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For example, the Catalyst 6000/6500 allows only up to two ingress SPAN sessions (traffic received or received and transmitted on source ports) and up to four egress SPAN sessions (only traffic transmitted on source ports). These limitations are based upon the direction of traffic that you capture. Session limitations Catalyst switches have limitations on how many simultaneous SPAN sessions you can run. While SPAN meets the requirements of many traffic capture environments, it does have a few limitations of which you need to be aware: Ingress SPAN session Refers to a SPAN session where at least one source interface is configured to mirror traffic received (rx) or configured to mirror traffic sent and received on an interface (both).Įgress SPAN session Refers to a SPAN session where all source interfaces are configured to mirror traffic transmitted (tx) only.ĭifferentiating between ingress and egress SPAN sessions is important because some Catalyst switches have different limitations as to the maximum number of ingress and egress SPAN sessions that you can run. This feature is sometimes referred to as VLAN SPAN or VSPAN.ĭepending on the direction of traffic that is mirrored, a SPAN session can be defined as one of the following session types: You can also specify a VLAN as a source, which means that the Catalyst switch automatically mirrors the traffic from all ports in the VLAN to the destination port. Looking at source ports, you can configure a SPAN session to mirror traffic either received, transmitted, or both transmitted and received on the source ports. With SPAN, only a single destination port can be configured, while multiple source ports can be configured per session.
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Figure 10-11 represents a SPAN session, which is essentially a relationship between a set of source ports and a single, unique destination port. In Figure 10-11, the traffic received on a group of source ports is mirrored out a single destination port to some form of network capture device. Figure 10-11 demonstrates SPAN operation. SPAN uses the concept of mirroring traffic from a set of source ports to a single destination port, which has a network capture tool connected to it. SPAN is the traditional method of monitoring LAN traffic on Cisco switches. To monitor traffic in a switched network, Cisco Catalyst switches provide three features: In switched networks, however, this does not happen because a switch only forwards unicast traffic out the port attached to the destination and not to any other ports. In traditional shared hub environments, capturing traffic from the local LAN is not a problem because all traffic is seen by all devices attached to the shared media. The ability to capture traffic on any network is important for both troubleshooting and monitoring purposes. To generate traffic and to verify that the traffic capturing features are working correctly, you need two hosts that simulate LAN communications, as well as another host that has some form of network capture software.
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In this scenario you learn how to use SPAN and VLAN access control lists to monitor traffic on a single switch and learn how to use RSPAN to monitor traffic across multiple switches. Figure 10-10 illustrates the topology used for this scenario. This capability is a very important troubleshooting and monitoring tool when verifying the data communications between devices in your LAN network. SPAN, RSPAN, and VACLs are all techniques that have been developed that enable administrators to troubleshoot and monitor LAN communications in a switched environment. In this scenario you learn how to capture traffic using the SPAN, RSPAN, and VACLs. To enable the ability to capture traffic sent and received on other switch ports, Cisco Catalyst switches include a feature called the switch port analyzer feature (SPAN), as well as remote SPAN (RSPAN) and VLAN access control lists (VACLs). Instead, Layer 2 unicast traffic between devices is isolated to the ports to which each device is connected, meaning a monitoring device connected to another port has no visibility of traffic. In the modern era of the LAN switch however, Ethernet segments are no longer shared.
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Traditionally, in hub-based LAN environments, traffic capture has been an easy process by simply attaching to the shared LAN segment, all traffic is seen by a monitoring device. The ability to capture traffic on the network is a fundamental requirement for any network engineer that requires low-level protocol monitoring and troubleshooting capabilities.