably I will take up the one which is interesting me now a days when I read literature on Data Center Networking.. I read quite a lot about leaf&Spine architecture, Data Center Fabric, Transformation of DC networks from Hierarchical nature to Flat.. And the most famous -  movement of traffic patterns from "North-South" to "East-West"]...