Internet DRAFT - draft-chen-isis-sl-overheads-reduction
draft-chen-isis-sl-overheads-reduction
Network Working Group Z. Chen
Internet-Draft Huawei
Intended status: Standards Track X. Xu
Expires: September 6, 2018 Alibaba
D. Cheng
Huawei
March 5, 2018
Overheads Reduction for IS-IS Enabled Spine-Leaf Networks
draft-chen-isis-sl-overheads-reduction-03
Abstract
When a Spine-Leaf topology adopts the Intermediate System to
Intermediate System (IS-IS) routing protocol, the Leaf node receives
Link State Packets (LSPs) from all the other nodes thus having the
entire routing information of the topology. This is usually
considered unnecessary and costly. This document describes a
solution to this problem by utilizing IS-IS's inherent multi-level
and area partition features, which requires that an IS-IS router
SHOULD check a level-1 LSP's area addresses before advertising it to
a neighbor.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 6, 2018.
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Copyright Notice
Copyright (c) 2018 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/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Solution Description . . . . . . . . . . . . . . . . . . . . 3
2.1. Area Address Assignment . . . . . . . . . . . . . . . . . 3
2.2. Area Address Checking . . . . . . . . . . . . . . . . . . 5
2.3. Default Route Advertising . . . . . . . . . . . . . . . . 7
3. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Overlapping Areas Use Case . . . . . . . . . . . . . . . 7
3.2. Maximum Area Addresses . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Spine-Leaf topology (a.k.a., CLOS topology) is widely used in today's
datacenter and campus networks. When the Spine-Leaf topology runs
the Intermediate System to Intermediate System (IS-IS) routing
protocol, each Leaf node receives Link State Packets (LSPs) from all
the other nodes thus having the entire routing information of the
topology. This is usually considered unnecessary and costly because
the Leaf node only needs to know its default gateways (i.e., the
Spine nodes it connects to) and the LSPs generated by the other Leaf
nodes bring little benefit for it to forward traffic.
To avoid Leaf nodes from learning the unnecessary LSPs from one
another, [IS-IS-SL-Extension] proposes a new TLV attached to the IS-
IS Hello (IIH) PDU to carry an router's role (i.e., Spine or Leaf) in
the topology. The Spine nodes then prevent all LSPs from being sent
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to the Leaf nodes, and each Leaf node sets the Spine nodes it
connects to as its default gateways.
This document proposes another solution to this problem, which
utilizes IS-IS's inherent multi-level and area partition features.
In particular, it requires that each Leaf node (configured as L1
router) SHOULD be assigned with a unique area address and each Spine
node (configured as L1/L2 router) MUST NOT advertise level-1 LSPs of
a given area to neighbors within another area. This prevents Leaf
nodes from receiving routing information from one another, without
introducing new message formats.
2. Solution Description
2.1. Area Address Assignment
+------------+ +------------+
| Spine-A | 10.10.10.0/24 | Spine-B |
| L1/L2 +----------------------+ L1/L2 |
| Area10/20 | .1 .2 | Area10/20 |
+---+--+-----+ +---+----+---+
.1 | | .1 .2 | | .1
| | 10.10.40.0/24 | |
| | +-----------------------------+ |
10.10.20.0/24 | | | | 10.10.30.0/24
| +--|-------------------------------+ |
| | 10.10.50.0/24 | |
.2 | | .1 .2 | | .2
+---+-----+--+ +-----+--+---+
| Leaf-A | | Leaf-B |
| L1 | | L1 |
| Area10 | | Area20 |
+-----+------+ +-----+------+
| |
| |
------+------- ------+-------
192.168.10.0/24 192.168.20.0/24
Figure 1: Topology Example
This section describes how to assign area addresses in the Spine-Leaf
topology, and illustrates why IS-IS routers SHOULD check the area
addresses before advertising level-1 LSPs. As shown in Figure 1,
there are two Spine nodes (i.e., Spine-A and Spine-B) and two Leaf
nodes (i.e., Leaf-A and Leaf-B). The System IDs of Spine-A, Spine-B,
Leaf-A, and Leaf-B are 1111.1111.1111.00 to 4444.4444.4444.00,
respectively.
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To prevent a Leaf node from learning the routing information of the
other ones, the following configurations are REQUIRED:
a. Leaf nodes SHOULD be configured as L1 routers and each of them
SHOULD be assigned a unique area address.
b. Spine nodes SHOULD be configured as L1/L2 routers and SHOULD be
assigned multiple area addresses with each being that of a given
Leaf node connected to it.
As a result, Leaf-A and Leaf-B in Figure 1 are configured as L1
routers and are assigned 10 and 20 as their area addresses,
respectively. Spine-A and Spine-B are configured as L1/L2 routers
and are assigned both 10 and 20 as their area addresses.
Level-1 Link State Database (Spine-A):
+--------------------+----------+--------+--------+------+--------+-----+
|LSPID |Seq Num |Checksum|Holdtime|Length|ATT/P/OL|Area |
+--------------------+----------+--------+--------+------+--------+-----+
|1111.1111.1111.00-00|0x0000006c|0x540b |743 |124 |0/0/0 |10/20|
+--------------------+----------+--------+--------+------+--------+-----+
|2222.2222.2222.00-00|0x0000006d|0x933b |1068 |124 |0/0/0 |10/20|
+--------------------+----------+--------+--------+------+--------+-----+
|3333.3333.3333.00-00|0x0000006b|0x1815 |402 |122 |0/0/0 |10 |
+--------------------+----------+--------+--------+------+--------+-----+
|4444.4444.4444.00-00|0x0000006a|0xf543 |431 |122 |0/0/0 |20 |
+--------------------+----------+--------+--------+------+--------+-----+
Level-2 Link State Database (Spine-A):
+--------------------+----------+--------+--------+------+--------+-----+
|LSPID |Seq Num |Checksum|Holdtime|Length|ATT/P/OL|Area |
+--------------------+----------+--------+--------+------+--------+-----+
|1111.1111.1111.00-00|0x0000006f|0x682f |743 |150 |0/0/0 |10/20|
+--------------------+----------+--------+--------+------+--------+-----+
|2222.2222.2222.00-00|0x00000063|0x30eb |1068 |150 |0/0/0 |10/20|
+--------------------+----------+--------+--------+------+--------+-----+
Figure 2: Link State Database of Spine-A
Under such configurations, however, Leaf-A still receives Leaf-B's
LSPs (and vice versa) even though they are in different areas. This
is because of the IS-IS definition that all routers in a specific
area SHOULD share the same level-1 Link State Database (LSDB). In
other words, IS-IS routers check area addresses during neighbor
establishment, but are regardless of area addresses when advertising
LSPs to a neighbor.
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The example in Figure 1 and the LSDB of Spine-A (in Figure 2) further
illustrate this. Since Spine-A and Leaf-B are both in area 20,
Spine-A will receive LSP 4444.4444.4444.00-00 from Leaf-B and store
the LSP into its level-1 LSDB. On the other hand, since Spine-A and
Leaf-A are both in area 10, Spine-A will advertise LSP
4444.4444.4444.00-00 to Leaf-A although Leaf-A and Leaf-B (generator
of the LSP) are in different areas. As a result, Leaf-A installs the
route 192.168.20.0/24 into its routing table (Figure 3), even though
it is an external area route.
Leaf-A Routing Table:
+---------------+-------+---+----+-----+----------+--------------+
|Destination |Proto |Pre|Cost|Flags|NextHop |Interface |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.10.0/24 |ISIS-L1|15 |20 |D |10.10.20.1|Ethernet0/0/0 |
| |ISIS-L1|15 |20 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.20.0/24 |Direct |0 |0 |D |127.0.0.1 |Ethernet0/0/0 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.30.0/24 |ISIS-L1|15 |20 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.40.0/24 |Direct |0 |0 |D |127.0.0.1 |Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.50.0/24 |ISIS-L1|15 |20 |D |10.10.20.1|Ethernet0/0/0 |
+---------------+-------+---+----+-----+----------+--------------+
|192.168.10.0/24|Direct |0 |0 |D |127.0.0.1 |GEthernet0/0/0|
+---------------+-------+---+----+-----+----------+--------------+
|192.168.20.0/24|ISIS-L1|15 |30 |D |10.10.20.1|Ethernet0/0/0 |
| |ISIS-L1|15 |30 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|127.0.0.0/8 |Direct |0 |0 |D |127.0.0.1 |InLoopBack0 |
+---------------+-------+---+----+-----+----------+--------------+
|0.0.0.0/0 |ISIS-L1|15 |10 |D |10.10.20.1|Ethernet0/0/0 |
| |ISIS-L1|15 |10 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
Figure 3: Routing Table of Leaf-A
Therefore, the solution proposed in this document requires that an
IS-IS router SHOULD check a level-1 LSP's area addresses before
advertising it to a neighbor (see Section 2.2).
2.2. Area Address Checking
Before advertising a level-1 LSP to a neighbor, an IS-IS router
SHOULD compare the area addresses associated with the LSP and the
ones associated with the neighbor. If they have at least one area
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address in common, the router SHOULD advertise the LSP to the
neighbor. Otherwise, the router MUST NOT advertise the LSP to the
neighbor.
In the former case, the router SHOULD remove every area addresse in
the LSP except the ones associated with the neighbor before the
advertisement. This makes the solution more compatible since the
Leaf nodes can be unaltered (see Section 3.2).
For instance, before Spine-A advertises LSP 1111.1111.1111.00-00 to
Leaf-A, it compares the LSP's area addresses (i.e., 10 and 20) with
Leaf-A's area address (i.e., 10). Since they have a common area
address 10, Spine-A SHOULD remove area address 20 from the LSP and
advertise the LSP to Leaf-A. On the other hand, before Spine-A
advertises LSP 4444.4444.4444.00-00 to Leaf-A, it checks their area
addresses and finds that they have no area address in common. So
Spine-A MUST NOT advertise the LSP to Leaf-A. As a result, Leaf-A
would not learn any routing information of Leaf-B, as shown in
Figure 4.
Leaf-A Routing Table:
+---------------+-------+---+----+-----+----------+--------------+
|Destination |Proto |Pre|Cost|Flags|NextHop |Interface |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.10.0/24 |ISIS-L1|15 |20 |D |10.10.20.1|Ethernet0/0/0 |
| |ISIS-L1|15 |20 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.20.0/24 |Direct |0 |0 |D |127.0.0.1 |Ethernet0/0/0 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.30.0/24 |ISIS-L1|15 |20 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.40.0/24 |Direct |0 |0 |D |127.0.0.1 |Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
|10.10.50.0/24 |ISIS-L1|15 |20 |D |10.10.20.1|Ethernet0/0/0 |
+---------------+-------+---+----+-----+----------+--------------+
|192.168.10.0/24|Direct |0 |0 |D |127.0.0.1 |GEthernet0/0/0|
+---------------+-------+---+----+-----+----------+--------------+
|127.0.0.0/8 |Direct |0 |0 |D |127.0.0.1 |InLoopBack0 |
+---------------+-------+---+----+-----+----------+--------------+
|0.0.0.0/0 |ISIS-L1|15 |10 |D |10.10.20.1|Ethernet0/0/0 |
| |ISIS-L1|15 |10 |D |10.10.40.2|Ethernet0/0/1 |
+---------------+-------+---+----+-----+----------+--------------+
Figure 4: Routing Table of Leaf-A
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2.3. Default Route Advertising
As defined in [RFC 1195], a L1/L2 router will indicate in its LSPs
that it is "attached" by setting the ATT bits. Therefore, each Leaf
node would set the Spine nodes as its default gateways and install a
default route in its routing table, as shown in Figure 4.
However, a specific IS-IS implementation in this case may not let the
L1/L2 router set the ATT bits, because it may speculate that the L1/
L2 router has lost connectivity to the level-2 backbone. To solve
this problem, operators can manually configure the L1/L2 router to
advertise a default route.
3. Compatibility
3.1. Overlapping Areas Use Case
In most deployments, an IS-IS router is assigned only one area
address, which will not be influenced by the area checking mechanism
proposed in this document. However, an IS-IS router might be
assigned more than one area addresses in some practical deployments
for the following reasons: 1) it is desirable to change the area
address of an area, 2) to merge two areas into one area, or 3) to
partition an area into two areas.
For instance, to change an area's address from X to Y, one can simply
add area address Y to all routers in the area, and then remove X from
them. Note that such operations would not disrupt live traffic in
the network.
Although the solution in this document requires IS-IS router to check
LSP's area addresses before advertising it, the above use cases are
still applicable and no compatible issue rises.
3.2. Maximum Area Addresses
The maximumAreaAddresses parameter in today's IS-IS implementation is
set to be 3 (or 0 which indicates 3) on consensus. Therefore, the
solution in this document also requires that Spine node SHOULD be
modified for supporting more area addresses. However, as LSPs sent
to a given neighbor only carry the area address(es) of the neighbor
(see Section 2.2), the solution does not require to modify Leaf
nodes.
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4. IANA Considerations
TBD.
5. Security Considerations
TBD.
6. Acknowledgements
TBD.
7. Normative References
[IS-IS-SL-Extension]
Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
Routing for Spine-Leaf Topology", draft-shen-isis-spine-
leaf-ext-05 (work in progress) , January 2018.
[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,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Authors' Addresses
Zhe Chen
Huawei
No. 156 Beiqing Rd
Beijing 100095
China
Email: chenzhe17@huawei.com
Xiaohu Xu
Alibaba
Email: xiaohu.xxh@alibaba-inc.com
Dean Cheng
Huawei
Email: dean.cheng@huawei.com
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