SPAN and RSPAN allow us to copy traffic from one interface to another. This is great if you want to send traffic to a sensor or if you want to take a closer look at it with a packet analyzer like Wireshark. SPAN is however limited to one switch, RSPAN is able to send traffic between switches but this traffic can’t be routed.
ERSPAN (Encapsulated Remote Switched Port Analyzer) solves this issue! It uses GRE encapsulation, this allows us to route SPAN traffic from a source to a destination. You can use ERSPAN on IOS XE, NX-OS and the Catalyst 6500/7600 switches. Unfortunately, It’s not supported on the “smaller” IOS switches and routers.
When you want to configure ERSPAN, there’s a couple of things you have to keep in mind. For the source session, we have to configure:
- Unique session ID.
- List of source interfaces or source VLANs that you want to monitor. Not all platforms support every possible source.
- What traffic we want to capture: tx, rx or both.
- Destination IP address for the GRE tunnel.
- Origin IP address which is used as the source for the GRE tunnel.
- Unique ERSPAN flow ID.
- Optional: you can specify attributes like the ToS (Type of Service), TTL, etc.
For the destination we have to specify:
- Unique session ID, doesn’t have to match with the source session.
- Destination interface(s) where you want to forward the traffic to.
- Source IP address: has to match with the origin IP address of the source session.
- Unique ERSPAN flow ID, has to match with the source session.
Let’s look at an example so we can see how ERSPAN works in action.
I will use the following topology for this example:
Above we have two routers, R1 and R2. On the left side there’s a host (H1) and on the right side, I have a machine running Wireshark. I will show you how to capture traffic on the Gigabit 2 interface of R2 and send it towards the Wireshark machine behind R2.
Let’s start with the configuration on R1:
R1(config)#monitor session 1 type erspan-source R1(config-mon-erspan-src)#source interface GigabitEthernet 2 rx R1(config-mon-erspan-src)#no shutdown R1(config-mon-erspan-src)#destination R1(config-mon-erspan-src-dst)#erspan-id 100 R1(config-mon-erspan-src-dst)#ip address 172.16.2.200 R1(config-mon-erspan-src-dst)#origin ip address 172.16.12.1
Above you can see that we capture incoming traffic on the Gigabit 2 interface of R1. We use ERSPAN ID 100, the source IP address will be 172.16.12.1 and the destination is 172.16.2.200 (Wireshark).
Here’s the configuration of R2:
R2(config)#monitor session 1 type erspan-destination R2(config-mon-erspan-dst)#no shutdown R2(config-mon-erspan-dst)#destination interface GigabitEthernet 2 R2(config-mon-erspan-dst)#source R2(config-mon-erspan-dst-src)#erspan-id 100 R2(config-mon-erspan-dst-src)#ip address 172.16.2.200
Above we configure the same ERSPAN ID, the destination IP address and the destination interface.
Let’s verify our work. First we will check the routers:
R1#show monitor session 1 Session 1 --------- Type : ERSPAN Source Session Status : Admin Enabled Source Ports : RX Only : Gi2 Destination IP Address : 172.16.2.200 Destination ERSPAN ID : 100 Origin IP Address : 172.16.12.1
R2#show monitor session 1 Session 1 --------- Type : ERSPAN Destination Session Status : Admin Enabled Destination Ports : Gi2 Source IP Address : 172.16.2.200 Source ERSPAN ID : 100
Above you can see the ERSPAN configuration. Let’s see if it works…I will send a ping from H1 to R1:
H1#ping 172.16.1.254 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.1.254, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/16 ms
These packets should be encapsulated by ERSPAN and forwarded towards our Wireshark machine. Here’s what the packet capture looks like:
Above you can see the source and destination IP addresses of our GRE tunnel. You can also see the GRE header and the ID (100) that we configured.
That’s all there is to it. We succesfully routed our ERSPAN traffic from one router to another.
Want to take a look for yourself? Here you will find the configuration of each device.