[nsp] Antwort: Re: [MPLS-OPS]: traceroute question

[Yves at Fauser.de]

1- Quote : I read this in the Cisco Pepelnjak / Guichard Book "MPLS and 
VPN Architektures" in the Note on Page 41. 
---- SNIP -----
Pelnultimate Hop Popping is used only for directly connected subnets or 
aggregate routes. In the case of directly connected interface, a Layer3 
lookup is necessary to obtain the correct next-hop information for a 
packet that is sent toward a directly connected destination. If the prefix 
is an aggregate, a Layer3 lookup also is necessary to find a more specific 
route that then is used to route the packet toward its correct 
destination. In all other cases, the Layer2 outbound packet information is 
available within the LFIB and, therfore, a Layer3 lookup is not necessary 
and the packet can be label switched."
---- SNIP ----
I did not find any other dokument yet, which states the same. Therefore 
this may be cisco specific.

2- Quote : Your right in your doubt, I thought the same. I thought maybe 
this is because PE2 has an LFIB entry, maybe it switches the packet 
directly outbound to the FireWall without doing any lookup as stated in 
Quote 1 by pepelnjak. But anyhow I would still expect to see the "TTL 
Exceed" from the P Device also.

3 -- 
PE2#sh tag forwarding-table tags 4731 detail
Local  Outgoing    Prefix              Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id        switched   interface
4731   Untagged    xxx.xxx.211.0/26    13571135   GE2/2.200 xxx.xxx.244.14
        MAC/Encaps=0/0, MTU=1504, Tag Stack{}
    Per-packet load-sharing

PE2#sh ip cef xxx.xxx.211.0 int
134.247.211.0/26, version 732, epoch 0, cached adjacency 134.247.244.14
0 packets, 0 bytes
  tag information set, unshareable
    local tag: 4731
  via xxx.xxx.244.14, 0 dependencies, recursive
    next hop xxx.xxx.244.14, GE-WAN2/2.200 via xxx.xxx.244.14/32
    valid cached adjacency
    tag rewrite with GE2/2.200, xxx.xxx.244.14, tags imposed: {}

I only x out the first 2 Bytes, the rest came directly from the device 
(PE2).

Cheers, Yves





"M. ELK" <elkou141061 at hotmail.com>
18.02.2003 07:54

 
        An:     Yves at Fauser.de, mpls-ops at mplsrc.com
        Kopie:  cisco-nsp at puck.nether.net
        Thema:  Re: [MPLS-OPS]: traceroute question


Yves

1- Quote
The external network is learned via static (E2) from PE2. Since the prefix 

is learned via static, there is no PHP on the P device
Unquote

Is it an observation specific to the setup U are using or it is
according to well know rule .
If the later , pls let me know the reference .

2- quote
this, since PE2 is sending the "TTL exceeded" message up the LSP to the
FireWall, which sends it back, as described in RFCs and in various books.
Unquote

the LSP end at PE2 . The PE2 is the edge of the MPLS domain and not the
Firewall .

3- Assume the prefix of the external netw is 10.0.0/8 ,
    PE2 advertise label L1 to P for such prefix .
    On PE2 , What is the action associated with incoming label L1 ?
    In other word ,what is the output of "sh tag f tags L1 details" .

Brgds





>From: Yves at Fauser.de
>To: mpls-ops at mplsrc.com
>CC: cisco-nsp at puck.nether.net
>Subject: [MPLS-OPS]: traceroute question
>Date: Mon, 17 Feb 2003 21:50:21 +0100
>
>Hi Folks,
>
>I have a question which might be an implementation specific issue, or an
>misunderstanding of some concepts from my part. here's the setup : - We
>have a small MPLS Network with 25 PEs and 5 Ps. All of them are Cisco7600
>(Catalyst6500) with 4-GE-WAN OSMs. We are still in the migration phase, 
so
>our Global-Routing-Table is still used for a bunch of prefixes. We use
>OSPF in a single area as IGP and TDP.
>
>Test WS  ---- PE1  -----  P  ----- PE2 ----|Sniffer|----- FireWall -----
>External Network
>
>The external network is learned via static (E2) from PE2. Since the 
prefix
>is learned via static, there is no PHP on the P device.
>Now if we do a traceroute from the Test WS (Global RT) to a host in the
>external network, we get an answer from every hop. What I don't 
understand
>is what we see in the Sniffer trace.
>In the trace the first thing we see is an "ICMP time exceeded" sourced by
>PE2 and send to the FireWall with the destination "Test WS". I understand
>this, since PE2 is sending the "TTL exceeded" message up the LSP to the
>FireWall, which sends it back, as described in RFCs and in various books.
>What I don't get is why we don't see the "ICMP Time Exceeded" send from
>the P device. If I understood the concepts right, we should see this in
>the trace, but we don't. Unfortunatly I didn't get the chance to do a
>Sniffer trace between the P and PE2 (lack of a Gigabit-Sniffer). Since we
>get a response from the P device in the traceroute output, either PE2 or 
P
>sends the "ICMP TTL Exceed" from the P device back to the workstation.
>
>So my main question is : - Should we see a "TTL Exceeded" with a source
>address of the P device and the workstation as destination in the Sniffer
>trace, or did I miss something in the concept.
>
>Thanks, Yves


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