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Date:   Thu, 7 May 2020 20:32:32 -0700
From:   Florian Fainelli <>
To:     Vladimir Oltean <>,,,
Subject: Re: [PATCH v3 net-next 4/4] net: dsa: sja1105: implement cross-chip
 bridging operations

On 5/3/2020 3:12 PM, Vladimir Oltean wrote:
> From: Vladimir Oltean <>
> sja1105 uses dsa_8021q for DSA tagging, a format which is VLAN at heart
> and which is compatible with cascading. A complete description of this
> tagging format is in net/dsa/tag_8021q.c, but a quick summary is that
> each external-facing port tags incoming frames with a unique pvid, and
> this special VLAN is transmitted as tagged towards the inside of the
> system, and as untagged towards the exterior. The tag encodes the switch
> id and the source port index.
> This means that cross-chip bridging for dsa_8021q only entails adding
> the dsa_8021q pvids of one switch to the RX filter of the other
> switches. Everything else falls naturally into place, as long as the
> bottom-end of ports (the leaves in the tree) is comprised exclusively of
> dsa_8021q-compatible (i.e. sja1105 switches). Otherwise, there would be
> a chance that a front-panel switch transmits a packet tagged with a
> dsa_8021q header, header which it wouldn't be able to remove, and which
> would hence "leak" out.
> The only use case I tested (due to lack of board availability) was when
> the sja1105 switches are part of disjoint trees (however, this doesn't
> change the fact that multiple sja1105 switches still need unique switch
> identifiers in such a system). But in principle, even "true" single-tree
> setups (with DSA links) should work just as fine, except for a small
> change which I can't test: dsa_towards_port should be used instead of
> dsa_upstream_port (I made the assumption that the routing port that any
> sja1105 should use towards its neighbours is the CPU port. That might
> not hold true in other setups).
> Signed-off-by: Vladimir Oltean <>

Reviewed-by: Florian Fainelli <>

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