ISSUE 90: SONiC-VPP LAG feature support

docs/HLD/vpp-lag.md

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+# SONIC VPP LAG Bridging support
+Rev v0.1
+
+<br/>
+<br/>
+
+## Table of Contents
+
+1. [Revisions](#item-1)
+2. [Scope](#item-2)
+3. [Definitions/Abbreviations](#item-3)
+4. [Introduction](#item-4)
+5. [LAG and Bridging](#item-5)
+
+<br/>
+<br/>
+
+<a id="item-1"></a>
+## Revisions
+
+| Rev | Date | Author(s) |
+|-----|------|-----------|
+|v0.1 | 25/02/2024 | Bendrapu Balareddy (Cisco), Sameer Nanajkar (Cisco) |
+
+
+<br/>
+<br/>
+
+<a id="item-2"></a>
+## Scope
+This document describes the high level design of integrating LAG and bridging functionality between SONIC and VPP. It provides
+ - LAG 
+ - 802.1Q bridging
+
+<br/>
+
+<a id="item-3"></a>
+## Definitions/Abbreviations
+**VPP**: Vector Packet Processing (VPP) technology based high performance packet processing stack that can run on commodity CPUs. For more details see [What is VPP](https://wiki.fd.io/view/VPP/What_is_VPP%3F)
+
+**LAG**:  Ling Aggregation Group
+
+
+<br/>
+<br/>
+
+<a id="item-4"></a>
+## Introduction
+LAG functionality is supported in SONiC with port channel interfaces. 
+VPP port channel supports modes as active-standby , active-active or LACP mode. But SONiC does not have a way program
+this through configuration. Hence the portchannel mode as default active-active by giving ROUND_ROBIN value. Similarly
+VPP supports different load balancing algorithms for distributing the traffic in active-active mode. But again SONIC
+does not support a way to configure any load balancing algorithm, hence it's programmed as BOND_API_LB_ALGO_L2 by default.
+If there is any defined requirement in future to use different load balancing algorithm to use by default, it can be
+changed accordingly.
+
+Some parameters the SONIC supports are not supported by VPP. The attributes like min-links for portchannel group, fast-rate
+etc are not supported by VPP.
+
+To enable LACP on VPP, the VPP should be rebuilt with LACP plugin added. Currently sonic-platform-vpp repo uses prebuilt
+vpp binary. Hence it needs some changes the way this repo is using the VPP
+
+
+<a id="item-5"></a>
+![LAG Bridging](../LAG-Dot1q-Bridging-Topo.png) 
+
+SONiC supports port channel interfaces. These interfaces can be configured for layer routing or as members of a vlan
+ - Access port
+   config vlan member add -u <portname> creates an un tagged bridge port. This will be translated to vpp in two steps
+   1. creates a bridge with given vlan id as bridge id if the bridge with given id doesnot exist.
+   2. set the port as member of bridge with appropriate tag rewrite
+ - Trunk port
+   config vlan member add  <portname> creates a tagged port bridge port. This will be translated to vpp in three steps
+   1. creates a bridge with given vlan id as bridge id if the bridge with given aid does not exist.
+   2. vpp trunk port operation is achieved through vlan subinterface. A subinterface is created for this vlan id. Note
+      that this subinterface is only specific to vpp and does not appear in SONiC control plane data bases.
+   3. set the vlan subinterface as bridge port
+
+
+## References
+
+[SONiC system architecture](https://github.com/sonic-net/SONiC/wiki/Architecture)\
+[What is VPP](https://s3-docs.fd.io/vpp/23.06/)
+