Hello everyone,
This post will introduce the MPLS problem.
Topic
1. As shown in the figure, when 4 routers use OSPF for interconnection and interconnection, the MPLS enterprise network LSP tunnel is established, but the establishment of the LSP tunnel is found to fail, then what are the factors that affect the establishment of the MPLS public network LSP? (At least write 4 points)
2. In the MPLS VPN network, why is the tracert command not applicable (*** is displayed in the middle.)?
What are the factors that affect the establishment of the MPLS public network LSP?
The factors that may affect the establishment of LSP are as follows:
1. By default, only LSPs are established for non-physical interface host routing and check whether the loopback interface address is 32 bits.
2. Check whether LDP protocol packets are filtered by ACL.
3. Check whether the transport address route established by the LDP session is reachable and whether the authentication configuration of the LDP session is correct.
4. Check whether the LDP LSR-ID conflicts
5. Check whether the multi-area scenario has done route summarization for the routes that need to establish LSPs or OSPF external routes
6. Check whether the LSP trigger strategy is correct and whether LSP filtering is configured.
7. Check whether MPLS and LDP are enabled on the interface
In the MPLS VPN network, why is the tracert command not applicable (*** is displayed in the middle.)?
The reason that the tracert command is not applicable is related to the processing mechanism of the TTL value of the packet in the MPLS VPN. In MPLS, there are two processing mechanisms for packet TTL, one is uniform mode and the other is Pipe mode. The mechanism of MPLS's handling of TTL is as follows:
In Uniform mode, the TTL value when adding a label to a packet is to subtract one from the TTL value of the IP packet and copy it to the TTL value of the label. When tracert detects hop by hop, it starts from ttl=1 and proceeds with ttl+1 for each hop. Path tracking, then when receiving a packet with a tag ttl=1, the device will generate a TTL timeout ICMP packet and reply to the source host. At this time, if the message has only one label, then the ICMP message with the TTL timeout will return the packet to the source host by searching the local route by default. At this time, if the local route does not have the route of the source host, an echo will appear **** If the message has two labels, then the ICMP message with TTL timeout will continue to be forwarded along the LSP of the destination address and return to the source host. At this time, there will be no response of ***.
In the scenario of cross-domain option B and option C2, when ASBR directly has one layer of the label, there will be an echo "***" problem. The solution to this problem is as follows:
1. Modify the ttl value processing mode to uniform (uniform is the default mode)
2. Execute the mpls command to enter the MPLS view. (On a cross-domain ASBR device)
3. Execute the command undo ttl expiration pop (use LSP to return ICMP response message)
In Pipe mode, when the message is labeled, the TTL value of the label is decoupled from the TTL of the IP message, and a label with a fixed TTL value is added, which usually defaults to 255, which leads to the failure of the message to be sent on the public network through tracert in the pipe mode The detection of the forwarding path hides the network structure of the public network.
That is all I want to share with you!
