Bridging & Broadcast Scenarios
Carlos Ribeiro
CTBC Telecom
Bridging on the Access Network
• Access network topology
– The Service Provider need to have full control over the
access network topology
– For the Spanning Tree Protocol, this means that the SP
must always be the root of the spanning tree topology
• Nightmare scenarios
– Multiple EFM-based SP connected together
– Multihomed EFM accesses
– Who is in charge of the resulting network?
Single Copy Broadcast
• Desired behavior
– Allows for very efficient use of bandwidth
– Well suited for multimedia applications
• Digital video distribution
• Digital audio distribution
• Conflicts with P2P emulation mode
– Broadcast frames need to be copied for every user!
• Potential issues
– Spanning Tree Topology, broadcast floods
Case studies
• Objectives
– Show how do some proposed topologies behave with
regards to single copy broadcast
• Assumptions
– EPON works in P2P emulation mode
– Single Copy Broadcast is enabled
• A single frame is sent to all stations downstream
– No special ‘routing’ behavior for SCB
• SCB is always on the downstream; upstream traffic is
always subject to P2P emulation rules
• If a P2P emulation port is disabled by the STP protocol,
then it will discard all broadcast frames received
Single Copy Broadcast
(without spanning tree)
Time diagram: frame duplication due to a loop
OLT
DS
Access
Network
1
scb
2
scb
US1
US2
ONU 1
1
2 ONU 2
User
managed
link
scb
uf
Single Copy
Broadcast frame
User Frame
scb
Both frames
are accepted


scb
scb
 Configuration:
• All ports work on P2P emulation mode
• Downstream is seen by all ports, even
with P2P emulation
 It won’t work without spanning tree
Single Copy Broadcast
(with spanning tree: case 1)
Time diagram: STP enabled, user link disabled
OLT
DS
Access
Network
1
2
US1
scb


uf12 
Unicast
frame is
received
by ONU2
Both frames
are accepted
uf12
US2
ONU 1
1
2 ONU 2
User
managed
link
scb
uf
Single Copy
Broadcast frame
User Frame
 Configuration:
• The user port is disabled by the STP
• Both P2P emulation ports are enabled
 The user loses the internal link
• Some user traffic may go through the
SP network, instead of the direct route!
Single Copy Broadcast
(with spanning tree: case 2)
Time diagram: STP enabled, one P2PE link disabled
OLT
DS
Access
Network
1
2
US1
scb
ONU 2 discards
the frame (link
is disabled)


scb
OLT receives
the frame
through ONU1
uf2O
uf2O
US2
ONU 1
1
2 ONU 2
User
managed
link
scb
uf
Single Copy
Broadcast frame
User Frame
 Configuration:
• One P2P emulation port is disabled
• The user link is still alive
 Consequence:
• Upstream traffic from the user network
goes to the other ONU to be forwarded
upstream
Summary
• Routing behavior is not absolutely needed
– P2P Emulation can work with SCB
• With STP enabled, loops are avoided, but...
– User traffic may flood through the SP network
– SP traffic may turn out to be carried through user links
• There are no guarantees that the root will always
be the SP
– There may be two SP providers connected through a
single customer!
Additional ideas
• Force VLANs in the access network
– All ports should apply SP-specific VLAN tags
– All frames which enter the access network with VLAN tags
applied should be tunneled with ‘VLAN-inside-VLAN’, or
802.1q inside 802.1q
– This implementation resembles a ‘administrative domain’
paradigm, as already implemented for routed networks
– Comment: to require 802.1q implementation may be beyond
802.3ah scope; but so is the requirement to use a router for
SE support.
• Special MAC addresses
– Reserve MAC addresses for the applications that use single
copy broadcast on the downstream
– Apply filtering to avoid loops on the network
VLANs inside VLANs
Service layer
Bridges, Routers...
OLT/Bridge
(802.1d compliant)
S1
ONU
S2
ONU
S1
S3
S2
ONU
S4
STP Domain
Inside the Access Network
S1
S2
S3
VLANs inside VLANs
SP Requirements
• A standard way to implement VLAN-inside-VLAN
– Encapsulation levels:
• 1 level of encapsulation only
• ‘n’ levels of encapsulation
• Arbitrary encapsulation
– Maximum frame size (depends on the maximum
encapsulation allowed)
• A standard way to provision and manage VLANs
– Standard MIBs
– Standard VLAN-to-VLAN mapping features