16-5
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 16 Configuring Resilient Ethernet Protocol
Understanding REP
Figure 16-4
shows neighbor offset numbers for a segment where E1 is the primary edge port and E2
is the secondary edge port. The red numbers inside the ring are numbers offset from the primary
edge port; the black numbers outside the ring show the offset numbers from the secondary edge port.
Note that you can identify all ports (except the primary edge port) by either a positive offset number
(downstream position from the primary edge port) or a negative offset number (downstream position
from the secondary edge port). If E2 became the primary edge port, its offset number would then
be 1, and E1 would be -1.
•
By entering the preferred keyword to select the port that you previously configured as the preferred
alternate port with the rep segment segment-id preferred interface configuration command.
Figure 16-4
Neighbor Offset Numbers in a Segment
When the REP segment is complete, all VLANs are blocked. When you configure VLAN load balancing,
you must also configure triggers in one of two ways:
•
Manually trigger VLAN load balancing at any time by entering the rep preempt segment
segment-id privileged EXEC command on the switch that has the primary edge port.
•
Configure a preempt delay time by entering the rep preempt delay seconds interface configuration
command. After a link failure and recovery, VLAN load balancing begins after the configured
preemption time period elapses. Note that the delay timer restarts if another port fails before the time
has elapsed.
Note
When VLAN load balancing is configured, it does not start working until triggered by either manual
intervention or a link failure and recovery.
When VLAN load balancing is triggered, the primary edge port sends a message to alert all interfaces in
the segment about the preemption. When the secondary port receives the message, it is reflected into the
network to notify the alternate port to block the set of VLANs specified in the message and to notify the
primary edge port to block the remaining VLANs.
You can also configure a particular port in the segment to block all VLANs. Only the primary edge port
initiates VLAN load balancing, which is not possible if the segment is not terminated by an edge port on
each end. The primary edge port determines the local VLAN load balancing configuration.
Reconfigure the primary edge port to reconfigure load balancing. When you change the load balancing
configuration, the primary edge port again waits for the rep preempt segment command or for the
configured preempt delay period after a port failure and recovery before executing the new
configuration. If you change an edge port to a regular segment port, the existing VLAN load balancing
status does not change. Configuring a new edge port might cause a new topology configuration.
E2
E1
-1
-2
-
3
-4
-5
-6
-7
-
8
-9
2
1
10
9
8
7
6
5
4
3
201
8
90
E1 = Prim
a
ry edge port
E2 =
S
econd
a
ry edge port
Off
s
et n
u
m
b
er
s
from the prim
a
ry edge port
Off
s
et n
u
m
b
er
s
from the
s
econd
a
ry edge
port (neg
a
tive n
u
m
b
er
s
)
