IS-IS for IPv6: Routing IPv6 with IS-IS
How IS-IS learned to carry IPv6 prefixes with two new TLVs, so one routing protocol can handle IPv6, IPv4 and OSI.
Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, DOI 10.17487/RFC5308, October 2008, <https://www.rfc-editor.org/info/rfc5308>.
@misc{rfc5308,
series = {Request for Comments},
number = 5308,
howpublished = {RFC 5308},
publisher = {RFC Editor},
doi = {10.17487/RFC5308},
url = {https://www.rfc-editor.org/info/rfc5308},
author = {C. Hopps},
title = {{Routing IPv6 with IS-IS}},
pagetotal = 7,
year = 2008,
month = oct,
} RFC text
Contents
Network Working Group C. Hopps
Request for Comments: 5308 Cisco Systems
Category: Standards Track October 2008
Routing IPv6 with IS-IS
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies a method for exchanging IPv6 routing
information using the IS-IS routing protocol. The described method
utilizes two new TLVs: a reachability TLV and an interface address
TLV to distribute the necessary IPv6 information throughout a routing
domain. Using this method, one can route IPv6 along with IPv4 and
OSI using a single intra-domain routing protocol.
1. Overview
IS-IS is an extendible intra-domain routing protocol. Each router in
the routing domain issues an Link State Protocol Data Unit (LSP) that
contains information pertaining to that router. The LSP contains
typed variable-length data, often referred to as TLVs (type-length-
values). We extend the protocol with two new TLVs to carry
information required to perform IPv6 routing.
In [RFC1195], a method is described to route both OSI and IPv4. We
utilize this same method with some minor changes to allow for IPv6.
To do so, we must define two new TLVs, namely "IPv6 Reachability" and
"IPv6 Interface Address", and a new IPv6 protocol identifier. In our
new TLVs, we utilize the extended metrics and up/down semantics of
[RFC5305].
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. IPv6 Reachability TLV
The "IPv6 Reachability" TLV is TLV type 236 (0xEC).
[RFC1195] defines two Reachability TLVs, "IP Internal Reachability
Information" and "IP External Reachability Information". We provide
the equivalent IPv6 data with the "IPv6 Reachability" TLV and an
"external" bit.
The "IPv6 Reachability" TLV describes network reachability through
the specification of a routing prefix, metric information, a bit to
indicate if the prefix is being advertised down from a higher level,
a bit to indicate if the prefix is being distributed from another
routing protocol, and OPTIONALLY the existence of Sub-TLVs to allow
for later extension. This data is represented by the following
structure:
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 = 236 | Length | Metric .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. Metric |U|X|S| Reserve | Prefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sub-TLV Len(*) | Sub-TLVs(*) ...
* - if present
U - up/down bit
X - external original bit
S - subtlv present bit
The above IPv6 Reachability TLV MAY appear any number of times
(including none) within an LSP. Link-local prefixes MUST NOT be
advertised using this TLV.
As is described in [RFC5305]: "The up/down bit SHALL be set to 0 when
a prefix is first injected into IS-IS. If a prefix is advertised
from a higher level to a lower level (e.g. level 2 to level 1), the
bit SHALL be set to 1, indicating that the prefix has traveled down
the hierarchy. Prefixes that have the up/down bit set to 1 may only
be advertised down the hierarchy, i.e., to lower levels".
If the prefix was distributed into IS-IS from another routing
protocol, the external bit SHALL be set to 1. This information is
useful when distributing prefixes from IS-IS to other protocols.
If the Sub-TLV bit is set to 0, then the octets of Sub-TLVs are not
present. Otherwise, the bit is 1 and the octet following the prefix
will contain the length of the Sub-TLV portion of the structure.
The prefix is "packed" in the data structure. That is, only the
required number of octets of prefix are present. This number can be
computed from the prefix length octet as follows:
prefix octets = integer of ((prefix length + 7) / 8)
Just as in [RFC5305], if a prefix is advertised with a metric larger
than MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST not be
considered during the normal Shortest Path First (SPF) computation.
This will allow advertisement of a prefix for purposes other than
building the normal IPv6 routing table.
If Sub-TLVs are present, they have the same form as normal TLVs, as
shown below.
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 | Value(*) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* - if present
Length indicates how many octets of value are present and can be 0.
3. IPv6 Interface Address TLV
The "IPv6 Interface Address" TLV is TLV type 232 (0xE8).
TLV 232 maps directly to "IP Interface Address" TLV in [RFC1195] .
We necessarily modify the contents to be 0-15 16-octet IPv6 interface
addresses instead of 0-63 4-octet IPv4 interface addresses.
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 = 232 | Length | Interface Address 1(*) .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. Interface Address 1(*) .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. Interface Address 1(*) .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .. Interface Address 1(*) .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Address 1(*) .. | Interface Address 2(*) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* - if present
We further restrict the semantics of this TLV depending on where it
is advertised. For Hello PDUs, the "Interface Address" TLV MUST
contain only the link-local IPv6 addresses assigned to the interface
that is sending the Hello. For LSPs, the "Interface Address" TLVs
MUST contain only the non-link-local IPv6 addresses assigned to the
IS.
4. IPv6 NLPID
The value of the IPv6 Network Layer Protocol ID (NLPID) is 142
(0x8E).
As with [RFC1195] and IPv4, if the IS supports IPv6 routing using
IS-IS, it MUST advertise this in the "NLPID" TLV by adding the IPv6
NLPID.
5. Operation
We utilize the same changes to [RFC1195] as made in [RFC5305] for the
processing of prefix information. These changes are both related to
the SPF calculation.
Since the metric space has been extended, we need to redefine the
MAX_PATH_METRIC (1023) from the original specification in [RFC1195].
This new value MAX_V6_PATH_METRIC is the same as in [RFC5305]
(0xFE000000). If, during the SPF, a path metric would exceed
MAX_V6_PATH_METRIC, it SHALL be considered to be MAX_V6_PATH_METRIC.
The order of preference between paths for a given prefix MUST be
modified to consider the up/down bit. The new order of preference is
as follows (from best to worst).
1. Level 1 up prefix
2. Level 2 up prefix
3. Level 2 down prefix
4. Level 1 down prefix
If multiple paths have the same best preference, then selection
occurs based on metric. Any remaining multiple paths SHOULD be
considered for equal-cost multi-path routing if the router supports
this; otherwise, the router can select any one of the multiple paths.
6. IANA Considerations
IANA has updated the IS-IS codepoint registry so that TLV codes 232
and 236 refer to this RFC.
IANA has also created the following new codepoint registry for Sub-
TLVs of TLV 236. The range of values for Type is 0-255. Allocations
within the registry require documentation of the use and requires
approval by the Designated Expert assigned by the IESG [RFC5226].
All codepoints are currently unassigned.
7. Security Considerations
This document raises no new security considerations. Security
considerations for the IS-IS protocol are covered in [ISO10589] and
in [RFC5304].
8. References
8.1. Normative References
[ISO10589] ISO, "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 Standard 10589:2002, Second Edition, 2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
Author's Address
Christian E. Hopps
Cisco Systems
170 W. Tasman Dr.
San Jose, California 95134
USA
EMail: chopps@cisco.com
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AI ELI5
On this page
§1Extending IS-IS for IPv6
IS-IS is an extendible intra-domain routing protocol. Each router in the routing domain issues a Link State Protocol Data Unit (LSP) with information about that router. The LSP holds typed, variable-length data called TLVs (type-length-values).
RFC 1195 describes how to route both OSI and IPv4 with IS-IS. This RFC reuses that method with minor changes. It defines two new TLVs, "IPv6 Reachability" and "IPv6 Interface Address", and a new IPv6 protocol identifier. The new TLVs use the extended metrics and up/down semantics of RFC 5305.
Routers already trade notes in a fixed format. This RFC just adds two new kinds of note so they can talk about the new addresses too.
§2The IPv6 Reachability TLV
The IPv6 Reachability TLV is TLV type 236 (0xEC). It is the IPv6 equivalent of the two IP reachability TLVs in RFC 1195 (internal and external), with an "external" bit in place of two separate TLVs.
It describes reachability by giving a routing prefix and metric information. It also carries an up/down bit (U), an external bit (X), a bit saying whether Sub-TLVs are present (S), and optionally the Sub-TLVs themselves for later extension. It MAY appear any number of times, including none, within an LSP. Link-local prefixes MUST NOT be advertised with it.
The prefix is packed: only the needed octets are present, computed as the integer of (prefix length + 7) / 8. If the Sub-TLV bit is 0, no Sub-TLV octets follow. If it is 1, the octet after the prefix holds the length of the Sub-TLV part, and Sub-TLVs have the same type, length, value form as ordinary TLVs.
Each reachability note says "you can get to this neighborhood through me, and here is how costly it is." It only writes down as much of the address as it needs.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | 236 | TLV type 236 (0xEC). |
LENGTH | Length | 1 byte | Length of the TLV value. | |
METRIC | Metric | 4 bytes | Metric for the prefix. | |
U | U | 1 bits | Up/down bit. | |
X | X | 1 bits | External original bit. | |
S | S | 1 bits | Sub-TLV present bit. | |
RESERVE | Reserve | 5 bits | Reserved bits. | |
PREFIX-LEN | Prefix Len | 1 byte | Length of the prefix in bits. |
§3Up/Down Bit, External Bit and Metric
As RFC 5305 says, the up/down bit SHALL be 0 when a prefix is first injected into IS-IS. If a prefix is advertised from a higher level to a lower level (for example level 2 to level 1), the bit SHALL be 1, meaning the prefix has traveled down the hierarchy. Prefixes with the bit set to 1 may only be advertised down the hierarchy.
If a prefix was distributed into IS-IS from another routing protocol, the external bit SHALL be set to 1. The RFC notes this is useful when distributing prefixes from IS-IS to other protocols.
As in RFC 5305, a prefix advertised with a metric larger than MAX_V6_PATH_METRIC (0xFE000000) MUST NOT be considered in the normal Shortest Path First (SPF) computation. This allows a prefix to be advertised for purposes other than building the normal IPv6 routing table.
One flag says "this note already went down a level, do not send it back up." Another says "I heard this from someone outside the club." A really huge cost means "do not use this for directions."
§4Interface Address TLV and the IPv6 NLPID
The IPv6 Interface Address TLV is TLV type 232 (0xE8). It maps directly to the "IP Interface Address" TLV of RFC 1195, except that it holds 0 to 15 interface addresses of 16 octets each, instead of 0 to 63 four-octet IPv4 addresses.
Its meaning depends on where it is advertised. In Hello PDUs it MUST contain only the link-local IPv6 addresses of the interface sending the Hello. In LSPs it MUST contain only the non-link-local IPv6 addresses assigned to the IS.
The IPv6 Network Layer Protocol ID (NLPID) is 142 (0x8E). As with IPv4, an IS that supports IPv6 routing using IS-IS MUST advertise this in the "NLPID" TLV.
A router lists its own addresses in a note. Addresses that only work on one wire go in the greeting. Addresses that work everywhere go in the big map update. It also says out loud that it speaks IPv6.
§5Path Metrics and Route Preference
The prefix processing follows the same changes to RFC 1195 that RFC 5305 made, and both concern the SPF calculation. Because the metric space is larger, the original MAX_PATH_METRIC (1023) is redefined as MAX_V6_PATH_METRIC, the same value as in RFC 5305 (0xFE000000). If a path metric would exceed it during SPF, it SHALL be treated as that value.
The order of preference between paths to the same prefix MUST take the up/down bit into account. From best to worst it is: Level 1 up, Level 2 up, Level 2 down, Level 1 down.
If several paths tie on preference, the choice is made by metric. Remaining ties SHOULD be considered for equal-cost multi-path routing if the router supports it. Otherwise the router can pick any one of them.
If there are several ways to reach the same place, routers have a ranking for which kind of note to trust first. Then they pick the cheapest route, and if there is still a tie they may split traffic or just pick one.
- 1Level 1 up — Most preferred path for a prefix.
- 2Level 2 up — Preferred after Level 1 up.
- 3Level 2 down — Preferred after Level 2 up.
- 4Level 1 down — Least preferred.
§6IANA and Security
IANA updated the IS-IS codepoint registry so TLV codes 232 and 236 refer to this RFC. It also created a new registry for Sub-TLVs of TLV 236, with a Type range of 0 to 255. Allocations require documentation of the use and approval by the Designated Expert assigned by the IESG. All codepoints are currently unassigned.
The RFC says it raises no new security considerations. Security considerations for IS-IS are covered in ISO 10589 and in RFC 5304.
There is now an official list where new sub-notes get registered, and it starts out empty. The RFC says it adds no new safety worries beyond what IS-IS already has.