Shortest Path Bridging
An Update on Bridging Technologies
Norman Finn
IEEE Tutorial, July 18, 2005
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
1
Caveat
• 802.1 has not spent a significant amount
of time working on P802.1ao Shortest Path
Bridging.
• The opinions expressed in this
presentation are those of the author, not
Cisco, and not IEEE 802.1.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
2
Spanning Tree Uses Sub-Optimal Paths
2
1
1
B
C
1
2
G
D
A
2
2
H
2
F
I
E
2
2
• Bridge A is the Root Bridge.
• Bridge E breaks the two spanning tree
loops by blocking the marked ports.
• Path from E to G is E-F-I-A-B-D-G.
• This clearly qualifies as “sub-optimal.”
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
3
Spanning Tree Per Bridge
2
1
1
B
C
1
2
G
D
A
2
2
H
F
I
2
E
2
2
• Instead of 1 spanning tree, we create 9
spanning trees.
Each bridge is the root of its own spanning
tree instance (MSTI).
802.1S (MSTP) supports 64 MSTIs already, 4k
with some effort.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
4
Spanning Tree Per Bridge
2
1
1
B
C
1
2
G
D
A
2
2
H
F
I
2
E
2
2
• Whenever Bridge A sends a frame, it uses
MSTI A.
• Of course, the MSTI with A as the root is
the optimal path away from A.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
5
Spanning Tree Per Bridge
2
1
1
B
C
1
2
G
D
A
2
2
H
F
I
2
E
2
2
• Similarly, traffic originating from Bridge E
uses MSTI E and thus takes the optimal
path from E.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
6
Problem solved??
• So, every frame takes the optimal path
through the network.
• Problem solved?
• Almost.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
7
Asymmetrical Spanning Trees
2
1
1
X
B
C
1
2
G
D
A
2
2
H
F
I
2
Y
E
2
X
2
• Station X sends a frame to Y. No bridges
know where Y is, so all flood over MSTI A.
• All bridges learn where X is.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
8
Asymmetrical Spanning Trees
2
1
1
A
X
(blocked)2
B
C
2
H
1
2
G
D
F
I
2
Y
E
2
X
2
• Y replies to X on MSTI E.
• Frame cannot get from I to A because
MSTI E is blocked.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
9
Asymmetrical Spanning Trees
2
1
1
X
B
C
1
2
G
D
A
2
2
H
F
I
2
E
Y
2
2
• But, if the two spanning trees are
symmetrical, then A-B-C-H-E is used for
both directions, and the learned addresses
work just fine.
• But how do we accomplish this?
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
10
Part 1: Symmetrical Port Path Costs
• The bridge advertises its link configured
costs in BPDUs.
• All bridges on a given LAN use the link
costs advertised by the CSTI Designated
Bridge.
• Also, bridge’s bridge priority must be the
same in all STIs that must be symmetrical.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
11
Part 2: Reflection Vector
• For each MSTI x, BPDU carries a vector
relating that MSTI to each of the other
MSTIs y.
• For n MSTIs, that takes n2 bits.
• Each bit says: “Along the path from the
Regional Root of MSTI x to this port, every
port on which this MSTI x information was
transmitted was a Regional Root Port for
MSTI y.”
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
12
The Reflection Vector
Y
N
R
Y
I
B
N
C
N
Y
D
Y
N
N
Y
H
Y
N
E
N
G
Y
R
N
Y
A
• The box is the color of the BPDU, the letter
inside shows what the vector says about
the other Root’s MSTI.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
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The Reflection Vector
Y
I
!
Y
!R N
B
N
C
N
Y
D
Y
N
N
Y
H
Y
N
R
N
G
Y
E
N
Y
!
A
• Bridge E sees from the Reflection Vector
that it must fix the problem. It selects a
new Root Port from among its Alternate
Ports.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
14
The Reflection Vector
Y
Y
R
Y
I
B
Y
C
N
N
D
Y
Y
N
N
H
Y
Y
R
N
G
E
N
Y
Y
A
• Bridge E advertises its decision.
• Now everything is OK and everybody
knows it.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
15
Adding back VLANs
• You need bits from the VLAN tag to mark
frames with which MSTI they’re using.
• VLAN tag are already used for identifying
broadcast domains (the current use for
VLANs).
• There are only 12 bits.
That is often enough.
When not, P802.1ah makes more bits available.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
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Summary so far
• The preceding was an attempt at an
existence proof that spanning tree
technology can:
Provide the optimum routes normally
associated with routing protocols;
Provide the same network characteristics, e.g.
in-order deliver, no multiple deliveries, etc., as
current STP-based networks; and
Preserve the data plane forwarding hardware
currently-deployed on STP-based bridges.
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
17
Where might this go?
Some commonly held beliefs:
GOOD
IS-IS or OSPF
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
18
Where might this go?
Some commonly held beliefs:
GOOD
IS-IS or OSPF
RSTP
BAD
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
19
Where might this go?
Some commonly held beliefs:
GOOD
IS-IS or OSPF
MSTP
COMPLEX
and BAD
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
20
But consider:
Distance Vector
Link State
IS-IS or OSPF
MSTP++
• MSTP augmented with Reflection Vector
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
21
But consider:
Temporary
loops
(TTL needed)
No
temporary
loops (no
TTL)
IS-IS or OSPF
MSTP
• Another difference
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
22
But consider:
Distance Vector
Temporary
loops
(TTL needed)
No
temporary
loops (no
TTL
Link State
RIP
IS-IS or OSPF
MSTP
• Combining them shows uninteresting RIP
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
23
But consider:
Distance Vector
Temporary
loops
(TTL needed)
No
temporary
loops (no
TTL
Link State
RIP
IS-IS or OSPF
Very
Interesting
MSTP
• Build STIs using Link State protocol?
Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
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Shortest Path Bridging
IEEE 802 Tutorial
July 18, 2005
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