Network Working Group A. Przygienda, Ed.
Internet-Draft C. Bowers
Intended status: Experimental Juniper
Expires: 12 June 2022 Y. Lee
A. Sharma
Comcast
R. White
Juniper
9 December 2021
IS-IS Flood Reflection
draft-ietf-lsr-isis-flood-reflection-07
Abstract
This document describes a backwards compatible, optional IS-IS
extension that allows the creation of IS-IS flood reflection
topologies. Flood reflection allows topologies in which L1 areas
provide transit forwarding for L2 using all available L1 nodes
internally. It accomplishes this by creating L2 flood reflection
adjacencies within each L1 area. Those adjacencies are used to flood
L2 LSPDUs, and they are used in the L2 SPF computation. However,
they are not used for forwarding within the flood reflection cluster.
This arrangement gives the L2 topology significantly better scaling
properties. As additional benefit, only those routers directly
participating in flood reflection have to support the feature. This
allows for the incremental deployment of scalable L1 transit areas in
an existing network, without the necessity of upgrading other routers
in the network.
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on 12 June 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Further Details . . . . . . . . . . . . . . . . . . . . . . . 9
4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Flood Reflection TLV . . . . . . . . . . . . . . . . . . 10
4.2. Flood Reflection Discovery Sub-TLV . . . . . . . . . . . 11
4.3. Flood Reflection Discovery Tunnel Type Sub-Sub-TLV . . . 12
4.4. Flood Reflection Adjacency Sub-TLV . . . . . . . . . . . 13
4.5. Flood Reflection Discovery . . . . . . . . . . . . . . . 14
4.6. Flood Reflection Adjacency Formation . . . . . . . . . . 15
5. Route Computation . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Tunnel Based Deployment . . . . . . . . . . . . . . . . . 16
5.2. No Tunnel Deployment . . . . . . . . . . . . . . . . . . 16
6. Redistribution of Prefixes . . . . . . . . . . . . . . . . . 17
7. Special Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8.1. New IS-IS TLV Codepoint . . . . . . . . . . . . . . . . . 18
8.2. Sub TLVs for TLV 242 . . . . . . . . . . . . . . . . . . 18
8.3. Sub-sub TLVs for Flood Reflection Discovery sub-TLV . . . 18
8.4. Sub TLVs for TLV 22, 23, 25, 141, 222, and 223 . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Informative References . . . . . . . . . . . . . . . . . 19
11.2. Normative References . . . . . . . . . . . . . . . . . . 19
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
This section introduces the problem space and outlines the solution.
Some of the terms may be unfamiliar to reader without extensive IS-IS
background and in such case a glossary is provided in Section 2 and
can be referenced.
Due to the inherent properties of link-state protocols the number of
IS-IS routers within a flooding domain is limited by processing and
flooding overhead on each node. While that number can be maximized
by well written implementations and techniques such as exponential
back-offs, IS-IS will still reach a saturation point where no further
routers can be added to a single flooding domain. In some L2
backbone deployment scenarios, this limit presents a significant
challenge.
The traditional approach to increasing the scale of an IS-IS
deployement is to break it up into multiple L1 flooding domains and a
single L2 backbone. This works well for designs where an L2 backbone
connects L1 access topologies, but it is limiting where a large L2 is
supposed to span large number of routers. In such scenarios, an
alternative approach is to consider multiple L2 flooding domains
connected together via L1 flooding domains. In other words, L2
flooding domains are connected by "L1/L2 lanes" through the L1 areas
to form a single L2 backbone again. Unfortunately, in its simplest
implementation, this requires the inclusion of most, or all, of the
transit L1 routers as L1/L2 to allow traffic to flow along optimal
paths through such transit areas. Consequently, this approach fails
to reduce the number of L2 routers involved, so it fails to increase
the scalability of the L2 backbone.
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+====+ +=======+ +=======+ +======-+ +====+
I R1 I I R10 +-------------+ R20 +---------------+ R30 I I R4 I
I L2 +--+ L1/L2 I I L1 I I L1/L2 +--+ L2 I
I I I + +--+ I +------------+ I I I
+====+ ++====+=+ | +===+===+ | +=+==+=++ +====+
| | | | | | |
| | | | | +-----------+ |
| +-------+ | | | | |
| | | | | | |
| | | | | | |
| | | | | | |
+====+ ++=====-+ | | +===+===+--+ | +======++ +====+
I R2 I I R11 I | | I R21 I | I R31 I I R5 I
I L2 +--+ L1/L2 +-------------+ L1 +---------------+ L1/L2 +--+ L2 I
I I I I | | I I | +-------+ I I I
+====+ ++=====-+ | | ++==+==++ | | +======++ +====+
| | | | | | | | |
| +---------------+ | | | | | |
| | | | | | | | |
| | +----------------+ | +-----------------+ |
| | | | | | | | |
+====+ ++=+==+=+ +-------+===+===+-----+ | ++=====++ +====+
I R3 I I R12 I I R22 I | + R32 I I R6 I
I L2 +--+ L1/L2 I I L1 +-------+ I L1/L2 +--+ L2 I
I I I +-------------+ +---------------+ I I I
+====+ +=======+ +=======+ +=======+ +====+
Figure 1: Example Topology of L1 with L2 Borders
Figure 1 is an example of a network where a topologically rich L1
area is used to provide transit between six different L2-only routers
(R1-R6). Note that the six L2-only routers do not have connectivity
to one another over L2 links. To take advantage of the abundance of
paths in the L1 transit area, all the intermediate systems could be
placed into both L1 and L2, but this essentially combines the
separate L2 flooding domains into a single one, triggering again
maximum L2 scale limitation we try to address in first place.
A more effective solution would allow to reduce the number of links
and routers exposed in L2, while still utilizing the full L1 topology
when forwarding through the network.
[RFC8099] describes Topology Transparent Zones (TTZ) for OSPF. The
TTZ mechanism represents a group of OSPF routers as a full mesh of
adjacencies between the routers at the edge of the group. A similar
mechanism could be applied to IS-IS as well. However, a full mesh of
adjacencies between edge routers (or L1/L2 nodes) significantly
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limits the scale of the topology. The topology in Figure 1 has 6 L1/
L2 nodes. Figure 2 illustrates a full mesh of L2 adjacencies between
the 6 L1/L2 nodes, resulting in (5 * 6)/2 = 15 L2 adjacencies. In a
somewhat larger topology containing 20 L1/L2 nodes, the number of L2
adjacencies in a full mesh rises to 190.
+----+ +-------+ +-------------------------------+-------+ +----+
| R1 | | R10 | | | R30 | | R4 |
| L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 |
| | | | | | | | |
+----+ ++-+-+--+-+ | +-+--+---++ +----+
| | | | | | | |
| +----------------------------------------------+ |
| | | | | | | |
| +-----------------------------------+ | | | |
| | | | | | | |
| +----------------------------------------+ | |
| | | | | | | |
+----+ ++-----+- | | | | -----+-++ +----+
| R2 | | R11 | | | | | | R31 | | R5 |
| L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 |
| | | | | | | | | | | |
+----+ ++------+------------------------------+ | | +----+-++ +----+
| | | | | | | |
| | | | | | | |
| +-------------------------------------------+ |
| | | | | | | |
| | | | +----------+ |
| | | | | | | |
| | | | +-----+ | |
| | | | | | | |
+----+ ++----+-+-+ | +-+-+--+-++ +----+
| R3 | | R12 | | L2 adjacency | R32 | | R6 |
| L2 +--+ L1/L2 +------------------------------------+ L1/L2 +--+ L2 |
| | | | | | | | |
+----+ +-------+----+ +-------+ +----+
Figure 2: Example topology represented in L2 with a full mesh of
L2 adjacencies between L1/L2 nodes
BGP, as specified in [RFC4271], faced a similar scaling problem,
which has been solved in many networks by deploying BGP route
reflectors [RFC4456]. We note that BGP route reflectors do not
necessarily have to be in the forwarding path of the traffic. This
incongruity of forwarding and control path for BGP route reflectors
allows the control plane to scale independently of the forwarding
plane.
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We propose here a similar solution for IS-IS. A simple example of
what a flood reflector control plane approach would look like is
shown in Figure 3, where router R21 plays the role of a flood
reflector. Each L1/L2 ingress/egress router builds a tunnel to the
flood reflector, and an L2 adjacency is built over each tunnel. In
this solution, we need only 6 L2 adjacencies, instead of the 15
needed for a full mesh. In a somewhat larger topology containing 20
L1/L2 nodes, this solution requires only 20 L2 adjacencies, instead
of the 190 need for a full mesh. Multiple flood reflectors can be
used, allowing the network operator to balance between resilience,
path utilization, and state in the control plane. The resulting L2
adjacency scale is R*n, where R is the number of flood reflectors
used and n is the number of L1/L2 nodes. This compares quite
favorably with n*(n-1)/2 L2 adjacencies required in a topologically
fully meshed L2 solution.
+----+ +-------+ +-------+ +----+
| R1 | | R10 | | R30 | | R4 |
| L2 +--+ L1/L2 +--------------+ +-----------------+ L1/L2 +--+ L2 |
| | | | L2 adj | | L2 adj | | | |
+----+ +-------+ over | | over +-------+ +----+
tunnel | | tunnel
+----+ +-------+ +--+---+--+ +-------+ +----+
| R2 | | R11 | | R21 | | R31 | | R5 |
| L2 +--+ L1/L2 +-----------+ L1/L2 +--------------+ L1/L2 +--+ L2 |
| | | | L2 adj | flood | L2 adj | | | |
+----+ +-------+ over |reflector| over +-------+ +----+
tunnel +--+---+--+ tunnel
+----+ +-------+ | | +-------+ +----+
| R3 | | R12 +--------------+ +-----------------+ R32 | | R6 |
| L2 +--+ L1/L2 | L2 adj L2 adj | L1/L2 +--+ L2 |
| | | | over over | | | |
+----+ +-------+ tunnel tunnel +-------+ +----+
Figure 3: Example topology represented in L2 with L2 adjacencies
from each L1/ L2 node to a single flood reflector
As illustrated in Figure 3, when R21 plays the role of flood
reflector, it provides L2 connectivity among all of the previously
disconnected L2 islands by reflooding all L2 LSPDUs. At the same
time, R20 and R22 in Figure 1 remain L1-only routers. L1-only
routers and L1-only links are not visible in L2. In this manner, the
flood reflector allows us provide L2 control plane connectivity in a
scalable manner.
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As described so far, the solution illustrated in Figure 3 relies only
on currently standardized IS-IS functionality. Without new
functionality, however, the data traffic will traverse only R21.
This will unnecessarily create a bottleneck at R21 since there is
still available capacity in the paths crossing the L1-only routers
R20 and R22 in Figure 1.
Hence, some new functionality is necessary to allow the L1/L2 edge
nodes (R10-12 and R30-32 in Figure 3) to recognize that the L2
adjacency to R21 should not be used for forwarding. The L1/L2 edge
nodes should forward traffic that would normally be forwarded over
the L2 adjacency to R21 over L1 links instead. This would allow the
forwarding within the L1 area to use the L1-only nodes and links
shown in Figure 1 as well. It allows networks to be built that use
the entire forwarding capacity of the L1 areas, while at the same
time introducing control plane scaling benefits provided by L2 flood
reflectors.
This document defines all extensions necessary to support flood
reflector deployment:
* A 'flood reflector adjacency' for all the adjacencies built for
the purpose of reflecting flooding information. This allows these
'flood reflectors' to participate in the IS-IS control plane
without being used in the forwarding plane. This is a purely
local operation on the L1/L2 ingress; it does not require
replacing or modifying any routers not involved in the reflection
process. Deployment-wise, it is far less tricky to just upgrade
the routers involved in flood reflection rather than have a flag
day on the whole IS-IS domain.
* An (optional) full mesh of tunnels between the L1/L2 routers,
ideally load-balancing across all available L1 links. This
harnesses all forwarding paths between the L1/L2 edge nodes
without injecting unneeded state into the L2 flooding domain or
creating 'choke points' at the 'flood reflectors' themselves. The
draft is agnostic as to the tunneling technology used but provides
enough information for automatic establishment of such tunnels.
The discussion of IS-IS adjacency formation and/or liveness
discovery on such tunnels is outside the scope of this draft and
is largely choice of the underlying implementation. A solution
without tunnels is also possible by applying judicious scoping of
reachability information between the levels as described in more
details later.
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* Some way to support reflector redundancy, and potentially some way
to auto-discover and advertise such adjacencies as flood reflector
adjacencies. Such advertisements may allow L2 nodes outside the
L1 to perform optimizations in the future based on this
information.
2. Glossary
This section is introduced with the intention of allowing quick
reference in the more detailed parts of the document to terms used
Flood Reflector:
Node configured to connect L2 only to flood reflector clients and
reflect (reflood) IS-IS L2 LSPs amongst them.
Flood Reflector Client:
Node configured to build flood reflector adjacencies and normal L2
nodes.
Flood Reflector Adjacency:
IS-IS L2 adjacency limited by one end being client and the other
reflector and agreeing on the same Flood Reflector Cluster ID.
Flood Reflector Cluster:
Collection of clients and flood reflectors configured with the
same cluster identifier. Cluster ID value of 0 SHOULD NOT be used
since it may be used in the future for special purposes.
Tunnel Deployment:
Deployment where flood reflector clients build a partial or full
mesh of tunnels in L1 to "shortcut" forwarding of L2 traffic
through the cluster.
No Tunnel Deployment:
Deployment where flood reflector clients redistribute L2
reachability into L1 to allow forwarding through the cluster
without underlying tunnels.
Tunnel Endpoint:
An endpoint that allows to establish a bi-directional tunnel
carrying ISIS control traffic or alternately serves as origin of
such tunnel.
L1 shortcut:
A tunnel between two clients visible in L1 only that is used as a
next-hop, i.e. to carry data traffic in tunnel deployment mode.
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3. Further Details
Several considerations should be noted in relation to such a flood
reflection mechanism.
First, this allows multi-area IS-IS deployments to scale without any
major modifications in the IS-IS implementation on most of the nodes
deployed in the network. Unmodified (traditional) L2 routers will
compute reachability across the transit L1 area using the flood
reflector adjacencies.
Second, the flood reflectors are not required to participate in
forwarding traffic through the L1 transit area. These flood
reflectors can be hosted on virtual devices outside the forwarding
topology.
Third, astute readers will realize that flooding reflection may cause
the use of suboptimal paths. This is similar to the BGP route
reflection suboptimal routing problem described in
[ID.draft-ietf-idr-bgp-optimal-route-reflection-28]. The L2
computation determines the egress L1/L2 and with that can create
illusions of ECMP where there is none. And in certain scenarios lead
to an L1/L2 egress which is not globally optimal. This represents a
straightforward instance of the trade-off between the amount of
control plane state and the optimal use of paths through the network
often encountered when aggregating routing information.
One possible solution to this problem is to expose additional
topology information into the L2 flooding domains. In the example
network given, links from router 01 to router 02 can be exposed into
L2 even when 01 and 02 are participating in flood reflection. This
information would allow the L2 nodes to build 'shortcuts' when the L2
flood reflected part of the topology looks more expensive to cross
distance wise.
Another possible variation is for an implementation to approximate
with the tunnel cost the cost of the underlying topology.
Redundancy can be achieved by building multiple flood reflectors in a
L1 area. Multiple flood reflectors do not need any synchronization
mechanisms amongst themselves, except standard IS-IS flooding and
database maintenance procedures.
4. Encodings
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4.1. Flood Reflection TLV
The Flood Reflection TLV is a new top-level TLV that MAY appear in L2
IIHs. The Flood Reflection TLV indicates the flood reflector cluster
(based on Flood Reflection Cluster ID) that a given router is
configured to participate in. It also indicates whether the router
is configured to play the role of either flood reflector or flood
reflector client. The Flood Reflection Cluster ID and flood
reflector roles advertised in the IIHs are used to ensure that flood
reflector adjacencies are only formed between a flood reflector and
flood reflector client, and that the Flood Reflection Cluster IDs
match. The Flood Reflection TLV has the following format:
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 |C| RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flood Reflection Cluster ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: The length, in octets, of the following fields.
C (Client): This bit is set to indicate that the router acts as a
flood reflector client. When this bit is NOT set, the router acts
as a flood reflector. On a given router, the same value of the
C-bit MUST be advertised across all interfaces advertising the
Flood Reflection TLV in IIHs.
RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.
Flood Reflection Cluster ID: Flood Reflection Cluster Identifier.
These same 32-bit value MUST be assigned to all of the flood
reflectors and flood reflector clients in the same L1 area. The
value MUST be unique across different L1 areas within the IGP
domain. In case of violation of those rules multiple L1 areas may
become a single cluster or a single area may split in flood
reflection sense and several mechanisms such as auto-discovery of
tunnels may not work correctly. On a given router, the same value
of the Flood Reflection Cluster ID MUST be advertised across all
interfaces advertising the Flood Reflection TLV in IIHs. When a
router discovers that a node is using multiple Cluster IDs based
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on its advertised TLVs and IIHs, the node MAY adequately log such
violations subject to rate limiting. This implies that a flood
reflector MUST NOT participate in more than a single L1 area. In
case of Cluster ID value of 0, the TLV containing it MUST be
ignored.
Sub-TLVs: Optional sub-TLVs. For future extensibility, the format
of the Flood Reflection TLV allows for the possibility of
including optional sub-TLVs. No sub-TLVs of the Flood Reflection
TLV are defined in this document.
The Flood Reflection TLV SHOULD NOT appear more than once in an IIH.
A router receiving multiple Flood Reflection TLVs in the same IIH
MUST use the values in the first TLV of the lowest numbered fragment
and it SHOULD adequately log such violations subject to rate
limiting.
4.2. Flood Reflection Discovery Sub-TLV
Flood Reflection Discovery sub-TLV is advertised as a sub-TLV of the
IS-IS Router Capability TLV-242, defined in [RFC7981]. The Flood
Reflection Discovery sub-TLV is advertised in L1 and L2 LSPs with
area flooding scope in order to enable the auto-discovery of flood
reflection capabilities. The Flood Reflection Discovery sub-TLV has
the following format:
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 |C| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flood Reflection Cluster ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: The length, in octets, of the following fields.
C (Client): This bit is set to indicate that the router acts as a
flood reflector client. When this bit is NOT set, the router acts
as a flood reflector.
RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.
Flood Reflection Cluster ID: The Flood Reflection Cluster Identifier
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is the same as that defined in the Flood Reflection TLV and obeys
the same rules.
The Flood Reflection Discovery sub-TLV SHOULD NOT appear more than
once in TLV 242. A router receiving multiple Flood Reflection
Discovery sub-TLVs in TLV 242 MUST use the values in the first sub-
TLV of the lowest numbered fragment and it SHOULD adequately log such
violations subject to rate limiting.
4.3. Flood Reflection Discovery Tunnel Type Sub-Sub-TLV
Flood Reflection Discovery Tunnel Type sub-sub-TLV is advertised
optionally as a sub-sub-TLV of the Flood Reflection Discovery Sub-
TLV, defined in Section 4.2. It allows the automatic creation of L2
tunnels to be used as flood reflector adjacencies and L1 shortcut
tunnels. The Flood Reflection Tunnel Type sub-sub-TLV has the
following format:
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 | Reserved |F|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Encapsulation Attribute |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: The length, in octets, of zero or more of the following
fields.
Reserved: SHOULD be 0 on transmission and ignored on reception.
F Flag: When set indicates flood reflection tunnel endpoint, when
clear, indicates possible L1 shortcut tunnel endpoint.
Tunnel Encapsulation Attribute: Carries encapsulation type and
further attributes necessary for tunnel establishment as defined
in [RFC9012]. Protocol type sub-TLV as defined in [RFC9012] MAY
be included but MUST when F flag is set include according type
that allows carrying of encapsulated IS-IS frames. Such tunnel
type MUST provide according mechanisms to carry up to
`originatingL2LSPBufferSize` sized IS-IS frames across.
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A flood reflector receiving Flood Reflection Discovery Tunnel Type
sub-sub-TLVs in Flood Reflection Discovery sub-TLV with F flag set
SHOULD use one or more of the specified tunnel endpoints to
automatically establish one or more tunnels that will serve as flood
reflection adjacency(-ies) to the clients advertising the endpoints.
A flood reflection client receiving multiple Flood Reflection
Discovery Tunnel Type sub-sub-TLVs in Flood Reflection Discovery sub-
TLV with F flag clear from other leaves MAY use one or more of the
specified tunnel endpoints to automatically establish one or more
tunnels that will serve as L1 tunnel shortcuts to the clients
advertising the endpoints.
In case of automatic flood reflection adjacency tunnels and in case
IS-IS adjacencies are being formed across L1 shortcuts all the
aforementioned rules in Section 4.5 apply as well.
Optional address validation procedures as defined in [RFC9012] MUST
be disregarded.
4.4. Flood Reflection Adjacency Sub-TLV
The Flood Reflection Adjacency sub-TLV is advertised as a sub-TLV of
TLVs 22, 23, 25, 141, 222, and 223. Its presence indicates that a
given adjacency is a flood reflector adjacency. It is included in L2
area scope flooded LSPs. Flood Reflection Adjacency sub-TLV has the
following format:
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 |C| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flood Reflection Cluster ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD
Length: The length, in octets, of the following fields.
C (Client): This bit is set to indicate that the router advertising
this adjacency is a flood reflector client. When this bit is NOT
set, the router advertising this adjacency is a flood reflector.
RESERVED: This field is reserved for future use. It MUST be set to
0 when sent and MUST be ignored when received.
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Flood Reflection Cluster ID: The Flood Reflection Cluster Identifier
is the same as that defined in the Flood Reflection TLV and obeys
the same rules.
The Flood Reflection Adjacency sub-TLV SHOULD NOT appear more than
once in a given TLV. A router receiving multiple Flood Reflection
Adjacency sub-TLVs in a TLV MUST use the values in the first sub-TLV
of the lowest numbered fragment and it SHOULD adequately log such
violations subject to rate limiting.
4.5. Flood Reflection Discovery
A router participating in flood reflection as client or reflector
MUST be configured as an L1/L2 router. It SHOULD originate the Flood
Reflection Discovery sub-TLV with area flooding scope in L1 and L2.
Normally, all routers on the edge of the L1 area (those having
traditional L2 adjacencies) will advertise themselves as route
reflector clients. Therefore, a flood reflector client will have
both traditional L2 adjacencies and flood reflector L2 adjacencies.
A router acting as a flood reflector MUST NOT have any traditional L2
adjacencies except with flood reflector clients. It will be an L1/L2
router only by virtue of having flood reflector L2 adjacencies. A
router desiring to act as a flood reflector SHOULD advertise itself
as such using the Flood Reflection Discovery sub-TLV in L1 and L2.
A given flood reflector or flood reflector client can only
participate in a single cluster, as determined by the value of its
Flood Reflection Cluster ID and should disregard other routers' TLVs
for flood reflection purposes if the cluster ID is not matching.
Upon reception of Flood Reflection Discovery sub-TLVs, a router
acting as flood reflector client SHOULD initiate a tunnel towards
each flood reflector with which it shares an Flood Reflection Cluster
ID using one or more of the tunnel encapsulations provided with F
flag being set. The L2 adjacencies formed over such tunnels MUST be
marked as flood reflector adjacencies. If the client or reflector
has a direct L2 adjacency with the according remote side it SHOULD
use it instead of instantiating a new tunnel.
In absence of auto-discovery an implementation MAY use statically
configured tunnels to create flood reflection adjacencies.
The IS-IS metrics for all flood reflection adjacencies in a cluster
SHOULD be uniform.
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Upon reception of Flood Reflection Discover TLVs, a router acting as
a flood reflector client MAY initiate tunnels with L1-only
adjacencies towards any of the other flood reflector clients with
lower router IDs in its cluster using encapsulations with F flag
clear. These tunnels MAY be used for forwarding to improve the load-
balancing characteristics of the L1 area. If the clients have a
direct L2 adjacency they SHOULD use it instead of instantiating a new
tunnel.
4.6. Flood Reflection Adjacency Formation
In order to simplify both implementations and network deployments,
this draft does not allow the formation of complex hierarchies of
flood reflectors and clients or allow multiple clusters in a single
L1 area. Consequently, all flood reflectors and flood reflector
clients in the same L1 area MUST share the same Flood Reflector
Cluster ID. Deployment of multiple cluster IDs in the same L1 area
are outside the scope of this document.
A flood reflector MUST only form flood reflection adjacencies with
flood reflector clients with matching Cluster ID. A flood reflector
MUST NOT form any traditional L2 adjacencies.
Flood reflector clients MUST only form flood reflection adjacencies
with flood reflectors with matching Cluster ID.
Flood reflector clients MAY form traditional L2 adjacencies with
flood reflector clients or nodes not participating in flood
reflection. When two clients form traditional L2 adjacency Cluster
ID is disregarded.
The Flood Reflector Cluster ID and flood reflector roles advertised
in the Flood Reflection TLVs in IIHs are used to ensure that flood
reflection adjacencies that are established meet the above criteria.
On change in either flood reflection role or cluster ID on IIH on the
local or remote side the adjacency has to be reset and re-established
if possible.
Once a flood reflection adjacency is established, the flood reflector
and the flood reflector client MUST advertise the adjacency by
including the Flood Reflection Adjacency Sub-TLV in the Extended IS
reachability TLV or MT-ISN TLV.
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5. Route Computation
To ensure loop-free routing, the route reflection client MUST follow
the normal L2 computation to determine L2 routes. This is because
nodes outside the L1 area will generally not be aware that flood
reflection is being performed. The flood reflection clients need to
produce the same result for the L2 route computation as a router not
participating in flood reflection.
5.1. Tunnel Based Deployment
In tunnel based option the reflection client, after L2 and L1
computation, MUST examine all L2 routes and replace all flood
reflector adjacencies with the correct underlying tunnel next-hop to
the egress.
5.2. No Tunnel Deployment
In case of deployment without underlying tunnels, the necessary L2
routes are distributed into the area, normally as L2->L1 routes. Due
to the rules in Section 4.6 the computation in the resulting topology
is relatively simple, the L2 SPF from a flood reflector client is
guaranteed to reach within a hop the Flood Reflector and in the
following hop the L2 egress to which it has a forwarding tunnel
again. All the flood reflector tunnel nexthops in the according L2
route can hence be removed and if the L2 route has no other ECMP L2
nexthops, the L2 route MUST be suppressed in the RIB by some means to
allow the less preferred L2->L1 route to be used to forward traffic
towards the advertising egress.
In the particular case the client has L2 routes which are not route
reflected, those will be naturally preferred (such routes normally
"hot-potato" route of the L1 area). However in the case the L2 route
through the flood reflector egress is "shorter" than such present non
flood reflected L2 routes, the node SHOULD ensure that such routes
are suppressed so the L2->L1 towards the egress still takes
preference. Observe that operationally this can be resolved in a
relatively simple way by configuring flood reflector adjacencies to
have a high metric, i.e. the flood reflector topology becomes "last
resort" and the leaves will try to "hot-potato" out the area as fast
as possible which is normally the desirable behavior.
In deployment scenarios where tunnels are not used, all L1/L2 edge
nodes MUST be ultimately flood reflector clients except during during
transition phase.
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6. Redistribution of Prefixes
When L2 prefixes need to be redistributed into L1 by the route
reflector clients a client that does not have any L2 flood reflector
adjacencies MUST NOT redistribute those routes into the area in case
of application of Section 5.2. The L2 prefixes advertisements
redistributed into L1 with flood reflectors SHOULD be normally
limited to L2 intra-area routes (as defined in [RFC7775]), if the
information exists to distinguish them from other other L2 prefix
advertisements.
On the other hand, in topologies that make use of flood reflection to
hide the structure of L1 areas while still providing transit
forwarding across them using tunnels, we generally do not need to
redistribute L1 prefixes advertisements into L2.
7. Special Considerations
In pathological cases setting the overload bit in L1 (but not in L2)
can partition L1 forwarding, while allowing L2 reachability through
flood reflector adjacencies to exist. In such a case a node cannot
replace a route through a flood reflector adjacency with a L1
shortcut and the client can use the L2 tunnel to the flood reflector
for forwarding while it MUST initiate an alarm and declare
misconfiguration.
A flood reflector with directly L2 attached prefixes should advertise
those in L1 as well since based on preference of L1 routes the
clients will not try to use the L2 flood reflector adjacency to route
the packet towards them. A very, very corner case is when the flood
reflector is reachable via L2 flood reflector adjacency (due to
underlying L1 partition) only in which case the client can use the L2
tunnel to the flood reflector for forwarding towards those prefixes
while it MUST initiate an alarm and declare misconfiguration.
A flood reflector SHOULD NOT set the attached bit on its LSPs.
Instead of modifying the computation procedures one could imagine a
flood reflector solution where the Flood Reflector would re-advertise
the L2 prefixes with a 'third-party' next-hop but that would have
less desirable convergence properties than the solution proposed and
force a fork-lift of all L2 routers to make sure they disregard such
prefixes unless in the same L1 domain as the Flood Reflector.
Depending on pseudo-node choice in case of a broadcast domain with
multiple flood reflectors attached this can lead to a partitioned LAN
and hence a router discovering such a condition MUST initiate an
alarm and declare misconfiguration.
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8. IANA Considerations
This document requests allocation for the following IS-IS TLVs and
Sub-TLVs.
8.1. New IS-IS TLV Codepoint
This document requests the following IS-IS TLV:
Value Name IIH LSP SNP Purge
----- --------------------------------- --- --- --- -----
TBD1 Flood Reflection y n n n
Suggested value for TBD1 is 161.
8.2. Sub TLVs for TLV 242
This document request the following registration in the "sub-TLVs for
TLV 242" registry.
Type Description
---- -----------
TBD2 Flood Reflection Discovery
Suggested value for TBD2 is 161.
8.3. Sub-sub TLVs for Flood Reflection Discovery sub-TLV
This document request the following registration in the "sub-sub-TLVs
for Flood Reflection Discovery sub-TLV" registry.
Type Description
---- -----------
TBD3 Flood Reflection Discovery Tunnel Encapsulation Attribute
Suggested value for TBD3 is 161.
8.4. Sub TLVs for TLV 22, 23, 25, 141, 222, and 223
This document requests the following registration in the "sub-TLVs
for TLV 22, 23, 25, 141, 222, and 223" registry.
Type Description 22 23 25 141 222 223
---- -------------------------------- --- --- --- --- --- ---
TBD4 Flood Reflector Adjacency y y n y y y
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Suggested value for TBD4 is 161.
9. Security Considerations
This document introduces tunnels carrying IS-IS control traffic via
tunnels. In case of statically configured tunnels a deployment
SHOULD provide enough security protection to prevent malicious
attackers from using the tunnel endpoints. For information used to
form dynamically discovered tunnels, it SHOULD be protected by the
the deployed IS-IS security mechanism preventing malicious nodes from
spoofing rogue information on behalf of other members.
10. Acknowledgements
The authors thank Shraddha Hegde, Peter Psenak, Acee Lindem, Robert
Raszuk and Les Ginsberg for their thorough review and detailed
discussions. Thanks are also extended to Michael Richardson for an
excellent routing directorate review.
11. References
11.1. Informative References
[ID.draft-ietf-idr-bgp-optimal-route-reflection-28]
Raszuk et al., R., "BGP Optimal Route Reflection", July
2019, <https://www.ietf.org/id/draft-ietf-idr-bgp-optimal-
route-reflection-28.txt>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC8099] Chen, H., Li, R., Retana, A., Yang, Y., and Z. Liu, "OSPF
Topology-Transparent Zone", RFC 8099,
DOI 10.17487/RFC8099, February 2017,
<https://www.rfc-editor.org/info/rfc8099>.
11.2. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7775] Ginsberg, L., Litkowski, S., and S. Previdi, "IS-IS Route
Preference for Extended IP and IPv6 Reachability",
RFC 7775, DOI 10.17487/RFC7775, February 2016,
<https://www.rfc-editor.org/info/rfc7775>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
Authors' Addresses
Tony Przygienda (editor)
Juniper
1137 Innovation Way
Sunnyvale, CA
United States of America
Email: prz@juniper.net
Chris Bowers
Juniper
1137 Innovation Way
Sunnyvale, CA
United States of America
Email: cbowers@juniper.net
Yiu Lee
Comcast
1800 Bishops Gate Blvd
Mount Laurel, NJ 08054
United States of America
Email: Yiu_Lee@comcast.com
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Alankar Sharma
Comcast
1800 Bishops Gate Blvd
Mount Laurel, NJ 08054
United States of America
Email: Alankar_Sharma@comcast.com
Russ White
Juniper
1137 Innovation Way
Sunnyvale, CA
United States of America
Email: russw@juniper.net
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