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	<title>Manual:VPLS Control Word - Revision history</title>
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	<updated>2026-04-27T13:49:28Z</updated>
	<subtitle>Revision history for this page on the wiki</subtitle>
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		<id>https://www.cablefree.net/support/radio/software/index.php?title=Manual:VPLS_Control_Word&amp;diff=823&amp;oldid=prev</id>
		<title>Administrator: Created page with &quot;__TOC__  ==Summary==  &lt;p id=&quot;shbox&quot;&gt;&lt;b&gt;Standards:&lt;/b&gt; &lt;code&gt;Pseudowire Fragmentation and Reassembly (RFC 4623)&lt;/code&gt;&lt;/p&gt;   VPLS allows remote sites to share an Ethernet broad...&quot;</title>
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		<updated>2015-12-14T13:44:09Z</updated>

		<summary type="html">&lt;p&gt;Created page with &amp;quot;__TOC__  ==Summary==  &amp;lt;p id=&amp;quot;shbox&amp;quot;&amp;gt;&amp;lt;b&amp;gt;Standards:&amp;lt;/b&amp;gt; &amp;lt;code&amp;gt;Pseudowire Fragmentation and Reassembly (RFC 4623)&amp;lt;/code&amp;gt;&amp;lt;/p&amp;gt;   VPLS allows remote sites to share an Ethernet broad...&amp;quot;&lt;/p&gt;
&lt;p&gt;&lt;b&gt;New page&lt;/b&gt;&lt;/p&gt;&lt;div&gt;__TOC__&lt;br /&gt;
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==Summary==&lt;br /&gt;
&lt;br /&gt;
&amp;lt;p id=&amp;quot;shbox&amp;quot;&amp;gt;&amp;lt;b&amp;gt;Standards:&amp;lt;/b&amp;gt; &amp;lt;code&amp;gt;Pseudowire Fragmentation and Reassembly (RFC 4623)&amp;lt;/code&amp;gt;&amp;lt;/p&amp;gt;&lt;br /&gt;
&lt;br /&gt;
&lt;br /&gt;
VPLS allows remote sites to share an Ethernet broadcast domain by connecting sites through &amp;#039;&amp;#039;&amp;#039;pseudo-wires(PW)&amp;#039;&amp;#039;&amp;#039; tunnels over a packet switching network (PSN). Since VPLS encapsulation adds additional overhead, each interface in LSP should be able to transmit large enough packet.&lt;br /&gt;
&lt;br /&gt;
Each ethernet chipset has hardware limitation on maximum packet size that it can transmit. Even now there are Ethernets that supports only one Vlan tag, meaning that maximum packet size without Ethernet header and checksum (L2MTU) is 1504 bytes. Obviously it is not enough to forward VPLS encapsulated Ethernet frame without fragmentation (at least 1524 L2MTU support is required).  See [[Manual:Maximum_Transmission_Unit_on_CableFree devices | MTU article]] for maximum supported L2MTUs on RouterBOADs.&lt;br /&gt;
&lt;br /&gt;
Since not even all CableFree devices support enough L2MTU to transmit VPLS encapsulated packet without fragmentation, RadioOS have added Pseudowire Fragmentation and Reassembly (PWE3) support according to RFC 4623 using 4-byte &amp;#039;&amp;#039;&amp;#039;Control Word (CW)&amp;#039;&amp;#039;&amp;#039;. &lt;br /&gt;
&lt;br /&gt;
==Control Word Usage==&lt;br /&gt;
&lt;br /&gt;
In RadioOS Control Word is used for packet fragmentation and reassembly inside VPLS tunnel and is done by utilizing optional &amp;#039;&amp;#039;&amp;#039;Control Word (CW)&amp;#039;&amp;#039;&amp;#039;.  CW is added between PW label (demultiplexor) and packet payload and adds additional 4-byte overhead.&lt;br /&gt;
&lt;br /&gt;
{{Note | Reordering OOO packets are not implemented, out of order fragments will be dropped}}&lt;br /&gt;
&lt;br /&gt;
Until RadioOS v5.5 CW was used always, but, for compatibility with other vendors that do not use CW, feature to turn off Control Word usage was added. &lt;br /&gt;
CW usage is controlled by one new parameter &amp;lt;var&amp;gt;use-control-word&amp;lt;/var&amp;gt; in [[Manual:Interface/VPLS#BGP_VPLS | /interface vpls bgp-vpls]] and [[Manual:Interface/VPLS#Cisco_Style_BGP_VPLS | /interface vpls cisco-bgp-vpls]]&lt;br /&gt;
&lt;br /&gt;
===VPLS Packet and CW Format===&lt;br /&gt;
&lt;br /&gt;
&lt;br /&gt;
[[File:VPLS_CW.png]]&lt;br /&gt;
&lt;br /&gt;
As you can see &amp;#039;&amp;#039;&amp;#039;Control Word&amp;#039;&amp;#039;&amp;#039; is divided into 5 fileds:&lt;br /&gt;
* 0000 - 4-bits identifies that packet packet is PW (not IP)&lt;br /&gt;
* Flags - 4bits&lt;br /&gt;
* Frag - 2bits value that indicates payload fragmentation.&lt;br /&gt;
* Len - 6bits&lt;br /&gt;
* Seq - 16bits sequence number used to detect packet loss / misordering.&lt;br /&gt;
According to RFC generation and processing of sequence number is optional.&lt;br /&gt;
&lt;br /&gt;
==Example Setup==&lt;br /&gt;
&lt;br /&gt;
To show CW usage we will use simple three router setup as illustrated below. &lt;br /&gt;
&lt;br /&gt;
This setup will not explain BGP and LDP configuration since its detailed explanation is found in other articles. [[#See Also | &amp;lt;code&amp;gt;Read here&amp;gt;&amp;gt;&amp;lt;/code&amp;gt;]]&lt;br /&gt;
&lt;br /&gt;
==See Also==&lt;br /&gt;
&lt;br /&gt;
* [[Manual:MPLSVPLS | Basic MPLS and LDP based VPLS]]&lt;br /&gt;
* [[Manual:BGP_based_VPLS | BGP based VPLS]]&lt;br /&gt;
* [[Manual:Cisco_VPLS | VPLS with Cisco routers]]&lt;br /&gt;
&lt;br /&gt;
&lt;br /&gt;
{{Cont}}&lt;br /&gt;
&lt;br /&gt;
[[Category:Manual|V]]&lt;br /&gt;
[[Category:MPLS|V]]&lt;br /&gt;
[[Category:Internetworking|V]]&lt;br /&gt;
[[Category:VPN|V]]&lt;br /&gt;
[[Category:Case_Studies|V]]&lt;/div&gt;</summary>
		<author><name>Administrator</name></author>
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