SRv6 Node Roles

Each node along the SRv6 packet path has a different functionality:

• Source node—A node that can generate an IPv6 packet with an SRH (an SRv6 packet), or an ingress node that can impose an SRH on an IPv6 packet.

• Transit node—A node along the path of the SRv6 packet (IPv6 packet and SRH). The transit node does not inspect the SRH. The destination address of the IPv6 packet does not correspond to the transit node.

• Endpoint node—A node in the SRv6 domain where the SRv6 segment is terminated. The destination address of the IPv6 packet with an SRH corresponds to the end point node. The segment endpoint node executes the function bound to the SID

SRv6 Head-End Behaviors

The SR Headend with Encapsulation behaviors are documented in the IETF RFC 8986 SRv6 Network Programming.

The SR Headend with Insertion head-end behaviors are documented in the following IETF draft:

https://datatracker.ietf.org/doc/draft-filsfils-spring-srv6-net-pgm-insertion/

This section describes a set of SR Policy headend behaviors. The following list summarizes them:

• H.Encaps—SR Headend Behavior with Encapsulation in an SRv6 Policy

• H.Encaps.Red—H.Encaps with Reduced Encapsulation

• H.Insert—SR Headend with insertion of an SRv6 Policy

• H.Insert.Red—H.Insert with reduced insertion

SRv6 Endpoint Behaviors

The SRv6 endpoint behaviors are documented in the IETF RFC 8986 SRv6 Network Programming.

The following is a subset of defined SRv6 endpoint behaviors that can be associated with a SID.

• End—Endpoint function. The SRv6 instantiation of a Prefix SID [RFC8402].

• End.X—Endpoint with Layer-3 cross-connect. The SRv6 instantiation of an Adj SID [RFC8402].

• End.DX6—Endpoint with decapsulation and IPv6 cross-connect (IPv6-L3VPN - equivalent to per-CE VPN label).

• End.DX4—Endpoint with decapsulation and IPv4 cross-connect (IPv4-L3VPN - equivalent to per-CE VPN label).

• End.DT6—Endpoint with decapsulation and IPv6 table lookup (IPv6-L3VPN - equivalent to per-VRF VPN label).

• End.DT4—Endpoint with decapsulation and IPv4 table lookup (IPv4-L3VPN - equivalent to per-VRF VPN label).

• End.DT46—Endpoint with decapsulation and specific IP table lookup (IP-L3VPN - equivalent to per-VRF VPN label).

• End.DX2—Endpoint with decapsulation and L2 cross-connect (L2VPN use-case).

• End.B6.Encaps—Endpoint bound to an SRv6 policy with encapsulation. SRv6 instantiation of a Binding SID.

• End.B6.Encaps.RED—End.B6.Encaps with reduced SRH. SRv6 instantiation of a Binding SID.

SRv6 Endpoint Behavior Variants

Depending on how the SRH is handled, different behavior variants are defined for the End and End.X behaviors. The End and End.X behaviors can support these variants, either individually or in combinations.

• Penultimate Segment Pop (PSP) of the SRH variant—An SR Segment Endpoint Nodes receive the IPv6 packet with the Destination Address field of the IPv6 Header equal to its SID address.

  A penultimate SR Segment Endpoint Node is one that, as part of the SID processing, copies the last SID from the SRH into the IPv6 Destination Address and decrements the Segments Left value from one to zero.

  The PSP operation takes place only at a penultimate SR Segment Endpoint Node and does not happen at non-penultimate endpoint nodes. When a SID of PSP-flavor is processed at a non-penultimate SR Segment Endpoint Node, the PSP behavior is not performed since Segments Left would not be zero.

  The SR Segment Endpoint Nodes advertise the SIDs instantiated on them via control plane protocols. A PSP-flavored SID is used by the Source SR Node when it needs to instruct the penultimate SR Segment Endpoint Node listed in the SRH to remove the SRH from the IPv6 header.

• Ultimate Segment Pop (USP) of the SRH variant—The SRH processing of the End and End.X behaviors are modified as follows:

  If Segments Left is 0, then:

  1. Update the Next Header field in the preceding header to the Next Header value of the SRH

  2. Decrease the IPv6 header Payload Length by 8*(Hdr Ext Len+1)

  3. Remove the SRH from the IPv6 extension header chain

  4. Proceed to process the next header in the packet

  One of the applications of the USP flavor is when a packet with an SRH is destined to an application on hosts with smartNICs implementing SRv6. The USP flavor is used to remove the consumed SRH from the extension header chain before sending the packet to the host.

• Ultimate Segment Decapsulation (USD) variant—The Upper-layer header processing of the End and End.X behaviors are modified as follows:

  • End behavior: If the Upper-layer Header type is 41 (IPv6), then:

    1. Remove the outer IPv6 Header with all its extension headers

    2. Submit the packet to the egress IPv6 FIB lookup and transmission to the new destination

    3. Else, if the Upper-layer Header type is 4 (IPv4)

    4. Remove the outer IPv6 Header with all its extension headers

    5. Submit the packet to the egress IPv4 FIB lookup and transmission to the new destination

    6. Else, process as per Section 4.1.1 (Upper-Layer Header) of IETF RFC 8986 SRv6 Network Programming

  • End.X behavior: If the Upper-layer Header type is 41 (IPv6) or 4 (IPv4), then:

    1. Remove the outer IPv6 Header with all its extension headers

    2. Forward the exposed IP packet to the L3 adjacency J

    3. Else, process as per Section 4.1.1 (Upper-Layer Header) of IETF RFC 8986 SRv6 Network Programming

  One of the applications of the USD flavor is the case of TI-LFA in P routers with encapsulation with H.Encaps. The USD flavor allows the last Segment Endpoint Node in the repair path list to decapsulate the IPv6 header added at the TI-LFA Point of Local Repair and forward the inner packet.

Usage Guidelines and Limitations

General Guidelines and Limitations

• Cisco IOS XR supports the following SRv6 SID behaviors and variants:

  • END with PSP

  • END.X with PSP

  • END.DT4

  • END.DT6

• SRv6 Underlay support includes:

  • IGP redistribution/leaking between levels

  • Prefix Summarization on ABR routers

  • IS-IS TI-LFA

  • Microloop Avoidance

  • Flex-algo

Configuring SRv6

To enable SRv6 globally, you should first configure a locator with its prefix. The IS-IS protocol announces the locator prefix in IPv6 network and SRv6 applications (like ISIS, BGP) use it to allocate SIDs.

The following usage guidelines and restrictions apply while configuring SRv6.

• All routers in the SRv6 domain should have the same SID block (network designator) in their locator.

• The locator length should be 64-bits long.

  • The SID block portion (MSBs) cannot exceed 40 bits. If this value is less than 40 bits, user should use a pattern of zeros as a filler.

  • The Node Id portion (LSBs) cannot exceed 24 bits.

• You can configure up to 8 locators to support SRv6 Flexible Algorithm. All locators prefix must share the same SID block (first 40-bits).

Router(config)# segment-routing srv6
Router(config-srv6)# locators
Router(config-srv6-locators)# locator myLoc1
Router(config-srv6-locator)# prefix 2001:db8:0:a2::/64

Router(config)# segment-routing srv6
Router(config-srv6)# encapsulation source-address 1::1
Router(config-srv6)# hop-limit 60

Router(config)# segment-routing srv6
Router(config-srv6)# logging locator status

Verifying SRv6 Manager

This example shows how to verify the overall SRv6 state from SRv6 Manager point of view. The output displays parameters in use, summary information, and platform specific capabilities.

Router# show segment-routing srv6 manager 
Parameters:
  Parameters:
  SRv6 Enabled: Yes
  SRv6 Operational Mode:
    Base:
      SID Base Block: 2001:db8::/40
  Encapsulation:
    Source Address:
      Configured: 1::1
      Default: 5::5
    Hop-Limit: Default 
    Traffic-class: Default
Summary:
  Number of Locators: 1 (1 operational)
  Number of SIDs: 4 (0 stale)
  Max SIDs: 64000
  OOR:
    Thresholds: Green 3200, Warning 1920
    Status: Resource Available
        History: (0 cleared, 0 warnings, 0 full)
    Block 2001:db8:0:a2::/64:
        Number of SIDs free: 65470
        Max SIDs: 65470
        Thresholds: Green 3274, Warning 1965
        Status: Resource Available
            History: (0 cleared, 0 warnings, 0 full)
Platform Capabilities:
  SRv6: Yes
  TILFA: Yes
  Microloop-Avoidance: Yes
  Endpoint behaviors:
    End (PSP)
    End.X (PSP)
    End.DX6
    End.DX4
    End.DT6
    End.DT4
    End.DX2
    uN (PSP/USD)
    uA (PSP/USD)
    uDT6
    uDT4
    uDX2
    uB6 (Insert.Red)
  Headend behaviors:
    T
    H.Insert.Red
    H.Encaps.Red
  Security rules:
    SEC-1
    SEC-2
    SEC-3
  Counters:
    CNT-1
    CNT-3
  Signaled parameters:
    Max-SL          : 3
    Max-End-Pop-SRH : 3
    Max-H-Insert    : 3 sids
    Max-H-Encap     : 3 sids
    Max-End-D       : 4
  Configurable parameters (under srv6):
    Encapsulation:
      Source Address: Yes
      Hop-Limit     : value=Yes, propagate=No
      Traffic-class : value=Yes, propagate=Yes
  Max SIDs: 64000
  SID Holdtime: 3 mins

Verifying SRv6 Locator

This example shows how to verify the locator configuration and its operational status.

Router# show segment-routing srv6 locator myLoc1 detail 
Name                 ID      Prefix                   Status
-------------------- ------- ------------------------ -------
myLoc1*              5       2001:db8:0:a2::/64       Up
  (*): is-default
  Interface:
    Name: srv6-myLoc1 
    IFH : 0x00000170
    IPv6 address: 2001:db8:0:a2::/64 
  Chkpt Obj ID: 0x2fc8
  Created: Apr 25 06:21:57.077 (00:03:37 ago)

Verifying SRv6 Local SIDs

This example shows how to verify the allocation of SRv6 local SIDs off locator(s).

Router# show segment-routing srv6 locator myLoc1 sid

SID                         Function     Context                           Owner               State  RW
--------------------------  -----------  ------------------------------    ------------------  -----  --
2001:db8:0:a2:1::           End (PSP)    'default':1                       sidmgr              InUse  Y
2001:db8:0:a2:40::          End.DT4      'VRF1'                            bgp-100             InUse  Y
2001:db8:0:a2:41::          End.X (PSP)  [Hu0/1/0/1, Link-Local]           isis-srv6           InUse  Y

The following example shows how to display detail information regarding an allocated SRv6 local SID.

Router# show segment-routing srv6 locator myLoc1 sid 2001:db8:0:a2:40:: detail

SID                         Function     Context                           Owner               State  RW
--------------------------  -----------  ------------------------------    ------------------  -----  --
2001:db8:0:a2:40::          End.DT4      'VRF1'                            bgp-100             InUse  Y
  SID context: { table-id=0xe0000011 ('VRF1':IPv4/Unicast) }
  Locator: myLoc1'
  Allocation type: Dynamic
  Created: Feb  1 14:04:02.901 (3d00h ago)

Similarly, you can display SID information across locators by using the show segment-routing sid command.

show Commands

You can use the following show commands to verify the SRv6 global and locator configuration: