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Segment-Routing with ISIS. Where to place Mapping server? how to prefer certain mapping server

LAB.PNG

 

In above topology area 1 and area 0 is SR-enabled but area 2 is LDP only. I leaking both PE's loopbck IP into area 1/2. I am using mapping server to assign and advertise prefix-sid for 10.10.10.10/32 ( Loop back of PE-10). From lab experience my conclusion is as follows.

  1. sid-prefix-map can not be propagated from Level-1 to Level-2 as ABR will strip off SID-Binding when it will generate L2-LSP from L1 LSP.
  2. sid-prefix-map can not be propagated from Level-2 to Level-1 by route-leaking. Route-leak will propagate the route but not the SID-Binding

Am I right ?

where to Place Mapping server ? --- mapping server in each level/area ?

If we have multiple mapping server with different prefix-sid mapping which will one be preferred

 

I have documented the above lab. Attaching it for reference

 

1 ACCEPTED SOLUTION

Accepted Solutions
Highlighted
Cisco Employee

Hi Samrjit,

 

Mapping Server or normal router prefix to SID mappings should propagate through out the ISIS domain across multiple levels.

In case of multiple mapping servers highest preference value (0-255) is preferred. Mapping server can be placed anywhere in the IGP SR domain.

 

 check if you have the domain-wide keyword

          segment-routing prefix-sid-map advertise-local [domain-wide]

 

I have pasted reference text from relevent IETF drafts. Can you test this in your lab?

 

https://tools.ietf.org/html/draft-ietf-spring-segment-routing-ldp-interop-15#page-11
3.2.3.  Interoperability of Multiple SRMSes and Prefix-SID advertisements

   In the case of SR/LDP interoperability through the use of a SRMS,
   mappings are advertised by one or more SRMS.

   SRMS function is implemented in the link-state protocol (such as IS-
   IS and OSPF).  Link-state protocols allow propagation of updates
   across area boundaries and therefore SRMS advertisements are
   propagated through the usual inter-area advertisement procedures in
   link-state protocols.

   Multiple SRMSs can be provisioned in a network for redundancy.
   Moreover, a preference mechanism may also be used among SRMSs so to
   deploy a primary/secondary SRMS scheme allowing controlled
   modification or migration of SIDs.

   The content of SRMS advertisement (i.e.: mappings) are a matter of
   local policy determined by the operator.  When multiple SRMSs are
   active, it is necessary that the information (mappings) advertised by
   the different SRMSs is aligned and consistent.  The following
   mechanism is applied to determine the preference of SRMS
   advertisements:

   If a node acts as an SRMS, it MAY advertise a preference to be
   associated with all SRMS SID advertisements sent by that node.  The
   means of advertising the preference is defined in the protocol
   specific drafts e.g.,[I-D.ietf-isis-segment-routing-extensions] ,
   [I-D.ietf-ospf-segment-routing-extensions], and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions].  The preference
   value is an unsigned 8 bit integer with the following properties:

      0 - Reserved value indicating advertisements from that node MUST
      NOT be used.

      1 - 255 Preference value (255 is most preferred)

   Advertisement of a preference value is optional.  Nodes which do not
   advertise a preference value are assigned a preference value of 128.

====================

https://datatracker.ietf.org/doc/draft-ietf-isis-segment-routing-extensions/?include_text=1

2.1.2.  Prefix-SID Propagation

   The Prefix-SID sub-TLV MUST be preserved when the IP Reachability TLV
   gets propagated across level boundaries.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels MUST set the R-flag.

   If the Prefix-SID contains a global index (L and V flags unset) and
   it is propagated as such (with L and V flags unset), the value of the
   index MUST be preserved when propagated between levels.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels MAY change the setting of the L and V flags in case a local
   label value is encoded in the Prefix-SID instead of the received
   value.



2.4.  SID/Label Binding TLV

   The SID/Label Binding TLV MAY be originated by any router in an IS-IS
   domain.
   <SNIP>
      S-Flag: If set, the SID/Label Binding TLV SHOULD be flooded across
      the entire routing domain.  If the S flag is not set, the SID/
      Label Binding TLV MUST NOT be leaked between levels.  This bit
      MUST NOT be altered during the TLV leaking.

      D-Flag: when the SID/Label Binding TLV is leaked from level-2 to
      level-1, the D bit MUST be set.  Otherwise, this bit MUST be
      clear.  SID/Label Binding TLVs with the D bit set MUST NOT be
      leaked from level-1 to level-2.  This is to prevent TLV looping
      across levels.

      A-Flag: Attached flag.  The originator of the SID/Label Binding
      TLV MAY set the A bit in order to signal that the prefixes and
      SIDs advertised in the SID/Label Binding TLV are directly
      connected to their originators.  The mechanisms through which the
      originator of the SID/Label Binding TLV can figure out if a prefix
      is attached or not are outside the scope of this document (e.g.:
      through explicit configuration).  If the Binding TLV is leaked to
      other areas/levels the A-flag MUST be cleared.


2.4.4.  Mapping Server Prefix-SID

   The Prefix-SID sub-TLV (suggested value 3) is defined in Section 2.1
   and contains the SID/index/label value associated with the prefix and
   range.  The Prefix-SID SubTLV MUST be present in the SID/Label
   Binding TLV unless the M-flag is set in the Flags field of the parent
   TLV.

   A node receiving an SRMS entry for a prefix MUST check the existence
   of such prefix in its link-state database prior to consider and use
   the associated SID.

 

View solution in original post

2 REPLIES 2
Highlighted
Cisco Employee

Hi Samrjit,

 

Mapping Server or normal router prefix to SID mappings should propagate through out the ISIS domain across multiple levels.

In case of multiple mapping servers highest preference value (0-255) is preferred. Mapping server can be placed anywhere in the IGP SR domain.

 

 check if you have the domain-wide keyword

          segment-routing prefix-sid-map advertise-local [domain-wide]

 

I have pasted reference text from relevent IETF drafts. Can you test this in your lab?

 

https://tools.ietf.org/html/draft-ietf-spring-segment-routing-ldp-interop-15#page-11
3.2.3.  Interoperability of Multiple SRMSes and Prefix-SID advertisements

   In the case of SR/LDP interoperability through the use of a SRMS,
   mappings are advertised by one or more SRMS.

   SRMS function is implemented in the link-state protocol (such as IS-
   IS and OSPF).  Link-state protocols allow propagation of updates
   across area boundaries and therefore SRMS advertisements are
   propagated through the usual inter-area advertisement procedures in
   link-state protocols.

   Multiple SRMSs can be provisioned in a network for redundancy.
   Moreover, a preference mechanism may also be used among SRMSs so to
   deploy a primary/secondary SRMS scheme allowing controlled
   modification or migration of SIDs.

   The content of SRMS advertisement (i.e.: mappings) are a matter of
   local policy determined by the operator.  When multiple SRMSs are
   active, it is necessary that the information (mappings) advertised by
   the different SRMSs is aligned and consistent.  The following
   mechanism is applied to determine the preference of SRMS
   advertisements:

   If a node acts as an SRMS, it MAY advertise a preference to be
   associated with all SRMS SID advertisements sent by that node.  The
   means of advertising the preference is defined in the protocol
   specific drafts e.g.,[I-D.ietf-isis-segment-routing-extensions] ,
   [I-D.ietf-ospf-segment-routing-extensions], and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions].  The preference
   value is an unsigned 8 bit integer with the following properties:

      0 - Reserved value indicating advertisements from that node MUST
      NOT be used.

      1 - 255 Preference value (255 is most preferred)

   Advertisement of a preference value is optional.  Nodes which do not
   advertise a preference value are assigned a preference value of 128.

====================

https://datatracker.ietf.org/doc/draft-ietf-isis-segment-routing-extensions/?include_text=1

2.1.2.  Prefix-SID Propagation

   The Prefix-SID sub-TLV MUST be preserved when the IP Reachability TLV
   gets propagated across level boundaries.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels MUST set the R-flag.

   If the Prefix-SID contains a global index (L and V flags unset) and
   it is propagated as such (with L and V flags unset), the value of the
   index MUST be preserved when propagated between levels.

   The level-1-2 router that propagates the Prefix-SID sub-TLV between
   levels MAY change the setting of the L and V flags in case a local
   label value is encoded in the Prefix-SID instead of the received
   value.



2.4.  SID/Label Binding TLV

   The SID/Label Binding TLV MAY be originated by any router in an IS-IS
   domain.
   <SNIP>
      S-Flag: If set, the SID/Label Binding TLV SHOULD be flooded across
      the entire routing domain.  If the S flag is not set, the SID/
      Label Binding TLV MUST NOT be leaked between levels.  This bit
      MUST NOT be altered during the TLV leaking.

      D-Flag: when the SID/Label Binding TLV is leaked from level-2 to
      level-1, the D bit MUST be set.  Otherwise, this bit MUST be
      clear.  SID/Label Binding TLVs with the D bit set MUST NOT be
      leaked from level-1 to level-2.  This is to prevent TLV looping
      across levels.

      A-Flag: Attached flag.  The originator of the SID/Label Binding
      TLV MAY set the A bit in order to signal that the prefixes and
      SIDs advertised in the SID/Label Binding TLV are directly
      connected to their originators.  The mechanisms through which the
      originator of the SID/Label Binding TLV can figure out if a prefix
      is attached or not are outside the scope of this document (e.g.:
      through explicit configuration).  If the Binding TLV is leaked to
      other areas/levels the A-flag MUST be cleared.


2.4.4.  Mapping Server Prefix-SID

   The Prefix-SID sub-TLV (suggested value 3) is defined in Section 2.1
   and contains the SID/index/label value associated with the prefix and
   range.  The Prefix-SID SubTLV MUST be present in the SID/Label
   Binding TLV unless the M-flag is set in the Flags field of the parent
   TLV.

   A node receiving an SRMS entry for a prefix MUST check the existence
   of such prefix in its link-state database prior to consider and use
   the associated SID.

 

View solution in original post

Highlighted

The domain-wide keyword is available in XE but what about XR? I don't see an equivalent command

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