MPLS and RSVP-TE
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MPLS Guide
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Use subject to Terms available at: www.nokia.com
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MPLS assigns a tunnel ID to the SR-TE LSP and a path ID to each new instantiation
of the primary path, as for an RSVP-TE LSP. These IDs represent the MBB path of
the same SR-TE LSP, which must coexist during the update of the primary path.
The router retains full control of the path of the LSP. CSPF is not supported;
therefore, the full or partially explicit path is instantiated as is and no other constraint
(such as SRLG, admin-group, hop-count, or bandwidth) is checked. Only the LSP
path label stack size is checked by MPLS against the maximum value configured for
the LSP after the TE database (TE-DB) hop-to-label translation returns the label
stack. See SR-TE LSP Path Computation for more information about this check.
The ingress LER performs the following steps to resolve the user-entered path
before programming it in the data path:
1. MPLS passes the path information to the TE-DB, which converts the list of hops
into a label stack by scanning the TE-DB for adjacency and node SID
information that belongs to the router or link identified by each hop address. If
the conversion is successful, the TE-DB will return the actual selected hop SIDs
plus labels as well as the configured path hop addresses that were used as the
input for this conversion.
Details of this step are as follows:
- A loose hop with an address matching any interface (loopback or not) of a
router (identified by router ID) is always translated to a node SID. If the
prefix matching the hop address has a node SID in the TE-DB, it will be
selected by preference. If not, the node SID of any loopback interface of the
same router that owns the hop address is selected. In the latter case, the
lowest IP address of that router that has a /32 prefix-SID is selected.
- A strict hop with an address matching any interface (loopback or not) of a
router (identified by router ID) is always translated to an adjacency SID. If
the hop address matches the host address reachable in a local subnet from
the previous hop, the adjacency SID of that adjacency is selected. If the hop
address matches a loopback interface, it is translated to the adjacency SID
of any link from the previous hop that terminates on the router owning the
loopback. The adjacency SID label of the selected link is used.
In both cases, it is possible to have multiple matching previous hops if the
interface is a LAN interface. If there are multiple hops, the adjacency SID
with the lowest interface address is selected.
Note: The concept of MBB is not exactly accurate in the context of an SR-TE LSP because
there is no signaling involved and therefore the new path information immediately overrides
the older one.
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