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MPLS and IP Routing
problem with using routes summarized by OSPF ABRs is that route summarization can
prevent label mappings from being propagated for the links internal to the area being
summarized, since a LSR will typically only propagate labels for FECs that exactly
match a routing table entry.
LSP Precedence and Interaction
LSPs can be LDP or RSVP-TE based, and are either direct or indirect with respect to a
given set. RSVP-TE based LSPs are preferred over LDP LSPs and direct LSPs are
preferred over indirect LSPs. Routed IP traffic always flows over an LSP, if one is
available. Therefore, if an LSP is established or torn down, routed IP traffic may flow
over a more preferred or next best LSP, respectively. These changes take place whenever
there is an OSPF routing topology change, LDP label advertisement event, or RSVP-TE
signaling action.
Traffic is never load-shared across LSPs over different types. For example, if multiple
LSPs exist for an FEC that has one RSVP-TE LSP and four equal-cost LDP LSPs, all IP
routed traffic for the FEC flows across the single RSVP-TE LSP (not load-shared among
the five active LSPs). If the RSVP-TE LSP is torn down, the IP routed traffic is then
load-shared across the four remaining equal-cost LDP LSPs.
Equal Cost LSPs
Traditional IP routers provide session-level IP traffic load distribution across equal cost
routed paths. When MPLS is enabled, multiple equal cost routed paths may result in
multiple active LSPs for a given FEC. If a label binding for the FEC exists, only the
multipath route entries for the FEC that have a label binding are included in the load
distribution forwarding table for the FEC. Thus, load distribution of IP traffic is
performed over MPLS LSPs or over traditional IP routed paths, but not both
simultaneously. MPLS LSPs are always preferred over IP routed paths.
The MPLS module supports distributing IP traffic to an FEC across a maximum of four
LSPs. If more than four LSPs are available for an FEC, only four LSP are used. Ingress
IP traffic is load-balanced across multiple LSPs using a hashing algorithm based on the
IP addresses of the packet. The IP hash algorithm is based on a hash of the source and
destination IP addresses. LSR traffic is load balanced across multiple LSPs using a hash
algorithm based only on the label stack and MAC addresses of the packet. The label
stack and MAC address hash algorithm is based on a hash of the source and destination
MAC addresses and the label stack.
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MPLS Module Installation and User Guide
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