Hi Andy!
Unfortunately, there is no public-facing document that describes how to map the sup_dst and sup_src_if values found in the N5K shim header to front-panel ports. However, mapping these values to front-panel interface is relatively straightforward.
In my lab, I have a Nexus 5548UP running NX-OS software release 7.3(2)N1(1) named LEAF-30.
LEAF-30# show module
Mod Ports Module-Type Model Status
--- ----- ----------------------------------- ---------------------- -----------
1 32 O2 32X10GE/Modular Universal Platfo N5K-C5548UP-SUP active *
3 0 O2 Daughter Card with L3 ASIC N55-D160L3-V2 ok
Mod Sw Hw World-Wide-Name(s) (WWN)
--- -------------- ------ ---------------------------------------------------
1 7.3(2)N1(1) 1.0 --
3 7.3(2)N1(1) 1.0 --
This switch is connected to a Nexus 7004 switch (named SPINE-2) through interface Ethernet1/3. The Nexus 7004 switch is visible as a CDP neighbor, which indicates that we are receiving CDP packets on Ethernet1/3.
LEAF-30# show cdp neighbors interface Ethernet1/3
Capability Codes: R - Router, T - Trans-Bridge, B - Source-Route-Bridge
S - Switch, H - Host, I - IGMP, r - Repeater,
V - VoIP-Phone, D - Remotely-Managed-Device,
s - Supports-STP-Dispute
Device-ID Local Intrfce Hldtme Capability Platform Port ID
SPINE-2(REDACTED)
Eth1/3 139 R S I s N7K-C7004 Eth4/48
This means we should see CDP packets in the control plane of the switch, as shown by the example below.
LEAF-30# ethanalyzer local interface inbound-hi display-filter cdp limit-captured-frames 0
Capturing on inband
2020-10-23 18:40:07.018010 84:78:ac:0b:60:46 -> 01:00:0c:cc:cc:cc CDP Device ID: SPINE-2(REDACTED) Port ID: Ethernet4/48
A detailed view of this packet with the internal N5K shim header decoded is shown below. Some fields are cut out for brevity:
LEAF-30# ethanalyzer local interface inbound-hi decode-internal display-filter cdp limit-captured-frames 0 detail
<snip>
Capturing on inband
N5K Protocol
NSH WORD 1: 0x01000000
CDCE DA: eid_lo: 0 ul: 0 ig: 1 eid_hi: 0 sw_id: 0 sswid: 0
NSH WORD 2: 0x00050200
CDCE DA: lid: 5, CDCE SA: eid_lo 0 ul: 1 ig: 0 eid_hi: 0 sid_hi: 0
NSH WORD 3: 0x1e000005
CDCE SA: sid_lo: 1e ssid: 0 lid: 5
NSH WORD 4: 0x00040020
sys_hdr_type: 0, tr_opt: 0, sup_src_if_mask: 0,
sup_code: 4, ftag: 0, l2mp_ttl: 32
NSH L2 Hdr DA: 01000CCCCCCC
NSH L2 Hdr SA: 8478AC0B6046
NSH L2 Hdr sup_dst: 0xc3
NSH L2 Hdr sup_src_if: 0x03
NSH L2 Hdr stag: 0xa00e
Frame 26 (243 bytes on wire, 243 bytes captured)
Ethernet II, Src: 84:78:ac:0b:60:46 (84:78:ac:0b:60:46), Dst: 01:00:0c:cc:cc:cc (01:00:0c:cc:cc:cc)
802.1Q Virtual LAN, PRI: 5, CFI: 0, ID: 14
Logical-Link Control
Cisco Discovery Protocol
Version: 2
TTL: 180 seconds
Checksum: 0x55a8 [correct]
Device ID: SPINE-2(REDACTED)
Port ID: Ethernet4/48
The internal header of this packet has a sup_src_if hex value of 0x03 (decimal value of 3). It also has a sup_dst hex value of 0xc3 (decimal value of 195), but this isn't terribly useful to us, as we already know what internal inband interface of the supervisor it ingresses since we performed our Ethanalyzer capture with the inbound-hi keyword.
In our scenario, we already know that this packet ingresses via Ethernet1/3. We can find the sup_src_if value of Ethernet1/3 with the show platform fwm info pif Ethernet1/3 command. The output of this command will display a "sup_src_dst_if" value, which will be the decimal equivalent of the sup_src_if hex value found in our Ethanalyzer capture. An example of this is shown below:
LEAF-30# show platform fwm info pif Ethernet1/3
<snip>
Eth1/3 pd: sup_src_dst_if 3 lif_blk -1--1
In scenarios wherein we know the sup_src_if hex value from Ethanalyzer, but don't know the front-panel interface that maps to this value, we can use the show platform fwm info pif all command and search for the sup_src_if's decimal value. For example, LEAF-30 is also receiving a CDP packet from another switch named SPINE-1:
LEAF-30# ethanalyzer local interface inbound-hi display-filter cdp limit-captured-frames 0
<snip>
Capturing on inband
2020-10-23 18:52:29.541020 84:78:ac:0c:7b:46 -> 01:00:0c:cc:cc:cc CDP Device ID: SPINE-1(REDACTED) Port ID: Ethernet4/47
LEAF-30# ethanalyzer local interface inbound-hi decode-internal display-filter cdp limit-captured-frames 0 detail
<snip>
N5K Protocol
NSH WORD 1: 0x01000000
CDCE DA: eid_lo: 0 ul: 0 ig: 1 eid_hi: 0 sw_id: 0 sswid: 0
NSH WORD 2: 0x00050200
CDCE DA: lid: 5, CDCE SA: eid_lo 0 ul: 1 ig: 0 eid_hi: 0 sid_hi: 0
NSH WORD 3: 0x1e000007
CDCE SA: sid_lo: 1e ssid: 0 lid: 7
NSH WORD 4: 0x00040020
sys_hdr_type: 0, tr_opt: 0, sup_src_if_mask: 0,
sup_code: 4, ftag: 0, l2mp_ttl: 32
NSH L2 Hdr DA: 01000CCCCCCC
NSH L2 Hdr SA: 8478AC0C7B46
NSH L2 Hdr sup_dst: 0xc3
NSH L2 Hdr sup_src_if: 0x04
NSH L2 Hdr stag: 0xa00e
Frame 50 (218 bytes on wire, 218 bytes captured)
Ethernet II, Src: 84:78:ac:0c:7b:46 (84:78:ac:0c:7b:46), Dst: 01:00:0c:cc:cc:cc (01:00:0c:cc:cc:cc)
802.1Q Virtual LAN, PRI: 5, CFI: 0, ID: 14
Logical-Link Control
Cisco Discovery Protocol
Version: 2
TTL: 180 seconds
Checksum: 0x870c [correct]
Device ID: SPINE-1(REDACTED)
Port ID: Ethernet4/47
Our sup_src_if hex value is 0x04, so the equivalent decimal value would be 4:
LEAF-30# show platform fwm info pif all | include "sup_src_dst_if 4"
Eth1/4 pd: sup_src_dst_if 4 lif_blk -1--1
Sure enough, CDP reports that it knows of a Nexus 7004 switch named SPINE-1 on Ethernet1/4:
LEAF-30# show cdp neighbors interface Ethernet1/4
Capability Codes: R - Router, T - Trans-Bridge, B - Source-Route-Bridge
S - Switch, H - Host, I - IGMP, r - Repeater,
V - VoIP-Phone, D - Remotely-Managed-Device,
s - Supports-STP-Dispute
Device-ID Local Intrfce Hldtme Capability Platform Port ID
SPINE-1(REDACTED)
Eth1/4 141 R S I s N7K-C7004 Eth4/47
I hope this helps - please let me know if you have any questions!
Thank you!
-Christopher
