Fast Convergence; Vlan Load Balancing - Cisco Catalyst 3560-X Software Configuration Manual
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Understanding REP
A segment port does not become operational if:
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Each port creates an adjacency with its immediate neighbor. Once the neighbor adjacencies are created,
the ports negotiate to determine one blocked port for the segment, the alternate port. All other ports
become unblocked. By default, REP packets are sent to a BPDU class MAC address. The packets can
also be sent to the Cisco multicast address, which is used only to send blocked port advertisement (BPA)
messages when there is a failure in the segment. The packets are dropped by devices not running REP.
Fast Convergence
Because REP runs on a physical link basis and not a per-VLAN basis, only one hello message is required
for all VLANs, reducing the load on the protocol. We recommend that you create VLANs consistently
on all switches in a given segment and configure the same allowed VLANs on the REP trunk ports. To
avoid the delay introduced by relaying messages in software, REP also allows some packets to be
flooded to a regular multicast address. These messages operate at the hardware flood layer (HFL) and
are flooded to the whole network, not just the REP segment. Switches that do not belong to the segment
treat them as data traffic. You can control flooding of these messages by configuring a dedicated
administrative VLAN for the whole domain.
The estimated convergence recovery time on fiber interfaces is between 50 ms and 200 ms for the local
segment with 200 VLANs configured. Convergence for VLAN load balancing is 300 ms or less.
VLAN Load Balancing
One edge port in the REP segment acts as the primary edge port; the other as the secondary edge port.
It is the primary edge port that always participates in VLAN load balancing in the segment. REP VLAN
balancing is achieved by blocking some VLANs at a configured alternate port and all other VLANs at
the primary edge port. When you configure VLAN load balancing, you can specify the alternate port in
one of three ways:
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Note
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
1-4
No neighbor has the same segment ID.
More than one neighbor has the same segment ID.
The neighbor does not acknowledge the local port as a peer.
By entering the port ID of the interface. To identify the port ID of a port in the segment, enter the
show interface rep detail interface configuration command for the port.
By entering the neighbor offset number of a port in the segment, which identifies the downstream
neighbor port of an edge port. The neighbor offset number range is –256 to +256; a value of 0 is
invalid. The primary edge port has an offset number of 1; positive numbers above 1 identify
downstream neighbors of the primary edge port. Negative numbers indicate the secondary edge port
(offset number -1) and its downstream neighbors.
You configure offset numbers on the primary edge port by identifying a port's downstream
position from the primary (or secondary) edge port. You would never enter an offset value of 1
because that is the offset number of the primary edge port itself.
Figure 1-4
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 of 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
Chapter 1
Configuring Resilient Ethernet Protocol
OL-25303-03
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