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Re: BGP-4 Label Encoding

  • From: Rajiv Asati <rajiva@cisco.com>
  • Date: Mon, 15 Jul 2002 14:17:03 -0400
  • Cc: <mpls-ops@mplsrc.com>
  • Resent-Date: Mon, 15 Jul 2002 15:20:48 -0400
  • To: "David Mechsner" <david_mechsner@web.de>
  • X-Sender: rajiva@dingdong.cisco.com

David,

Please see inline..

At 07:05 AM 7/12/2002, David Mechsner wrote:
Hy guys,
 
I'm a student and want to learn how BGP-4 transport MPLS Label values. (RFC 3107 / RFC 2283)
I use the Ethereal  tool 'text2pcap' for generating a sample cature file.
 
Until now I generated a file for this hex code:

0000 00 01 63 44 47 81 00 05 00 f3 c4 1c 88 47 18 6f ................
0010 1d ff 45 c0 ff 3b 1e 9f 00 00 ff 06 fd 1c 0a 00 ................
0020 00 01 0a 00 00 02 00 b3 2a fb 39 7b f7 bd a5 43 ................
0030 e4 50 50 18 3e 38 e7 b1 00 00 ff ff ff ff ff ff ................
0040 ff ff ff ff ff ff ff ff ff ff 00 ff 02 00 00 00 ................
0050 ff 40 01 01 00 40 02 00 40 03 04 0a 00 00 04 c0 ................
0060 0e ff 00 01 04 04 0a 00 00 03 01 01 0a 04 00 01 ................
0070 11 00                                          ................
 

Decoding MP_REACH_NLRI:

c0 0e ff 00 01 04 04 0a 00 00 03 01 01 0a 04 00 01 11 00

c0      =       flag
0e      =       MP_REACH_NLRI
ff      =       Length is 255B
00 01   =       Address-family is IPv4
04      =       SAFI is 4 i.e. MPLS labeled prefix
04      =       length of NEXT_HOP address is 4B
0a 00 00 03
        =       10.0.0.3/32
01      =       SNPA    <<<<=====
01      =       SNPA length
0a 04 00 01 11 00

Curious about the presence of a SNPA field ? How come you have it ?

I have seen that to be typically 00.
is it a cisco router sending this BGP update ?

Would you please send me the complete message (in hex)?

Rajiv

 
This is a BGP UPDATE Message with Path Attributes: ORIGIN, AS_PATH, NEXT_HOP, MP_REACH_NLRI
 
MP_REACH_NLRI includes the NLRI where the label is positioned:
 
+---------------------------+
| Length (1 octet)          |
+---------------------------+
| Label (3 octets)          |
+---------------------------+
.............................
+---------------------------+
| Prefix (variable)         |
+---------------------------+
 
My NLRI is 0x 04 00 01 11 00  but Ethereal doesn't visible that.
 
legth             = 4 octet
label             = 17 (dec)
bottom of stack   = 1
prefix            = 0
 
What do I wrong?
 
Thanks David