The MPLS-OPS Archive[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index][Thread Index][Author Index][Subject Index] Antwort: Re: traceroute question
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
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@Fauser.de >To: mpls-ops@mplsrc.com >CC: cisco-nsp@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 _________________________________________________________________ Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail ------- The MPLS-OPS Mailing List Subscribe/Unsubscribe: http://www.mplsrc.com/mplsops.shtml Archive: http://www.mplsrc.com/mpls-ops_archive.shtml |
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