VRF-Aware ICMP Ping and LSP PingTrace Mechanisms


VRF-Aware ICMP Ping and LSP Ping/Trace Mechanisms

VRF-aware ping differs from traditional ping and trace by using the VRF routing table for route lookup, instead of the global routing table, when sending the probe packets. This is mainly used for private addresses. If no source IP address is given, the router uses the first interface that is associated with the VRF.

You can choose a source IP address by using the extended ping command. The specificity of the VRF routing information presents an important consequence: Not all middle routers along the path have such routing information available for the source and destination. Thus, any middle router is unable to respond directly to the VRF interface that sourced the UDP packets. The implementation of VRF-aware ping does not rely on a response from the middle LSRs. In the case of a failure along the path between the PEs, the probe ICMP packet is lost (no response would reach the source) because no ICMP packet can be emitted in response to an error on an ICMP packet.

If a configuration error, such as MPLS not being configured on an outbound interface that results in an improper label pop operation in the middle of the LSP, occurs VRF-aware ping still works because the packet can be delivered and responded to via IP. The operator in this example assumes that the network and corresponding service are still functioning. MPLS LSP ping and trace, discussed in the next section, perform the following tasks: verifying the health of the LSP path, returning detailed information as to the probable defect cause, and further localizing the defect.

MPLS LSP ping/traceroute provides diagnostics and troubleshooting capability for MPLS LSPs. When an LSP fails to deliver traffic, the failure cannot always be detected by the MPLS control plane. For the MPLS data plane verification, as a natural progression, the IP data plane verification tools (that is, ping and traceroute) are extended to work on the MPLS networks. MPLS ping/ traceroute is modeled after the ping/traceroute paradigm. MPLS ping enables the verification of the LSP connectivity and the integrity of the MPLS network.

Ping mode can test the integrity of the connectivity via the verification on the forward equivalence class (FEC) entity between the ping origin and the egress node for this particular FEC. This test is carried out by sending an MPLS echo request along the same data path as other packets belonging to this FEC. When the ping packet reaches the end of the path, it is sent to the control plane of the egress LSR, which then verifies that it is indeed an egress for the FEC. The MPLS echo request contains information about the FEC whose MPLS path is being verified.

MPLS traceroute is used for hop-by-hop fault localization and LSP path tracing. In the trace route LSP verification, the packet is sent to the control plane of each transit LSR, which performs various checks including one that determines whether it is a transit LSR for this path. Furthermore, each transit LSR also returns extra information related to the FEC being tested (such as the label bound to the FEC). This information helps in checking the control plane against the data plane (for example, in checking whether the local forwarding information matches what the routing protocols determined as the path). The traceroute operation is performed via a manipulation on the time to live (TTL) that is decremented to avoid loops.

These tools provide the foundation for the MPLS OAM capabilities and facilitate the operation of the MPLS network.




MPLS and Next-Generation Networks(c) Foundations for NGN and Enterprise Virtualization
MPLS and Next-Generation Networks: Foundations for NGN and Enterprise Virtualization
ISBN: 1587201208
EAN: 2147483647
Year: 2006
Pages: 162

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net