Advertising L2 Bundle Member Link Attributes in IS-IS Cisco Systems
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Routing Area Networking Working Group Sample There are deployments where the Layer 3 interface on which IS-IS operates is a Layer 2 interface bundle. Existing IS-IS advertisements only support advertising link attributes of the Layer 3 interface. If entities external to IS-IS wish to control traffic flows on the individual physical links which comprise the Layer 2 interface bundle link attribute information about the bundle members is required. This document introduces the ability for IS-IS to advertise the link attributes of layer 2 (L2) bundle members. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
Introduction There are deployments where the Layer 3 interface on which an IS-IS adjacency is established is a Layer 2 interface bundle, for instance a Link Aggregation Group (LAG) . This reduces the number of adjacencies which need to be maintained by the routing protocol in cases where there are parallel links between the neighbors. Entities external to IS-IS such as Path Computation Elements (PCE) may wish to control traffic flows on individual members of the underlying Layer 2 bundle. In order to do so link attribute information about individual bundle members is required. The protocol extensions defined in this document provide the means to advertise this information. This document introduces a new TLV to advertise link attribute information for each of the L2 bundle members which comprise the Layer 3 interface on which IS-IS operates. introduces a new link attribute - adjacency segment identifier (Adj-SID) - which can be used as an instruction to forwarding to send traffic over a specific link. This document introduces additional sub-TLVs to advertise Adj-SIDs for L2 Bundle members. Note that the new advertisements defined in this document are intended to be provided to external (to IS-IS) entities. The following items are intentionally not defined and/or are outside the scope of this document:
  • What link attributes will be advertised. This is determined by the needs of the external entities.
  • A minimum or default set of link attributes.
  • How these attributes are configured
  • How the advertisements are used
  • What impact the use of these advertisements may have on traffic flow in the network
  • How the advertisements are passed to external entities
L2 Bundle Member Attributes TLV A new TLV is introduced to advertise L2 Bundle member attributes. Although much of the information is identical to and uses the same sub-TLVs included in Extended IS-Neighbor advertisements (TLVs 22 and 222), a new TLV is used so that changes to the advertisement of the L2 Bundle member link attributes does not trigger unnecessary action by the Decision process. Advertisement of this information implies that the identified link is a member of the L2 Bundle associated with the identified Parent L3 Neighbor and that the member link is operationally up. Therefore advertisements MUST be withdrawn if the link becomes operationally down or it is no longer a member of the identified L2 Bundle. This new TLV utilizes the sub-TLV space defined for TLVs 22, 23, 141, 222, and 223. The following new TLV is introduced:
  • L2 Bundle Member Attributes
    • Type: 25 (suggested - to be assigned by IANA)
    • Length: Number of octets to follow
  • Parent L3 Neighbor Descriptor
    • L3 Neighbor System ID + pseudonode ID (7 octets)
    • Flags: 1 octet field of following flags: where:
      P-flag:
      When set to 1 one of the sub-TLVs described in Section 2.1 immediately follows the flags field. If the P-flag is set to 0, then none of the sub-TLVs described in Section 2.1 are present.
      Other bits:
      MUST be zero when originated and ignored when received.
  • One or more of the following:
    • L2 Bundle Attribute Descriptors
      • Length of L2 Bundle Attribute Descriptor (1 octet)
        • NOTE: This includes all fields described below.
        Number of L2 Bundle Member Descriptors (1 octet) L2 Bundle Member Link Local Identifiers
        • (4 * Number of L2 Bundle Member Descriptors octets)
        • NOTE: An L2 Bundle Member Descriptor is a Link Local Identifier as defined in .
        sub-TLV(s) A sub-TLV may define an attribute common to all of the bundle members listed or a sub-TLV may define an attribute unique to each bundle member. Use of these two classes of sub-TLVs is described in the following sections.
Parallel L3 Adjacencies When there exist multiple L3 adjacencies to the same neighbor additional information is required to uniquely identify the L3 Neighbor. One and only one of the following three sub-TLVs is used to uniquely identify the L3 adjacency:
  • IPv4 Interface Address (sub-TLV 6 defined in )
  • IPv6 Interface Address (sub-TLV 12 defined in )
  • Link Local/Remote Identifiers (sub-TLV 4 defined in )
When the P-bit is set in the flags field in the Parent L3 Neighbor Descriptor one and only one of the above sub-TLVs MUST be present. The chosen sub-TLV MUST immediately follow the flags field described in Section 2. These sub-TLVs MAY be omitted if no parallel adjacencies to the neighbor exist.
Shared Attribute sub-TLVs These sub-TLVs advertise a single copy of an attribute (e.g. link bandwidth). The attribute applies to all of the L2 Bundle Members in the set advertised under the preceding  L2 Bundle Member Attribute Descriptor. No more than one copy of a given sub-TLV in this category may appear in the set of sub-TLVs under the preceding L2 Bundle Member Attribute Descriptor. If multiple copies of a given sub-TLV are present all copies MUST be ignored. The set of L2 Bundle Member Descriptors which may be advertised under a single L2 Bundle Member Attribute Descriptor is therefore limited to bundle members which share the set of attributes advertised in the shared attribute sub-TLVs. All existing sub-TLVs defined in the IANA Sub-TLVs for TLVs 22, 23, 141, 222, and 223 registry are in the category of shared attribute sub-TLVs unless otherwise specified in this document.
Advertising L2 Bundle Member Adj-SIDs defines sub-TLVs to advertise Adj-SIDs for L3 adjacencies. However these sub-TLVs only support a advertisement of a single Adj-SID. As it is expected that each L2 Bundle member will have unique Adj-SIDs in many deployments it is desirable to define a new sub-TLV which allows more efficient encoding of a set of Adj-SIDs in a single sub-TLV. Two new sub-TLVs are therefore introduced to support advertising Adj-SIDs for L2 Bundle members. The format of the new sub-TLVs is similar to that used for L3 adjacencies, but is optimized to allow advertisement of a set of Adj-SIDs (one per L2 Bundle Member) in a single sub-TLV. The two new sub-TLVs defined in the following sections do not fall into the category of shared attribute sub-TLVs.
L2 Bundle Member Adjacency Segment Identifier sub-TLV This sub-TLV is used to advertise Adj-SIDs for L2 Bundle Members associated with a parent L3 adjacency which is Point-to-Point. The following format is defined for this sub-TLV:
  • Type: 41 (suggested value to be assigned by IANA) (1 octet)
  • Length: variable (1 octet)
  • Flags: 1 octet field of following flags:
  • where:
    F-Flag:
    Address-Family flag. If unset, then the Adj-SID refers to an L2 Bundle Member with outgoing IPv4 encapsulation. If set then the Adj-SID refers to an L2 Bundle Member with outgoing IPv6 encapsulation.
    V-Flag:
    Value flag. If set, then the Adj-SID carries a value. By default the flag is SET.
    L-Flag:
    Local Flag. If set, then the value/index carried by the Adj-SID has local significance. By default the flag is SET.
    S-Flag.
    Set Flag. When set, the S-Flag indicates that the Adj-SID refers to a set of L2 Bundle Members (and therefore MAY be assigned to other L2 Bundle Members as well).
    P-Flag.
    Persistent flag. When set, the P-Flag indicates that the Adj-SID is persistently allocated, i.e., the Adj-SID value remains consistent across router restart and/or interface flap.
    Other bits:
    MUST be zero when originated and ignored when received.
Weight:
1 octet. The value represents the weight of the Adj-SID for the purpose of load balancing. The use of the weight is defined in .
L2 Bundle Member Adj-SID Descriptors.
There MUST be one descriptor for each of the L2 Bundle Members advertised under the preceding L2 Bundle Member Attribute Descriptor. Each descriptor consists of one of the following fields:
SID/Index/Label:
according to the V and L flags, it contains either:
  • A 3 octet local label where the 20 rightmost bits are used for encoding the label value. In this case the V and L flags MUST be set.
  • A 4 octet index defining the offset in the SID/Label space advertised by this router. See . In this case V and L flags MUST be unset.
L2 Bundle Member LAN Adjacency Segment Identifier sub-TLV This sub-TLV is used to advertise Adj-SIDs for L2 Bundle Members associated with a parent L3 adjacency which is a LAN adjacency. In LAN subnetworks, the Designated Intermediate System (DIS) is elected and originates the Pseudonode-LSP (PN-LSP) including all neighbors of the DIS. When Segment Routing is used, each router in the LAN MAY advertise the Adj-SID of each of its neighbors on the LAN. Similarly, for each L2 Bundle Member a router MAY advertise an Adj-SID to each neighbor on the LAN. The following format is defined for this sub-TLV: Type: 42 (suggested value to be assigned by IANA) (1 octet) Length: variable (1 octet) Neighbor System ID: 6 octets
  • Flags: 1 octet field of following flags:
  • where:
    F-Flag:
    Address-Family flag. If unset, then the Adj-SID refers to an L2 Bundle Member with outgoing IPv4 encapsulation. If set then the Adj-SID refers to an L2 Bundle Member with outgoing IPv6 encapsulation.
    V-Flag:
    Value flag. If set, then the Adj-SID carries a value. By default the flag is SET.
    L-Flag:
    Local Flag. If set, then the value/index carried by the Adj-SID has local significance. By default the flag is SET.
    S-Flag.
    Set Flag. When set, the S-Flag indicates that the Adj-SID refers to a set of L2 Bundle Members (and therefore MAY be assigned to other L2 Bundle Members as well).
    P-Flag.
    Persistent flag. When set, the P-Flag indicates that the Adj-SID is persistently allocated, i.e., the Adj-SID value remains consistent across router restart and/or interface flap.
    Other bits:
    MUST be zero when originated and ignored when received.
Weight:
1 octet. The value represents the weight of the Adj-SID for the purpose of load balancing. The use of the weight is defined in .
L2 Bundle Member LAN Adj-SID Descriptors.
There MUST be one descriptor for each of the L2 Bundle Members advertised under the preceding L2 Bundle Member Attribute Descriptor. Each descriptor consists of one of the following fields:
SID/Index/Label:
according to the V and L flags, it contains either:
  • A 3 octet local label where the 20 rightmost bits are used for encoding the label value. In this case the V and L flags MUST be set.
  • A 4 octet index defining the offset in the SID/Label space advertised by this router. See . In this case V and L flags MUST be unset.
IANA Considerations This document adds the following new TLV to the IS-IS TLV Codepoints registry.
  • Value: 25 (suggested - to be assigned by IANA)
  • Name: L2 Bundle Member Attributes
The name of the Sub-TLVs for TLVs 22, 23, 141, 222, and 223 registry needs to be changed to Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 registry. An additional column needs to be added to the registry to indicate which sub-TLVs may appear in the new L2 Bundle Member Attributes TLV. The column for TLV 25 has one of the following three values:
y:
sub-TLV may appear in TLV 25 but MUST NOT be shared by multiple L2 Bundle Members
y(s):
sub-TLV may appear in TLV 25 and MAY be shared by multiple L2 Bundle Members
n:
sub-TLV MUST NOT appear in TLV 25
The following table indicates the appropriate settings for all currently defined sub-TLVs as regards their use in the new L2 Bundle Member Attributes TLV. Appropriate Settings for All Defined Sub-TLVs as regards their use in the new L2 Bundle Member Attributes TLV
Type Description 25
3 Administrative group (color) y(s)
4 Link Local/Remote Identifiers y(s)
6 IPv4 interface address y(s)
8 IPv4 neighbor address y(s)
9 Maximum link bandwidth y(s)
10 Maximum reservable link bandwidth y(s)
11 Unreserved bandwidth y(s)
12 IPv6 Interface Address y(s)
13 IPv6 Neighbor Address y(s)
14 Extended Administrative Group y(s)
18 TE Default metric y(s)
19 Link-attributes y(s)
20 Link Protection Type y(s)
21 Interface Switching Capability Descriptor y(s)
22 Bandwidth Constraints y(s)
23 Unconstrained TE LSP Count y(s)
24 Remote AS number n
25 IPv4 remote ASBR Identifier n
26 IPv6 remote ASBR Identifier n
27 Interface Adjustment Capability Descriptor (IACD) y(s)
28 MTU n n
29 SPB-Metric y(s)
30 SPB-A-OALG y(s)
33 Unidirectional Link Delay y
34 Min/Max Unidirectional Link Delay y
35 Unidirectional Delay Variation y
36 Unidirectional Link Loss y
37 Unidirectional Residual Bandwidth y
38 Unidirectional Available Bandwidth y
39 Unidirectional Utilized Bandwidth y
40 RTM Capability n
This document adds the following new sub-TLVs to the sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 registry.
  • Value: 41 (suggested - to be assigned by IANA)
  • Name: L2 Bundle Member Adj-SID
  • This sub-TLV is allowed in the following TLVs:
22 23 25 141 222 223
n n y n n n
  • Value: 42 (suggested to be assigned by IANA)
  • Name: L2 Bundle Member LAN Adj-SID
  • This sub-TLV is allowed in the following TLVs:
22 23 25 141 222 223
n n y n n n
Security Considerations The IS-IS protocol has supported the advertisement of link attribute information, including link identifiers, for many years. The advertisements defined in this document are identical to existing advertisements defined in , , , and - but are associated with L2 links which are part of a bundle interface on which the IS-IS protocol operates. There are therefore no new security issues introduced by the extensions in this document. As always, if the protocol is used in an environment where unauthorized access to the physical links on which IS-IS PDUs are sent occurs then attacks are possible. The use of authentication as defined in and is recommended to prevent such attacks.
Contributors The following people gave a substantial contribution to the content of this document and should be considered as co-authors:
Acknowledgements The authors would like to thank Jon Mitchell for his careful review.
References Normative References Intermediate system to Intermediate system intra-domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode Network Service (ISO 8473) International Organization for Standardization IEEE Standard for Local and Metropolitan Area Networks - Link Aggregation. Institute of Electrical and Electronics Engineers Informational References
Example Encoding Below is an example encoding of L2 Bundle advertisements in a case where we have two parallel adjacencies to the same neighbor whose system-id is 1234.1234.1234.00. The two L2 bundles have the following sets of attributes: L3 Adjacency #1 L3 IPv4 local link address: 192.0.2.1 Four bundle members with the following attributes:
Num Link Local ID Bandwidth Adj-SID/Weight
1 0x11111111 1G 0x11111/1
2 0x11112222 1G 0x11112/1
3 0x11113333 10G 0x11113/1
4 0x11114444 10G 0x11114/1
L3 Adjacency #2 L3 IPv4 local link address: 192.0.2.2 Three bundle members with the following attributes:
Num Link Local ID Bandwidth Adj-SID/Weight
1 0x22221111 10G 22221/1
2 0x22222222 10G 22222/1
3 0x22223333 10G 22223/1
This requires two TLVs, one for each L3 adjacency. TLV for Adjacency #1: Parent L3 Neighbor Descriptor IPv4 Interface Address sub-TLV L2 Bundle Attribute Descriptors Maximum Link Bandwidth sub-TLV L2 Bundle Member Adjacency Segment Identifier sub-TLV L2 Bundle Attribute Descriptors Maximum Link Bandwidth sub-TLV L2 Bundle Member Adjacency Segment Identifier sub-TLV TLV for Adjacency #2 Parent L3 Neighbor Descriptor IPv4 Interface Address sub-TLV L2 Bundle Attribute Descriptors Maximum Link Bandwidth sub-TLV L2 Bundle Member Adjacency Segment Identifier sub-TLV