LSR Working Group A. Wang
Internet-Draft China Telecom
Intended status: Standards Track Z. Hu
Expires: February 27, 2021 Huawei Technologies
August 26, 2020

Passive Interface Attribute


This document describes the mechanism that can be used to differentiate the passive interfaces from the normal interfaces within ISIS domain.

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Table of Contents

1. Introduction

Passive interfaces are used commonly within operator or enterprise networks, especially at the boundary of different IGP domains. Using passive interface can keep the address that associated with it is reachable within the domain it belongs to but no other link information is leaked to the other side in another domain.

For operator which runs different IGP domains that interconnect with each other, there is desire that to get the inter-as topology information as that described in [I-D.ietf-idr-bgpls-inter-as-topology-ext]. If the router that run BGP-LS within one IGP domain can distinguish the passive interfaces(also the links between two boundary) from the other normal interfaces, then it is easy for them to report these link in the "Stub Link NLRI" via the BGP-LS.

OSPF has the mechanism as described in [RFC2328] to label the passive interface, but ISIS protocol has no such attribute to label the passive interface.

This document introduces the mechanism that can be used in such situation, to label the passive interface via the newly defined passive interface attribute.

2. Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] .

3. Scenario Description

Figure 1 illustrates the topology scenario when ISIS/OSPF is running in different domain. B1, B3 are border routers within IGP domain A, B2, B4 are border routers within domain B. S1-S4 are the internal routers within domain A, T1-T4 are the internal routers within domain B. The two domain are interconnected via the links between B1/B2 and B3/B4.

Passive interfaces are enabled in the links between B1/B2 and B3/B4 respectively. For domain B, the T1 router that runs OSPF can extract the passives links from the normal links and report it to IP SDN controller via the BGP-LS protocol. But for domain A, the S2 router that runs ISIS can only judge the passive interfaces from other characteristics, such as no IGP neighbor on this link. Such judgement can extract these passive links but it is not exactly, because it covers also the situation when there is some issues to establish the ISIS adjacency but not the passive interface.

The passive interfaces are also often used in the edge router which connects the server, for example in the router S1/S4 and T2/T4 in Figure 1. Knowing these interfaces are correctly configured will also benefit the management of these interfaces.

The method to label these passive interface explicitly is necessary then.

                 +----+IP SDN Controller+----+
                 |    +-----------------+    |
                 |                           |
                 |BGP-LS                     |BGP-LS
                 |                           |
 +---------------+-----+               +-----+--------------+
 | +--+        +-++   ++-+           +-++   +|-+        +--+|
 | |S1+--------+S2+---+B1+-----------+B2+---+T1+--------+T2||
 | +-++   N1   +-++   ++-+           +-++   ++++   N2   +-++|
 |   |           |     |               |     ||           | |
 |   |           |     |               |     ||           | |
 | +-++        +-++   ++-+           +-++   ++++        +-++|
 | |S4+--------+S3+---+B3+-----------+B4+---+T3+--------+T4||
 | +--+        +--+   ++-+           +-++   ++-+        +--+|
 |                     |               |                    |
 |                     |               |                    |
 |  Domain A(ISIS)     |               |  Domain B(OSPF)    |
 +---------------------+               +--------------------+

             Figure 1: Inter-AS Domain Scenarios

4. Passive Interface Attribute

[RFC7794] defines the "IPv4/IPv6 Extended Reachability Attribute Flags" sub-TLV to advertise the additional flags associated with a given prefix advertisement. Currently, only X(Bit 0),R(Bit 1),N(Bit 2), E(Bit 3) flags are defined, here we propose another bit(Bit 4 is desired) to be assigned by the IANA for the passive interface attribute, as illustrated in the following Figure2:

          0 1 2 3 4 5 6 7...
         |X|R|N|E|P       ...
               Figure 2: Prefix Attribute Flags
         P-flag: Passive Flag(Bit 4)
                Set for local interface that is configured as passive interface.

When the interfaces on one router be configured as the passive interface, the P-flag bit will be set in the "IPv4/IPv6 Extended Reachability Attribute Flags" sub-TLV. This sub-TLV will be included in the TLV 135, TLV 235, TLV 236 and TLV 237 as necessary and be flooded within the ISIS domain.

The router receives such advertisement can then easily distinguish the passive interfaces from the normal interface, and report them to the SDN controller if it run the BGP-LS protocol.

5. Security Considerations

Security concerns for ISIS are addressed in [RFC5304] and[RFC5310]

Advertisement of the additional information defined in this document introduces no new security concerns.

6. IANA Considerations

IANA is requested to allocate the P-bit (bit position 4 is desired) from the "Bit Values for Prefix Attribute Flags Sub-TLV" registry.

7. Acknowledgement

Thanks Shunwan Zhang, Tony Li, Les Ginsberg and Robert Raszuk for their suggestions and comments on this idea.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication", RFC 5304, DOI 10.17487/RFC5304, October 2008.
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R. and M. Fanto, "IS-IS Generic Cryptographic Authentication", RFC 5310, DOI 10.17487/RFC5310, February 2009.
[RFC7794] Ginsberg, L., Decraene, B., Previdi, S., Xu, X. and U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794, March 2016.

8.2. Informative References

[I-D.ietf-idr-bgpls-inter-as-topology-ext] Wang, A., Chen, H., Talaulikar, K. and S. Zhuang, "BGP-LS Extension for Inter-AS Topology Retrieval", Internet-Draft draft-ietf-idr-bgpls-inter-as-topology-ext-08, April 2020.

Authors' Addresses

Aijun Wang China Telecom Beiqijia Town, Changping District Beijing, 102209 China EMail:
Zhibo Hu Huawei Technologies Huawei Bld., No.156 Beiqing Rd. Beijing, 100095 China EMail: