Cisco 2950 Software Configuration Guide - Page 485
Reflector Port, SPAN Traffic, SPAN and RSPAN Interaction with Other Features
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Chapter 25 Configuring SPAN and RSPAN Understanding SPAN and RSPAN Reflector Port The reflector port is the mechanism that copies packets onto an RSPAN VLAN. The reflector port forwards only the traffic from the RSPAN source session with which it is affiliated. Any device connected to a port set as a reflector port loses connectivity until the RSPAN source session is disabled. The reflector port has these characteristics: • It is a port set to loopback. • It cannot be an EtherChannel group, it does not trunk, and it cannot do protocol filtering. • It can be a physical port that is assigned to an EtherChannel group, even if the EtherChannel group is specified as a SPAN source. The port is removed from the group while it is configured as a reflector port. • A port used as a reflector port cannot be a SPAN source or destination port, nor can a port be a reflector port for more than one session at a time. • It is invisible to all VLANs. • The native VLAN for looped-back traffic on a reflector port is the RSPAN VLAN. • The reflector port loops back untagged traffic to the switch. The traffic is then placed on the RSPAN VLAN and flooded to any trunk ports that carry the RSPAN VLAN. • Spanning tree is automatically disabled on a reflector port. • A reflector port receives copies of sent and received traffic for all monitored source ports. If a reflector port is oversubscribed, it could become congested. This could affect traffic forwarding on one or more of the source ports. If the bandwidth of the reflector port is not sufficient for the traffic volume from the corresponding source ports, the excess packets are dropped. A 10/100 port reflects at 100 Mbps. A Gigabit port reflects at 1 Gbps. SPAN Traffic You can use local SPAN to monitor all network traffic, including multicast and bridge protocol data unit (BPDU) packets, and CDP, VTP, DTP, STP, PagP, and LACP packets. You cannot use RSPAN to monitor Layer 2 protocols. See the "RSPAN Configuration Guidelines" section on page 25-12 for more information. In some SPAN configurations, multiple copies of the same source packet are sent to the SPAN destination port. For example, a bidirectional (both Rx and Tx) SPAN session is configured for the sources a1 Rx monitor and the a2 Rx and Tx monitor to destination port d1. If a packet enters the switch through a1 and is switched to a2, both incoming and outgoing packets are sent to destination port d1. SPAN and RSPAN Interaction with Other Features SPAN interacts with these features: • Spanning Tree Protocol (STP)-A destination port or a reflector port does not participate in STP while its SPAN or RSPAN session is active. The destination or reflector port can participate in STP after the SPAN or RSPAN session is disabled. On a source port, SPAN does not affect the STP status. STP can be active on trunk ports carrying an RSPAN VLAN. • Cisco Discovery Protocol (CDP)-A SPAN destination port does not participate in CDP while the SPAN session is active. After the SPAN session is disabled, the port again participates in CDP. 78-11380-10 Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide 25-5