OSPF Control Plane Extensions

All routers supporting OSPF control plane extensions must advertise support of the new algorithm ‟Backup-constrained-SPF” of value 2 in the SR-Algorithm TLV, which is advertised in the Router Information Opaque LSA. This is in addition to the default supported algorithm ‟IGP-metric-based-SPF” of value 0. The following shows the encoding of the prefix SID sub-TLV to indicate a node SID of type backup and to indicate the modified SPF algorithm in the SR Algorithm field. The values used in the Flags field and in the Algorithm field are SR OS proprietary.

The new Algorithm (0x2) field and values are used by this feature.

0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flags     |   Reserved    |      MT-ID    |Algorithm (0x2)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     SID/Index/Label (variable)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Table 1 lists OSPF control plane extension flag values.

Table 1. OSPF Control Plane Extension Fields
Field	Value
Type	2
Length	variable
Flags	1 octet field

The following flags are defined; the ‟B” flag is new:

     0  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |  |NP|M |E |V |L | B|  |
   +--+--+--+--+--+--+--+--+

Table 2 describes OSPF control plane extension flags.

Table 2. OSPF Control Plane Extension Flags
Flag	Description
NP-Flag	No-PHP flag If set, the penultimate hop must not pop the prefix SID before delivering the packet to the node that advertised the prefix SID.
M-Flag	Mapping Server Flag If set, the SID is advertised from the Segment Routing Mapping Server functionality as described in I-D.filsfils-spring-segment-routing-ldp-interop.
E-Flag	Explicit-Null Flag If set, any upstream neighbor of the prefix SID originator must replace the prefix SID with a prefix SID having an Explicit-NULL value (0 for IPv4) before forwarding the packet.
V-Flag	Value/Index Flag If set, the prefix SID carries an absolute value. If not set, the prefix SID carries an index.
L-Flag	Local/Global Flag If set, the value/index carried by the prefix SID has local significance. If not set, then the value/index carried by this sub-TLV has global significance.
B-Flag	This flag is used by the Protection using backup node SID feature. If set, the SID is a backup SID for the prefix. This value is SR OS proprietary.
Other bits	Reserved These must be zero when sent and are ignored when received.
MT-ID	Multi-Topology ID, as defined in RFC 4915.
Algorithm	One octet identifying the algorithm the prefix SID is associated with. A value of (0x2) indicates the modified SPF algorithm, which removes from the topology the node that is backed up by the backup node SID. This value is SR OS proprietary.
SID/Index/Label	Based on the V and L flags, it contains either: a 32-bit index defining the offset in the SID/Label space advertised by this router a 24-bit label where the 20 rightmost bits are used for encoding the label value