Cisco - Port Mirror (SPAN / RSPAN / ERSPAN)

Introduction:

Switch port Analyzer (SPAN) is an efficient, high performance traffic monitoring system. It duplicated network traffic to one or more monitor interfaces as it transverse the switch. SPAN is used for troubleshooting connectivity issues and calculating network utilization and performance, among many others. There are three types of SPANs supported on Cisco products, which are illustrated in below diagram.

Types of SPAN:

SPAN

Local SPAN: Mirrors traffic from one or more interface on the switch to one or more interfaces on the same switch.
Remote SPAN (RSPAN): An extension of SPAN called remote SPAN or RSPAN. RSPAN allows you to monitor traffic from source ports distributed over multiple switches, which means that you can centralize your network capture devices. RSPAN works by mirroring the traffic from the source ports of an RSPAN session onto a VLAN that is dedicated for the RSPAN session. This VLAN is then trunked to other switches, allowing the RSPAN session traffic to be transported across multiple switches. On the switch that contains the destination port for the session, traffic from the RSPAN session VLAN is simply mirrored out the destination port.
Encapsulated remote SPAN (ERSPAN): encapsulated Remote SPAN (ERSPAN), as the name says, brings generic routing encapsulation (GRE) for all captured traffic and allows it to be extended across Layer 3 domains.

ERSPAN is a Cisco proprietary feature and is available only to Catalyst 6500, 7600, Nexus, and ASR 1000 platforms to date. The ASR 1000 supports ERSPAN source (monitoring) only on Fast Ethernet, Gigabit Ethernet, and port-channel interfaces.

Configuration Example:
Configuring Local SPAN: Local SPAN configures using “monitor session” command specifying source and destination on the same switch.

Switch1# configure terminal
Switch1(config)# monitor session 1 source interface fastEthernet0/2
Switch1(config)# monitor session 1 destination interface fastEthernet0/24
Switch1(config)#end

Local SPAN configuration syntax on Cisco IOS release 12.2(33)SXH and beyond as shown below.

monitor session 1 type local
source int fa0/2
destination int fa0/24

RSPAN

Step1: In order to configure RSPAN you need to have an RSPAN VLAN, those VLANs have special properties and can’t be assigned to any access ports. To create a VLAN for RSPAN on Cisco IOS, you must create the VLAN via the config-vlan configuration mode, as opposed to using the older VLAN database configuration mode. During the process of defining VLAN parameters, you must specify that the new VLAN is an RSPAN VLAN by configuring the remote-span VLAN configuration command.

Switch1# configure terminal
Switch1(config)# vlan 200
Switch1(config-vlan)# remote-span
Switch1(config-vlan)# end
Switch1# show vlan remote-span

Switch2# configure terminal
Switch2(config)# vlan 200
Switch2(config-vlan)# remote-span
Switch2(config-vlan)# end
Switch2# show vlan remote-span
Remote SPAN VLANs

Step2: Then configure the RSPAN on Source switch: Unlike SPAN, where the source and destination ports exist on the same switch, the source and destination ports for an RSPAN session reside on different switches. This requires a separate RSPAN source session to be configured, as well as a separate RSPAN destination session to be configured.

Switch1# configure terminal
Switch1(config)# monitor session 1 source interface fastEthernet0/2 rx
Switch1(config)# monitor session 1 destination remote vlan 200
reflector-port fastEthernet0/24
Switch1(config)# exit

Switch1# show monitor
Session 1
---------
Type                 : Remote Source Session
Source Ports     :
   Rx                 : Fa0/2
Reflector Port    : Fa0/24
Dest RSPAN VLAN  : 200

Step3: Configure the RSPAN on destination switch:
Switch2# configure terminal
Switch2(config)# monitor session 1 source remote vlan 200
Switch2(config)# monitor session 1 destination interface fastEthernet0/3
Switch2(config)# exit

The RSPAN VLAN should be allowed in ALL trunks between the involved switches (Source and Destination switches in this case); if you have enabled "pruning" in your network, remove the RSPAN VLAN from the pruning, with the command: “switchport trunk pruning vlan remove <RSPAN VLAN ID>” under the interface configure as trunk.

ERSPAN

In this example we will capture received traffic on the ASR 1002 (GigabitEthernet0/1/0) and send to Catalyst 6509 Gig2/2/1. This traffic will simply be captured, encapsulated in GRE by ASR 1002 natively by the QFP chipset and routed over to the Catalyst 6509. A sniffing station on the 6500 attached to GE2/2/1 will see the complete Ethernet frame (L2 to L7) information.
Configuring source interface, direction of traffic, and ERSPAN session ID on the ASR 1002.

ASR1002(config)# monitor session 1 type erspan-source
ASR1002(config-mon-erspan-src)# source interface gig0/1/0 rx
ASR1002(config-mon-erspan-src)# no shutdown
ASR1002(config-mon-erspan-src)# destination
ASR1002(config-mon-erspan-src-dst)# erspan-id 101
ASR1002(config-mon-erspan-src-dst)# ip address 10.1.1.1
ASR1002(config-mon-erspan-src-dst)# origin ip address 172.16.1.1

Configuring Catalyst 6509 to receive traffic from the source session on the ASR 1002

SW6509(config)# monitor session 2 type erspan-destination
SW6509(config-mon-erspan-dst)# destination interface gigabitEthernet2/2/1
SW6509(config-mon-erspan-dst)# no shutdown
SW6509(config-mon-erspan-dst)# source
SW6509(config-mon-erspan-dst-src)# erspan-id 101
SW6509(config-mon-erspan-dst-src)# ip address 10.1.1.1

You can use the show monitor session command to verify the configuration:

ASR1002#sh monitor session 1
Session 1
---------
Type                             : ERSPAN Source Session
Status                           : Admin Enabled
Source Ports                  :
RX Only                         : Gi0/1/0
Destination IP Address   : 10.1.1.1
MTU                              : 1464
Destination ERSPAN ID  : 101
Origin IP Address           : 172.16.1.1

To monitor the statistics of monitored traffic, you need to use "show platform hardware qfp active feature erspan state" command:

ASR1002#show platform hardware qfp active feature erspan state
ERSPAN State:
Status                : Active
Complexes         : 1
CPPs                 : 1
Capabilites          :
Max sessions      : 1024
Max outputs        : 128
Encaps type        : ERSPAN type-II
GRE protocol       : 0x88BE
MTU                    : 1464
IP TOS                : 0
IP TTL                 : 255
COS                   : 0
System Statistics:
DROP src session replica :                  0 /                 0
DROP term session replica:                 0 /                 0
DROP receive malformed   :                 0 /                 0
DROP receive invalid ID  :                    0 /                 0
DROP recycle queue full  :                   0 /                 0
DROP no GPM memory       :               0 /                 0
DROP no channel memory   :               0 /                 0
Client Debug Config:
Enabled: Info, Warn
Data Path Debug Config:
0x00000000
ASR1002#

Note:  10.1.1.1 is SW6509’s loopback. 172.16.1.1 Is ASR1002's loopback. ip address in destination session and ip address in source session should match. If they don't- that is causing the drops you see.

Taken From:

• https://supportforums.cisco.com/document/139236/understanding-spanrspanand-erspan