- There are two types
of cycles in a network:
- Odd cycles:
the number of nodes in
the cycle is odd
Example: this cycle contains 5 nodes
- Even cycles:
the number of nodes in
the cycle is even
Example: this cycle contains 6 nodes
- Odd cycles:
the number of nodes in
the cycle is odd
- Observation:
- In an even cylce:
- There is exactly one bridge that have two paths with the same distance to the root bridge
Example:
- In an even cylce:
- My nomemclature:
- For simplicity,
I (not in any book)
will call
this node:
- the last node in the even cycle
- For simplicity,
I (not in any book)
will call
this node:
- Fact:
- The last node in the
even cycle can
tell that:
- It is the last node in an even cycle
How:
- The last node in the
even cycle can
tell that:
- Goal:
- We want to obtain a tree (= cycle-free) logical network !!!
Therefore:
- When the last node in an
even cycle detects that
it is the
last node:
- The last node in an even cycle must block one of its ports (to break the cycle ) !!!
- Depending
on the order in which the
2 configuration messages
are received, you can have
one of two possible states:
- If the
configuration message
from the left bridge (L)
arrives first:
then:
- The left port will be the root port
- If the
configuration message
from the right bridge (R)
arrives first:
then:
- The right port will be the root port
- If the
configuration message
from the left bridge (L)
arrives first:
- How to break an
even cycle:
by block the
non-root port
** Bridge receives a configuration message on port Port; /* ===================================================== How to detect the last node in an even cycle ===================================================== */ if ( ( RootID in the control message == myRoot ) AND ( Distance in message == myDistance - 1 ) ) then { if ( Status(Port) != Root ) Status(Port) = BLOCKED; // Block the non-root port ! }
- Example:
- The bridge 5 just received
a configuration message with
equal distance to
root node on a
non-root port:
Now the bridge 5 can tell that:
- It is the last node in an even cycle !!!
- Bridge 5 will
block the
receiving port:
The cycle is now broken !!!
- The bridge 5 just received
a configuration message with
equal distance to
root node on a
non-root port:
- Observation:
- In an odd cylce:
- There will be two bridges that
have the same maximum distance
to the root bridge
Example:
I.e.:
- There are 2 last nodes in the cycle
- There will be two bridges that
have the same maximum distance
to the root bridge
- In an odd cylce:
- How to
determine if
a bridge is
a last node in
an odd cycle:
- A last node
in an odd cycle
will receive a
configuration message with:
Distance in configuration message == myDistanceto the same root bridge
- Example:
- A last node
in an odd cycle
will receive a
configuration message with:
- Note:
- There are 2
last nodes in an
odd cycle
- Both last nodes can (and will) determine that it is (one of) a last node in an odd cycles
Caveat:
- Only one of the bridges must block its port !!!
Graphically:
(You must not block both bridges, or else you will disconnect some LAN !!!)
- There are 2
last nodes in an
odd cycle
- IEEE 802.11D rule
to determine the
block port:
- The bridge with a large ID value must make its non-root port into a blocked port
Example:
- Algorithm is pseudo code:
Bridge received a configuration message Msg on port Port; /* ================================================ How to detect the last node in an odd cycle ================================================ */ if ( ( Msg(RootID) == myRoot ) AND ( Msg(Distance) == myDistance ) ) { /* ================================================ Block the port of the bridge with the large ID ================================================ */ if ( myID > Msg(SourceID) ) Status(Port) = BLOCKED; }
- A concrete example:
- Bridge 5
and bridge 3
are 4 hops away from the
root bridge:
- When bridge 3 receives
a configuration message from
node 5:
Brigde 3 will find out that it is one of the last nodes in an odd cycle:
However:
- Brigde 3 will ignore the message because its ID is smaller than bridge 5
Graphically:
- But,
when bridge 5 receives
a configuration message from
node 3:
Brigde 5 will find out that it is one of the last nodes in an odd cycle:
In this case:
- Brigde 5 will change the status of the incoming port to blocked
Graphically:
because its ID is larger than bridge 3
- Result:
The cycle is also broken
- Bridge 5
and bridge 3
are 4 hops away from the
root bridge: