Ipv6 dhcp relay and remote identifier (option 37)

[Roger Saugestad]

Hi

I’m working on a lab for prefix delegation on cat 4500, sup7, cat4500e-universalk9.SPA.03.02.01.SG.150-2.SG1.bin.

Prefix delegation works fine, but I’m having problems with decoding the remote-id option inserted by the 4500.

The document “Implementing DHCP for IPv6” (located here: http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/ip6-dhcp.html#wp1054194 )

States “The remote-ID option provides information to the DHCPv6 server, including port information, the system's DUID, and the VLAN ID.”

When capturing dhcp6 relay messages in the network I can find VLAN ID, mac address of the VLAN interface, but have trouble finding slot/port information fields in the message. The remote-id below is from a client attached to int vlan 512, and connected to fa2/37.

Remote-ID: 02002d090200000a00030001c47d4f73a0bf

HEX2DEC     512                                 VLAN ID of incomming DHCP packet on the 4500

HEX                                c47d4f73a0bf MAC address on int vlan 512

HEX2DEC        2                                Slot 2

But I can't find the port information, my guess would be that it's located after slot info, but "0xD09" doesn't make sense to me.

Has someone figured this out or know where to find documentation?

Regards

Roger Saugestad.

[Roger Saugestad]

Hi

To apply some more information on this topic, I have collected remote-id samples on different ports. Results posted below.

                         000000000011111111112222222222333333

                         012345678901234567890123456789012345

Fa2/3     Remote-ID:     020023000200000a00030001c47d4f73a0bf

Fa2/10    Remote-ID:     02002a000200000a00030001c47d4f73a0bf

Fa2/11    Remote-ID:     02002b000200000a00030001c47d4f73a0bf

Fa2/15    Remote-ID:     02002f000200000a00030001c47d4f73a0bf

Fa2/16    Remote-ID:     020020080200000a00030001c47d4f73a0bf

Fa2/17    Remote-ID:     020021080200000a00030001c47d4f73a0bf

Fa2/18    Remote-ID:     020022080200000a00030001c47d4f73a0bf

Fa2/31    Remote-ID:     02002f080200000a00030001c47d4f73a0bf

Fa2/32    Remote-ID:     020028090200000a00030001c47d4f73a0bf

Fa2/37    Remote-ID:     02002d090200000a00030001c47d4f73a0bf

Fa2/39    Remote-ID:     02002f090200000a00030001c47d4f73a0bf

Fa2/40    Remote-ID:     0200280a0200000a00030001c47d4f73a0bf

Field 0 - 3:     Vlan ID

Field 4 - 4:     Module

Field 5 - 8:     Interface

Field 8 - 15:   Unknown (to me)

Field 16 -35:  DUID

If I isolate the part I think is related to interface number the list looks like this:

Interface     Module      Port

Fa2/3     2      3000

Fa2/10    2      a000

Fa2/11    2      b000

Fa2/15    2      f000

Fa2/16    2      0080

Fa2/17    2      1080

Fa2/18    2      2080

Fa2/31    2      f080

Fa2/32    2      8090

Fa2/37    2      d090

Fa2/39    2      f090

Fa2/40    2      80a0

I understand the numbering up to interface "15" == f. But from port 16 I lose track of the logics behind the numbering.

Any help would be appreciated

Regards

Roger Saugestad

[shwethab · Accepted solution]

Hi Roger,

Heres the interpretation of the fields in remote id:

                                     000000000011111111112222222222333333

                                     012345678901234567890123456789012345

Fa2/3     Remote-ID:     020023000200000a00030001c47d4f73a0bf

Fa2/10    Remote-ID:     02002a000200000a00030001c47d4f73a0bf

Fa2/11    Remote-ID:     02002b000200000a00030001c47d4f73a0bf

Fa2/15    Remote-ID:     02002f000200000a00030001c47d4f73a0bf

Fa2/16    Remote-ID:     020020080200000a00030001c47d4f73a0bf

Fa2/17    Remote-ID:     020021080200000a00030001c47d4f73a0bf

Fa2/18    Remote-ID:     020022080200000a00030001c47d4f73a0bf

Fa2/31    Remote-ID:     02002f080200000a00030001c47d4f73a0bf

Fa2/32    Remote-ID:     020028090200000a00030001c47d4f73a0bf

Fa2/37    Remote-ID:     02002d090200000a00030001c47d4f73a0bf

Fa2/39    Remote-ID:     02002f090200000a00030001c47d4f73a0bf

Fa2/40    Remote-ID:     0200280a0200000a00030001c47d4f73a0bf

Field 0 - 3:     Remote ID type - for ethernet it is 2, if type is 2 remaining fields are interpreted as below

Field 4 - 7:     interface information (detailed below)

Field 8 - 11:   Vlan id

Field 12-15:   Length of the DUID that follows

Field 16 -35:  DUID

Field 4-7 consists of 2 bytes that are interpreted as

  4-5 (in binary): - interface_info  -> slot : 4(ssss); module : 1(m); port :3(ppp);
  6-7 (in binary): - extension_info  -> slot : 4(SSSS); module : 1(M); port :3(PPP);

field 4-5: form the lower order information

field 6-7: form the higher order information

slot : field 6  and field 4 are stiched together to form slot (SSSSssss)

module: first bit of field 7 and first bit of field 5 are stiched together (Mm)

port: last 3 bits of field 7 and last 3 bits of 5 are stiched together (PPPppp)

fastethernetX/Y

X = slot

Y = module*8 + port

for e.g. to arrive at fa2/3 = 0x2300 - 2 is the slot, module is 0, port = 3 so it is fa2/3

0x280a -> slot = 2, module = 3, port = 16 , fa 2/ (3*8 + 16) = fa2/40

HTH,

Shwetha