This document provides the steps to perform an ELAM on the Nexus 7000 F2 modules, explains the most relevant outputs, and how to interpret their results. Please refer to the following document for an overview on ELAM:
In this example a host on Vlan10 (10.1.1.101) port Eth6/4 sends an ICMP request to a host also on Vlan10 (10.1.1.102) off port Eth6/3. We will use ELAM to capture this single packet between the hosts. It's important to remember that ELAM allows us to capture a single frame.
To perform an ELAM on the Nexus7000, you need to first attach to the appropriate module. This requires the network-admin privilege.
N7K# attach module 6
Attaching to module 6 ...
To exit type 'exit', to abort type '$.'
Determine the ingress FE
We expect the traffic to ingress the switch on port Eth6/4. Checking the modules in the system we can see that module 6 is an F2 module. Remember, the Nexus 7000 is fully distributed and the modules, not the supervisor, are responsible for making the forwarding decision for dataplane traffic.
For F2 modules, we want to perform the ELAM on the L2 forwarding engine (FE) with internal codename Clipper. Note that the L2 FE data bus (DBUS) contains original header information before the L2 and L3 lookup and the result bus (RBUS) contains the results after both L3 and L2 lookups.
The F2 module has 12 forwarding engines per module. We need to determine which Clipper ASIC is the FE for port Eth6/4. We can use the following command to verify:
From the output above, we can see that Eth6/4 is on Clipper (L2LKP) instance 0.
module-6# elam asic clipper instance 0
Configure the Trigger
The Clipper ASIC supports ELAM triggers for multiple frame types. The ELAM trigger must align to the frame type. If the frame is an IPv4 frame then the trigger must also be IPv4. An IPv4 frame will not be captured with an "other" trigger. The same logic applies to IPv6. You can see the different frame types supported by Clipper below:
module-6(clipper-l2-elam)# trigger dbus ?
arp ARP Frame Format
fc Fc hdr Frame Format
ipv4 IPV4 Frame Format
ipv6 IPV6 Frame Format
other L2 hdr Frame Format
pup PUP Frame Format
rarp Rarp hdr Frame Format
valid On valid packet
For NX-OS you can utilize the question mark to help parse out the ELAM trigger. You'll notice that there are several options available for F2 ELAM.
module-6(clipper-l2-elam)# trigger dbus ipv4 ingress if ?
destination-ipv4-address destination ipv4 address
destination-mac-address Inner destination mac address
source-index Source index
source-ipv4-address source ipv4 address
source-mac-address Inner source mac address
For this example we want to capture the frame based off source and destination IPv4 address so we will only specify those values.
Clipper requires a trigger to be set for the DBUS and the RBUS. Different from M-series modules, there is no requirement to specify a packet buffer instance which helps simplify the RBUS trigger.
module-6(clipper-l2-elam)# trigger dbus ipv4 ingress if source-ipv4-address 10.1.1.101 destination-ipv4-address 10.1.1.102
module-6(clipper-l2-elam)# trigger rbus ingress if trig
Start the Capture
Now that the ingress FE has been selected and we've configured our trigger, we can start the capture
We can check the status of the ELAM via the status command.
From the DBUS data above we can validate the frame was received on Vlan10 (vlan: 0xa) with a source MAC of 0050.56a1.1a01 and a destination MAC of 0050.56a1.1aef. We can also see that this is an IPv4 frame sourced from 10.1.1.101 destined to 10.1.1.102. There are several other fields not included in this output such as TOS value, IP flags, IP length, L2 frame length, etc... that are also often useful to check.
We can also validate what port the frame was received on via the source-index (the source LTL). For Nexus 7000, we can map an LTL to a port or group of ports via the following command:
N7K# show system internal pixm info ltl 0x3
The above output shows that source-index of 0x3 maps to port Eth6/4. This confirms that the frame was received on Eth6/4.
From the RBUS data we can validate that the frame was switched on Vlan10 (vlan-id: 0xa). We can confirm the egress port from the di-ltl-index (destination LTL):
N7K# show system internal pixm info ltl 0x2
The above output shows that the di-ltl-index of 0x2 maps to port Eth6/3. This confirms that the frame was switched out Eth6/3.
Another command to remember is show system internal pixm info ltl-region, which will show how the switch has allocated the pool of LTL's. This is useful to understand the purpose of an LTL if it does not match to a physical port. A good example is a drop LTL: