All routers supporting OSPF control plane extensions must advertise support of the new algorithm ‟Backup-constrained-SPF” by setting the value of the Algorithm bit to 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: OSPF control plane extension fields lists OSPF control plane extension flag values.
| 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: OSPF control plane extension flags describes 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 RFC 8661. |
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:
|