BGP Prefix Independent Convergence
draft-ietf-rtgwg-bgp-pic-01
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| Authors | Ahmed Bashandy , Clarence Filsfils , Prodosh Mohapatra | ||
| Last updated | 2016-06-21 | ||
| Replaces | draft-bashandy-rtgwg-bgp-pic | ||
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draft-ietf-rtgwg-bgp-pic-01
Network Working Group A. Bashandy, Ed.
Internet Draft C. Filsfils
Intended status: Informational Cisco Systems
Expires: December 2016 P. Mohapatra
Sproute Networks
June 20, 2016
BGP Prefix Independent Convergence
draft-ietf-rtgwg-bgp-pic-01.txt
Abstract
In the network comprising thousands of iBGP peers exchanging millions
of routes, many routes are reachable via more than one next-hop.
Given the large scaling targets, it is desirable to restore traffic
after failure in a time period that does not depend on the number of
BGP prefixes. In this document we proposed an architecture by which
traffic can be re-routed to ECMP or pre-calculated backup paths in a
timeframe that does not depend on the number of BGP prefixes. The
objective is achieved through organizing the forwarding data
structures in a hierarchical manner and sharing forwarding elements
among the maximum possible number of routes. The proposed technique
achieves prefix independent convergence while ensuring incremental
deployment, complete automation, and zero management and provisioning
effort. It is noteworthy to mention that the benefits of BGP-PIC are
hinged on the existence of more than one path whether as ECMP or
primary-backup.
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................4
1.2. Terminology...............................................4
2. Overview.......................................................5
3. Constructing the Shared Hierarchical Forwarding Chain..........7
3.1. Example 1: Primary-Backup Path Scenario...................8
3.2. Example 2: Platforms with Limited Levels of Hierarchy.....9
4. Forwarding Behavior...........................................13
5. Forwarding Chain Adjustment at a Failure......................15
5.1. BGP-PIC core.............................................16
5.2. BGP-PIC edge.............................................17
5.2.1. Adjusting forwarding Chain in egress node failure...17
5.2.2. Adjusting Forwarding Chain on PE-CE link Failure....17
5.3. Handling Failures for Flattended Forwarding Chains.......18
6. Properties....................................................19
6.1. Coverage.................................................19
6.1.1. A remote failure on the path to a BGP next-hop......19
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6.1.2. A local failure on the path to a BGP next-hop.......19
6.1.3. A remote iBGP next-hop fails........................20
6.1.4. A local eBGP next-hop fails.........................20
6.2. Performance..............................................20
6.2.1. Perspective.........................................20
6.3. Automated................................................21
6.4. Incremental Deployment...................................22
7. Dependency....................................................22
7.1. Hierarchical Hardware FIB................................22
7.2. Availability of more than one primary or secondary BGP next-
hops..........................................................22
7.3. Pre-Computation of a secondary BGP next-hop..............23
8. Security Considerations.......................................23
9. IANA Considerations...........................................23
10. Conclusions..................................................23
11. Acknowledgments..............................................25
12. References...................................................23
12.1. Normative References....................................23
12.2. Informative References..................................24
1. Introduction
As a path vector protocol, BGP is inherently slow due to the
serial nature of reachability propagation. BGP speakers exchange
reachability information about prefixes[2][3] and, for labeled
address families, namely AFI/SAFI 1/4, 2/4, 1/128, and 2/128, an
edge router assigns local labels to prefixes and associates the
local label with each advertised prefix such as L3VPN [8], 6PE
[9], and Softwire [7] using BGP label unicast technique[4]. A BGP
speaker then applies the path selection steps to choose the best
path. In modern networks, it is not uncommon to have a prefix
reachable via multiple edge routers. In addition to proprietary
techniques, multiple techniques have been proposed to allow for
BGP to advertise more than one path for a given prefix
[6][11][12], whether in the form of equal cost multipath or
primary-backup. Another more common and widely deployed scenario
is L3VPN with multi-homed VPN sites with unique Route
Distinguisher.
This document proposes a hierarchical and shared forwarding chain
organization that allows traffic to be restored to pre-calculated
alternative equal cost primary path or backup path in a time
period that does not depend on the number of BGP prefixes. The
technique relies on internal router behavior that is completely
transparent to the operator and can be incrementally deployed and
enabled with zero operator intervention.
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1.1. 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 RFC-2119
[1].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to
be interpreted as carrying RFC-2119 significance.
1.2. Terminology
This section defines the terms used in this document. For ease of
use, we will use terms similar to those used by L3VPN [8]
o BGP prefix: It is a prefix P/m (of any AFI/SAFI) that a BGP
speaker has a path for.
o IGP prefix: It is a prefix P/m (of any AFI/SAFI) that is learnt
via an Interior Gateway Protocol, such as OSPF and ISIS, has a
path for. The prefix may be learnt directly through the IGP or
redistributed from other protocol(s)
o CE: It is an external router through which an egress PE can
reach a prefix P/m.
o Ingress PE, "iPE": t is a BGP speaker that learns about a prefix
through a IBGP peer and chooses an egress PE as the next-hop for
the prefix..
o Path: It is the next-hop in a sequence of unique connected
nodes starting from the current node and ending with the
destination node or network identified by the prefix.
o Recursive path: It is a path consisting only of the IP address
of the next-hop without the outgoing interface. Subsequent
lookups are needed to determine the outgoing interface.
o Non-recursive path: It is a path consisting of the IP address
of the next-hop and one outgoing interface
o Primary path: It is a recursive or non-recursive path that can
be used all the time. A prefix can have more than one primary
path
o Backup path: It is a recursive or non-recursive path that can
be used only after some or all primary paths become unreachable
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o Leaf: A leaf is container data structure for a prefix or local
label. Alternatively, it is the data structure that contains
prefix specific information.
o IP leaf: Is the leaf corresponding to an IPv4 or IPv6 prefix
o Label leaf. It is the leaf corresponding to a locally allocated
label such as the VPN label on an egress PE [8].
o Pathlist: It is an array of paths used by one or more prefix to
forward traffic to destination(s) covered by a IP prefix. Each
path in the pathlist carries its "path-index" that identifies
its position in the array of paths. A pathlist may contain a
mix of primary and backup paths
o OutLabel-List: Each labeled prefix is associated with an
OutLabel-List. The OutLabel-List is an array of one or more
outgoing labels and/or label actions where each label or label
action has 1-to-1 correspondence to a path in the pathlist.
Label actions are: push the label, pop the label, or swap the
incoming label with the label in the Outlabel-Array entry. The
prefix may be an IGP or BGP prefix
o Adjacency: It is the layer 2 encapsulation leading to the layer
3 directly connected next-hop
o Dependency: An object X is said to be a dependent or Child of
object Y if Object Y cannot be deleted unless object X is no
longer a dependent/child of object Y
o Route: It is a prefix with one or more paths associated with
it. Hence the minimum set of objects needed to construct a
route is a leaf and a pathlist.
2. Overview
The idea of BGP-PIC is based on two pillars
o A shared hierarchal Forwarding Chain
o A forwarding plane that supports multiple levels of indirection
To illustrate the two pillars above, we will use an example of a
simple multihomed L3VPN [8] prefix in a BGP-free core running LDP
[5] or segment routing over MPLS forwarding plane [14].
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+--------------------------------+
| |
| ePE2
| | \
| | \
| | \
iPE | CE.......VRF "Blue"
| | / (VPN-IP1)
| | / (VPN-IP2)
| LDP/Segment-Routing Core | /
| ePE1
| |
+--------------------------------+
Figure 1 VPN prefix reachable via multiple PEs
Referring to Figure 1, suppose the iPE (the ingress PE) receives
NLRIs for the VPN prefixes VPN-IP1 and VPN-IP2 from two egress PEs,
ePE1 and ePE2 with next-hop BGP-NH1 and BGP-NH2, respectively.
Assume that ePE1 advertise the VPN labels VPN-L11 and VPN-L12 while
ePE2 advertise the VPN labels VPN-L21 and VPN-L22 for VPN-IP1 and
VPN-IP2, respectively. Suppose that BGP-NH1 and BGP-NH2 are resolved
via the IGP prefixes IGP-IP1 and IGP-P2, where each happen to have 2
ECMP paths with IGP-NH1 and IGP-NH2 reachable via the interfaces I1
and I2, respectively. Suppose that local labels (whether LDP[5] or
segment routing [14]) on the downstream LSRs for IGP-IP1 are IGP-L11
and IGP-L12 while for IGP-P2 are IGP-L21 and IGP-L22.
Based on the information about NLRIs and the resolving IGP prefixes,
a hierarchical forwarding chain can be constructed as shown in
Figure 2.
IP Leaf: Pathlist: IP Leaf: Pathlist:
-------- +-------+ -------- +----------+
VPN-IP1-->|BGP-NH1|-->IGP-IP1(BGP NH1)--->|IGP NH1,I1|--->Adjacency1
| |BGP-NH2|-->.... | |IGP NH2,I2|--->Adjacency2
| +-------+ | +----------+
| |
| |
v v
OutLabel-List: OutLabel-List:
+----------------------+ +----------------------+
|VPN-L11 (VPN-IP1, NH1)| |IGP-L11 (IGP-IP1, NH1)|
|VPN-L12 (VPN-IP1, NH2)| |IGP-L12 (IGP-IP1, NH2)|
+----------------------+ +----------------------+
Figure 2 Shared Hierarchical Forwarding Chain at iPE
The forwarding chain depicted in Figure 2 illustrates the first
pillar, which is sharing and hierarchy. We can see that the BGP
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pathlist consisting of BGP-NH1 and BGP-NH2 is shared by all NLRIs
reachable via ePE1 and ePE2. As such, it is possible to make changes
to the pathlist without having to make changes to the NLRIs. For
example, if BGP-NH2 becomes unreacreachable, there is no need to
modify any of the possibly large number of NLRIs. Instead only the
shared pathlist needs to be modified. Likewise, due to the
hierarchical structure of the forwarding chain, it is possible to
make modifications to the IGP routes without having to make any
changes to the BGP NLRIs. For example, if the interface "I2" goes
down, only the shared IGP pathlist needs to be updated, but none of
the IGP prefixes sharing the IGP pathlist nor the BGP NLRIs using
the IGP prefixes for resolution need to be modified.
Figure 2 can also be used to illustrate the second BGP-PIC pillar.
Having a deep forwarding chain such as the one illustrated in Figure
2 requires a forwarding plane that is capable of accessing multiple
levels of indirection in order to calculate the outgoing
interface(s) and next-hops(s). While a deeper forwarding chain
minimizes the re-convergence time on topology change, there will
always exist platforms with limited capabilities and hence imposing
a limit on the depth of the forwarding chain. The example in Section
3.2 illustrates how to gracefully trade off convergence speed with
the number of hierarchical levels to support platforms with
different capabilities.
3. Constructing the Shared Hierarchical Forwarding Chain
Constructing the forwarding chain is an application of the two
pillars described in Section 2.
The whole process starts when BGP downloads a prefix to FIB. The
prefix contains one or more outgoing paths. For certain labeled
prefixes, such as VPN [8] prefixes, each path may be associated with
an outgoing label and the prefix itself may be assigned a local
label. The list of outgoing paths defines a pathlist. If such
pathlist does not already exist, then FIB creates a new pathlist,
otherwise the existing pathlist is used. The BGP prefix is added as
a dependent of the pathlist.
The previous step constructs the upper part of the hierarchical
forwarding chain. The forwarding chain is completed by resolving the
paths of the pathlist. A BGP path usually consists of a next-hop.
The next-hop is resolved by finding a matching IGP prefix.
The end result is a hierarchical shared forwarding chain where the
BGP pathlist is shared by all BGP prefixes that use the same list of
paths and the IGP prefix is shared by all pathlists that have a path
resolving via that IGP prefix. It is noteworthy to mention that the
forwarding chain is constructed without any operator intervention at
all.
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The remainder of this section illustrates two examples. The first
example illustrates the applicability of BGP-PIC in a primary-backup
path deployment. The second example illustrates how BGP-PIC can be
applied in cases where the forwarding plane supports limited number
of indirections.
3.1. Example 1: Primary-Backup Path Scenario
Consider the egress PE ePE1 in the case of the multi-homed VPN
prefixes in the BGP-free core depicted in Figure 1. Suppose ePE1
determines that the primary path is the external path but the backup
path is the iBGP path to the other PE ePE2 with next-hop BGP-NH2.
ePE2 constructs the forwarding chain depicted in Figure 3. We are
only showing a single VPN prefix for simplicity. But all prefixes
that are multihomed to ePE1 and ePE2 share the BGP pathlist.
BGP OutLabel Array
VPN-L11 +---------+
(Label-leaf)---+---->|Unlabeled|
| +---------+
| | VPN-L21 |
| | (swap) |
| +---------+
| ^
| | BGP Pathlist
| | +------------+ Connected route
| | | CE-NH |------>(to the CE)
| | |path-index=0|
| | +------------+
V | | VPN-NH2 |
VPN-IP1 -----------------+------>| (backup) |------>IGP Leaf
(IP prefix leaf) |path-index=1| (Towards ePE2)
+------------+
Figure 3 : VPN Prefix Forwarding Chain with eiBGP paths on egress PE
The example depicted in Figure 3 differs from the example in Figure
2 in two main aspects. First, as long as the primary path towards
the CE (external path) is useable, it will be the only path used for
forwarding while the OutLabel-List contains both the unlabeled label
(primary path) and the VPN label (backup path) advertised by the
backup path ePE2. The second aspect is presence of the label leaf
corresponding to the VPN prefix. This label leaf is used to match
VPN traffic arriving from the core. Note that the label leaf shares
the OutLabel-List and the pathlist with the IP prefix.
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3.2. Example 2: Platforms with Limited Levels of Hierarchy
This example uses a case of inter-AS option C [8] where there are 3
levels of hierarchy. Figure 4 illustrates the sample topology. To
force 3 levels of hierarchy, the ASBRs on the ingress domain (domain
1) advertise the core routers of the egress domain (domain 2) to the
ingress PE (iPE) via BGP-LU [4] instead of redistributing them into
the IGP of domain 1. The end result is that the ingress PE (iPE) has
2 levels of recursion for the VPN prefix VPN-IP1 and VPN2-P2.
Domain 1 Domain 2
+----------------+ +-------------+
| | | |
| LDP/SR Core | | LDP/SR core |
| | | |
| ASBR11------ASBR21.......ePE1\
| | \ / | . . | \
| | \ / | . . | \
| | \/ | .. | \VPN-IP1
| | /\ | . . | /
| | / \ | . . | /
| | / \ | . . | /
iPE ASBR12------ASBR22.......ePE2
| | | | \
| | | | \
| | | | \
| | | | /VPN-IP2
| | | | /
| | | | /
| ASBR13------ASBR23.......ePE3/
| | | |
| | | |
+----------------+ +-------------+
<============== <========= <============
Advertise PE2x Advertise Redistribute
Using iBGP-LU PE2x Using IGP into
eBGP-LU BGP
Figure 4 Sample 3-level hierarchy topology
We will make the following assumptions about connectivity
o In "domain 2", both ASBR21 and ASBR22 can reach both ePE1 and
ePE2 using the same distance
o In "domain 2", only ASBR23 can reach ePE3
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o In "domain 1", iPE (the ingress PE) can reach ASBR11, ASBR12, and
ASBR13 via IGP using the same distance.
We will make the following assumptions about the labels
o The VPN labels advertised by ePE1 and ePE2 for prefix VPN-IP1 are
VPN-L11 and VPN-L21, respectively
o The VPN labels advertised by ePE2 and ePE3 for prefix VPN-IP2 are
VPN-L22 and VPN-L32, respectively
o The labels advertised by ASBR11 to iPE using BGP-LU [4] for the
egress PEs ePE1 and ePE2 are LASBR11(ePE1) and LASBR11(ePE2),
respectively.
o The labels advertised by ASBR12 to iPE using BGP-LU [4] for the
egress PEs ePE1 and ePE2 are LASBR12(ePE1) and LASBR12(ePE2),
respectively
o The label advertised by ASBR11 to iPE using BGP-LU [4] for the
egress PE ePE3 is LASBR13(ePE3)
o The IGP labels advertised by the next hops directly connected to
iPE towards ASBR11, ASBR12, and ASBR13 in the core of domain 1
are IGP-L11, IGP-L12, and IGP-L13, respectively.
The diagram in Figure 5 illustrates the forwarding chain in iPE
assuming that the forwarding hardware in iPE supports 3 levels of
hierarchy. The leaves corresponding to the ABSRs on domain 1
(ASBR11, ASBR12, and ASBR13) are at the bottom of the hierarchy.
There are few important points:
o Because the hardware supports the required depth of hierarchy,
the sizes of a pathlist equal the size of the label list
associated with the leaves using this pathlist
o The index inside the pathlist entry indicates the label that will
be picked from the Outlabel-List if that path is chosen by the
forwarding engine hashing function.
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Outlabel-List Outlabel-List
For VPN-IP1 For VPN-IP2
+------------+ +--------+ +-------+ +------------+
| VPN-L11 |<---| VPN-IP1| |VPN-IP2|-->| VPN-L22 |
+------------+ +---+----+ +---+---+ +------------+
| VPN-L21 | | | | VPN-L32 |
+------------+ | | +------------+
| |
V V
+---+---+ +---+---+
| 0 | 1 | | 0 | 1 |
+-|-+-\-+ +-/-+-\-+
| \ / \
| \ / \
| \ / \
| \ / \
v \ / \
+-----+ +-----+ +-----+
+----+ ePE1| |ePE2 +-----+ | ePE3+-----+
| +--+--+ +-----+ | +--+--+ |
v | / v | v
+-------------+ | / +-------------+ | +-------------+
|LASBR11(ePE1)| | / |LASBR11(ePE2)| | |LASBR13(ePE3)|
+-------------+ | / +-------------+ | +-------------+
|LASBR12(ePE1)| | / |LASBR12(ePE2)| | Outlabel-List
+-------------+ | / +-------------+ | For ePE3
Outlabel-List | / Outlabel-List |
For ePE1 | / For ePE2 |
| / |
| / |
| / |
v / v
+---+---+ Shared Pathlist +---+ Pathlist
| 0 | 1 | For ePE1 and ePE2 | 0 | For ePE3
+-|-+-\-+ +-|-+
| \ |
| \ |
| \ |
| \ |
v \ v
+------+ +------+ +------+
+---+ASBR11| |ASBR12+--+ |ASBR13+---+
| +------+ +------+ | +------+ |
v v v
+-------+ +-------+ +-------+
|IGP-L11| |IGP-L12| |IGP-L13|
+-------+ +-------+ +-------+
Figure 5 : Forwarding Chain for hardware supporting 3 Levels
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Now suppose the hardware on iPE (the ingress PE) supports 2 levels
of hierarchy only. In that case, the 3-levels forwarding chain in
Figure 5 needs to be "flattended" into 2 levels only.
Outlabel-List Outlabel-List
For VPN-IP1 For VPN-IP2
+------------+ +-------+ +-------+ +------------+
| VPN-L11 |<---|VPN-IP1| | VPN-IP2|--->| VPN-L22 |
+------------+ +---+---+ +---+---+ +------------+
| VPN-L21 | | | | VPN-L32 |
+------------+ | | +------------+
| |
| |
| |
Flattened | | Flattened
pathlist V V pathlist
+===+===+ +===+===+===+ +=============+
+--------+ 0 | 1 | | 0 | 0 | 1 +---->|LASBR11(ePE2)|
| +=|=+=\=+ +=/=+=/=+=\=+ +=============+
v | \ / / \ |LASBR12(ePE2)|
+=============+ | \ +-----+ / \ +=============+
|LASBR11(ePE1)| | \/ / \ |LASBR13(ePE3)|
+=============+ | /\ / \ +=============+
|LASBR12(ePE1)| | / \ / \
+=============+ | / \ / \
| / \ / \
| / + + \
| + | | \
| | | | \
v v v v \
+------+ +------+ +------+
+----|ASBR11| |ASBR12+---+ |ASBR13+---+
| +------+ +------+ | +------+ |
v v v
+-------+ +-------+ +-------+
|IGP-L11| |IGP-L12| |IGP-L13|
+-------+ +-------+ +-------+
Figure 6 : Flattening 3 levels to 2 levels of Hierarchy on iPE
Figure 6 represents one way to "flatten" a 3 levels hierarchy into
two levels. There are few important points:
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o The flattened pathlists have label lists associated with them.
The size of the label list associated with the flattened pathlist
equals the size of the pathlist. Hence it is possible that an
implementation includes these label lists in the flattened
pathlist itself
o Because of "flattening", the size of a flattened pathlist may not
be equal to the size of the label lists of leaves using the
flattened pathlist.
o The indices inside a flattened pathlist still indicate the label
index in the Outlabel-Lists of the leaves using that pathlist.
Because the size of the flattened pathlist may be different from
the size of the label lists of the leaves, the indices may be
repeated
o Let's take a look at the flattened pathlist used by the prefix
"VPN-IP2", The pathlist associated with the prefix "VPN-IP2" has
three entries.
o The first and second entry have index "0". This is because
both entries correspond to ePE2. Hence when hashing performed
by the forwarding engine results in using first or the second
entry in the pathlist, the forwarding engine will pick the
correct VPN label "VPN-L22", which is the label advertised by
ePE2 for the prefix "VPN-IP2"
o The third entry has the index "1". This is because the third
entry corresponds to ePE3. Hence when the hashing is performed
by the forwarding engine results in using the third entry in
the flattened pathlist, the forwarding engine will pick the
correct VPN label "VPN-L32", which is the label advertised by
"ePE3" for the prefix "VPN-IP2"
4. Forwarding Behavior
This section explains how the forwarding plane uses the hierarchical
shared forwarding chain to forward a packet.
When a packet arrives at a router, it matches a leaf. A labeled
packet matches a label leaf while an IP packet matches an IP prefix
leaf. The forwarding engines walks the forwarding chain starting
from the leaf until the walk terminates on an adjacency. Thus when a
packet arrives, the chain is walked as follows:
1. Lookup the leaf based on the destination address or the label at
the top of the packet
2. Retrieve the parent pathlist of the leaf
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3. Pick the outgoing path from the list of resolved paths in the
pathlist. The method by which the outgoing path is picked is
beyond the scope of this document (i.e. flow-preserving hash
exploiting entropy within the MPLS stack and IP header). Let the
"path-index" of the outgoing path be "i".
4. If the prefix is labeled, use the "path-index" "i" to retrieve
the ith label "Li" stored the ith entry in the OutLabel-List and
apply the label action of the label on the packet (e.g. for VPN
label on the ingress PE, the label action is "push").
5. Move to the parent of the chosen path "i"
6. If the chosen path "i" is recursive, move to its parent prefix
and go to step 2
7. If the chosen path "i" is non-recursive move to its parent
adjacency
8. Encapsulate the packet in the L2 string specified by the
adjacency and send the packet out.
Let's apply the above forwarding steps to the forwarding chain
depicted in Figure 2 in Section 2. Suppose a packet arrives at
ingress PE iPE from an external neighbor. Assume the packet matches
the VPN prefix VPN-IP1. While walking the forwarding chain, the
forwarding engine applies a hashing algorithm to choose the path and
the hashing at the BGP level yields path 0 while the hashing at the
IGP level yields path 1. In that case, the packet will be sent out
of interface I2 with the label stack "IGP-L12,VPN-L11".
Now let's try and apply the above steps to the flattened forwarding
chain illustrated in Figure 6.
o Suppose a packet arrives at "iPE" and matches the VPN prefix
"VPN-IP2"
o The forwarding engine walks to the parent of the "VPN_P2", which
is the flattened pathlist and applies a hashing algorithm to pick
a path
o Suppose the hashing by the forwarding engine picks the second
entry in the flattened pathlist associated with the leaf "VPN-
IP2".
o Because the second entry has the index "0", the label "VPN-L22"
is pushed on the packet
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o At the same time, the forwarding engine picks the second label
from the Outlabel-Array associated with the flattened pathlist.
Hence the next label that is pushed is "LASBR12(ePE2)"
o The forwarding engine now moves to the parent of the flattened
pathlist corresponding to the second entry. The parent is the IGP
label leaf corresponding to "ASBR12"
o So the packet is forwarded towards the ASBR "ASBR12" and the IGP
label at the top will be "L12"
Based on the above steps, a packet arriving at iPE and destined to
the prefix VPN-L22 reaches its destination as follows
o iPE sends the packet along the shortest path towards ASBR12 with
the following label stack starting from the top: {L12,
LASBR12(ePE2), VPN-L22}.
o The penultimate hop of ASBR12 pops the top label "L12". Hence the
packet arrives at ASBR12 with the label stack {LASBR12(ePE2),
VPN-L22} where "LASBR12(ePE2)" is the top label.
o ASBR12 swaps "LASBR12(ePE2)" with the label "LASBR22(ePE2)",
which is the label advertised by ASBR22 for the ePE2 (the egress
PE).
o ASBR22 receives the packet with "LASBR22(ePE2)" at the top.
o Hence ASBR22 swaps "LASBR22(ePE2)" with the IGP label for ePE2
advertised by the next-hop towards ePE2 in domain 2, and sends
the packet along the shortest path towards ePE2.
o The penultimate hop of ePE2 pops the top label. Hence ePE2
receives the packet with the top label VPN-L22 at the top
o ePE2 pops "VPN-L22" and sends the packet as a pure IP packet
towards the destination VPN-IP2.
5. Forwarding Chain Adjustment at a Failure
The hierarchical and shared structure of the forwarding chain
explained in Section 2 allows modifying a small number of
forwarding chain objects to re-route traffic to a pre-calculated
equal-cost or backup path without the need to modify the possibly
very large number of BGP prefixes. In this section, we go over
various core and edge failure scenarios to illustrate how FIB
manager can utilize the forwarding chain structure to achieve BGP
prefix independent convergence.
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5.1. BGP-PIC core
This section describes the adjustments to the forwarding chain when
a core link or node fails but the BGP next-hop remains reachable.
There are two case: remote link failure and attached link failure.
Node failures are treated as link failures.
When a remote link or node fails, IGP on the ingress PE receives
advertisement indicating a topology change so IGP re-converges to
either find a new next-hop and/or outgoing interface or remove the
path completely from the IGP prefix used to resolve BGP next-hops.
IGP and/or LDP download the modified IGP leaves with modified
outgoing labels for labeled core.
When a local link fails, FIB manager detects the failure almost
immediately. The FIB manager marks the impacted path(s) as unusable
so that only useable paths are used to forward packets. Hence only
IGP pathlists with paths using the failed local link need to be
modified. All other pathlists are not impacted. Note that in this
particular case there is actually no need even to backwalk to IGP
leaves to adjust the OutLabel-Lists because FIB can rely on the
path-index stored in the useable paths in the pathlist to pick the
right label.
It is noteworthy to mention that because FIB manager modifies the
forwarding chain starting from the IGP leaves only, BGP pathlists
and leaves are not modified. Hence traffic restoration occurs within
the time frame of IGP convergence, and, for local link failure,
assuming a backup path has been precomputed, within the timeframe of
local detection (e.g. 50ms). Examples of solutions that pre-
computing backup paths are IP FRR [16] remote LFA [17], Ti-LFA [15]
and MRT [18] or eBGP path having a backup path [10].
Let's apply the procedure to the forwarding chain depicted in Figure
2. Suppose a remote link failure occurs and impacts the first ECMP
IGP path to the remote BGP next-hop. Upon IGP convergence, the IGP
pathlist used by the BGP next-hop is updated to reflect the new
topology (one path instead of two). As soon as the IGP convergence
is effective for the BGP next-hop entry, the new forwarding state is
immediately available to all dependent BGP prefixes. The same
behavior would occur if the failure was local such as an interface
going down. As soon as the IGP convergence is complete for the BGP
next-hop IGP route, all its BGP depending routes benefit from the
new path. In fact, upon local failure, if LFA protection is enabled
for the IGP route to the BGP next-hop and a backup path was pre-
computed and installed in the pathlist, upon the local interface
failure, the LFA backup path is immediately activated (sub-50msec)
and thus protection benefits all the depending BGP traffic through
the hierarchical forwarding dependency between the routes.
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5.2. BGP-PIC edge
This section describes the adjustments to the forwarding chains as a
result of edge node or edge link failure.
5.2.1. Adjusting forwarding Chain in egress node failure
When an edge node fails, IGP on neighboring core nodes send route
updates indicating that the edge node is no longer reachable. IGP
running on the iBGP peers instructs FIB to remove the IP and label
leaves corresponding to the failed edge node from FIB. So FIB
manager performs the following steps:
o FIB manager deletes the IGP leaf corresponding to the failed edge
node
o FIB manager backwalks to all dependent BGP pathlists and marks
that path using the deleted IGP leaf as unresolved
o Note that there is no need to modify BGP leaves because each path
in the pathlist carries its path index and hence the correct
outgoing label will be picked. Consider for example the
forwarding chain depicted in Figure 2. If the 1st BGP path
becomes unresolved, then the forwarding engine will only use the
second path for forwarding. Yet the pathindex of that single
resolved path will still be 1 and hence the label VPN-L12 will be
pushed.
5.2.2. Adjusting Forwarding Chain on PE-CE link Failure
Suppose the link between an edge router and its external peer fails.
There are two scenarios (1) the edge node attached to the failed
link performs next-hop self and (2) the edge node attached to the
failure advertises the IP address of the failed link as the next-hop
attribute to its iBGP peers.
In the first case, the rest of iBGP peers will remain unaware of the
link failure and will continue to forward traffic to the edge node
until the edge node attached to the failed link withdraws the BGP
prefixes. If the destination prefixes are multi-homed to another
iBGP peer, say ePE2, then FIB manager on the edge router detecting
the link failure applies the following steps:
o FIB manager backwalks to the BGP pathlists marks the path through
the failed link to the external peer as unresolved
o Hence traffic will be forwarded used the backup path towards ePE2
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o For labeled traffic
o The Outlabel-List attached to the BGP leaf already contains
an entry corresponding to the backup path.
o The label entry in OutLabel-List corresponding to the
internal path to backup egress PE has swap action to the
label advertised by backup egress PE
o For an arriving label packet (e.g. VPN), the top label is
swapped with the label advertised by backup egress PE and the
packet is sent towards that backup egress PE
o For unlabeled traffic, packets are simply redirected towards
backup egress PE.
In the second case where the edge router uses the IP address of the
failed link as the BGP next-hop, the edge router will still perform
the previous steps. But, unlike the case of next-hop self, IGP on
failed edge node informs the rest of the iBGP peers that IP address
of the failed link is no longer reachable. Hence the FIB manager on
iBGP peers will delete the IGP leaf corresponding to the IP prefix
of the failed link. The behavior of the iBGP peers will be identical
to the case of edge node failure outlined in Section 5.2.1.
It is noteworthy to mention that because the edge link failure is
local to the edge router, sub-50 msec convergence can be achieved as
described in [10].
Let's try to apply the case of next-hop self to the forwarding chain
depicted in Figure 3. After failure of the link between ePE1 and CE,
the forwarding engine will route traffic arriving from the core
towards VPN-NH2 with path-index=1. A packet arriving from the core
will contain the label VPN-L11 at top. The label VPN-L11 is swapped
with the label VPN-L21 and the packet is forwarded towards ePE2.
5.3. Handling Failures for Flattended Forwarding Chains
As explained in the Example in Section 3.2 if the number of
hierarchy levels of a platform cannot support the native number of
hierarchy levels of a recursive forwarding chain, the instantiated
forwarding chain is constructed by flattening two or more levels.
Hence a 3 levels chain in Figure 5 is flattened into the 2 levels
chain in Figure 6.
While reducing the benefits of BGP-PIC, flattening one hierarchy
into a shallower hierarchy does not always result in a complete loss
of the benefits of the BGP-PIC. To illustrate this fact suppose
ASBR12 is no longer reachable in domain 1. If the platform supports
the full hierarchy depth, the forwarding chain is the one depicted
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in Figure 5 and hence the FIB manager needs to backwalk one level to
the pathlist shared by "ePE1" and "ePE2" and adjust it. If the
platform supports 2 levels of hierarchy, then a useable forwarding
chain is the one depicted in Figure 6. In that case, if ASBR12 is no
longer reachable, the FIB manager has to backwalk to the two
flattened pathlists and update both of them.
The main observation is that the loss of convergence speed due to
the loss of hierarchy depth depends on the structure of the
forwarding chain itself. To illustrate this fact, let's take two
extremes. Suppose the forwarding objects in level i+1 depend on the
forwarding objects in level i. If every object on level i+1 depends
on a separate object in level i, then flattening level i into level
i+1 will not result in loss of convergence speed. Now let's take the
other extreme. Suppose "n" objects in level i+1 depend on 1 object
in level i. Now suppose FIB flattens level i into level i+1. If a
topology change results in modifying the single object in level i,
then FIB has to backwalk and modify "n" objects in the flattened
level, thereby losing all the benefit of BGP-PIC. Experience shows
that flattening forwarding chains usually results in moderate loss
of BGP-PIC benefits. Further analysis is needed to corroborate and
quantify this statement.
6. Properties
6.1. Coverage
All the possible failures, except CE node failure, are covered,
whether they impact a local or remote IGP path or a local or remote
BGP next-hop as described in Section 5. This section provides
details for each failure and now the hierarchical and shared FIB
structure proposed in this document allows recovery that does not
depend on number of BGP prefixes.
6.1.1. A remote failure on the path to a BGP next-hop
Upon IGP convergence, the IGP leaf for the BGP next-hop is updated
upon IGP convergence and all the BGP depending routes leverage the
new IGP forwarding state immediately.
This BGP resiliency property only depends on IGP convergence and is
independent of the number of BGP prefixes impacted.
6.1.2. A local failure on the path to a BGP next-hop
Upon LFA protection, the IGP leaf for the BGP next-hop is updated to
use the precomputed LFA backup path and all the BGP depending routes
leverage this LFA protection.
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This BGP resiliency property only depends on LFA protection and is
independent of the number of BGP prefixes impacted.
6.1.3. A remote iBGP next-hop fails
Upon IGP convergence, the IGP leaf for the BGP next-hop is deleted
and all the depending BGP Path-Lists are updated to either use the
remaining ECMP BGP best-paths or if none remains available to
activate precomputed backups.
This BGP resiliency property only depends on IGP convergence and is
independent of the number of BGP prefixes impacted.
6.1.4. A local eBGP next-hop fails
Upon local link failure detection, the adjacency to the BGP next-hop
is deleted and all the depending BGP pathlists are updated to either
use the remaining ECMP BGP best-paths or if none remains available
to activate precomputed backups.
This BGP resiliency property only depends on local link failure
detection and is independent of the number of BGP prefixes impacted.
6.2. Performance
When the failure is local (a local IGP next-hop failure or a local
eBGP next-hop failure), a pre-computed and pre-installed backup is
activated by a local-protection mechanism that does not depend on
the number of BGP destinations impacted by the failure. Sub-50msec
is thus possible even if millions of BGP routes are impacted.
When the failure is remote (a remote IGP failure not impacting the
BGP next-hop or a remote BGP next-hop failure), an alternate path is
activated upon IGP convergence. All the impacted BGP destinations
benefit from a working alternate path as soon as the IGP convergence
occurs for their impacted BGP next-hop even if millions of BGP
routes are impacted.
6.2.1. Perspective
The following table puts the BGP PIC benefits in perspective
assuming
o 1M impacted BGP prefixes
o IGP convergence ~ 500 msec
o local protection ~ 50msec
o FIB Update per BGP destination ~ 100usec conservative,
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~ 10usec optimistic
o BGP Convergence per BGP destination ~ 200usec conservative,
~ 100usec optimistic
Without PIC With PIC
Local IGP Failure 10 to 100sec 50msec
Local BGP Failure 100 to 200sec 50msec
Remote IGP Failure 10 to 100sec 500msec
Local BGP Failure 100 to 200sec 500msec
Upon local IGP next-hop failure or remote IGP next-hop failure, the
existing primary BGP next-hop is intact and usable hence the
resiliency only depends on the ability of the FIB mechanism to
reflect the new path to the BGP next-hop to the depending BGP
destinations. Without BGP PIC, a conservative back-of-the-envelope
estimation for this FIB update is 100usec per BGP destination. An
optimistic estimation is 10usec per entry.
Upon local BGP next-hop failure or remote BGP next-hop failure,
without the BGP PIC mechanism, a new BGP Best-Path needs to be
recomputed and new updates need to be sent to peers. This depends on
BGP processing time that will be shared between best-path
computation, RIB update and peer update. A conservative back-of-the-
envelope estimation for this is 200usec per BGP destination. An
optimistic estimation is 100usec per entry.
6.3. Automated
The BGP PIC solution does not require any operator involvement. The
process is entirely automated as part of the FIB implementation.
The salient points enabling this automation are:
o Extension of the BGP Best Path to compute more than one primary
([11]and [12]) or backup BGP next-hop ([6] and [13]).
o Sharing of BGP Path-list across BGP destinations with same
primary and backup BGP next-hop
o Hierarchical indirection and dependency between BGP pathlist and
IGP pathlist
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6.4. Incremental Deployment
As soon as one router supports BGP PIC solution, it benefits from
all its benefits without any requirement for other routers to
support BGP PIC.
7. Dependency
This section describes the required functionality in the forwarding
and control planes to support BGP-PIC described in this document
7.1. Hierarchical Hardware FIB
BGP PIC requires a hierarchical hardware FIB support: for each BGP
forwarded packet, a BGP leaf is looked up, then a BGP Pathlist is
consulted, then an IGP Pathlist, then an Adjacency.
An alternative method consists in "flattening" the dependencies when
programming the BGP destinations into HW FIB resulting in
potentially eliminating both the BGP Path-List and IGP Path-List
consultation. Such an approach decreases the number of memory
lookup's per forwarding operation at the expense of HW FIB memory
increase (flattening means less sharing hence duplication), loss of
ECMP properties (flattening means less pathlist entropy) and loss of
BGP PIC properties.
7.2. Availability of more than one primary or secondary BGP next-
hops
When the primary BGP next-hop fails, BGP PIC depends on the
availability of a pre-computed and pre-installed secondary BGP next-
hop in the BGP Pathlist.
The existence of a secondary next-hop is clear for the following
reason: a service caring for network availability will require two
disjoint network connections hence two BGP next-hops.
The BGP distribution of the secondary next-hop is available thanks
to the following BGP mechanisms: Add-Path [11], BGP Best-External
[6], diverse path [12], and the frequent use in VPN deployments of
different VPN RD's per PE. It is noteworthy to mention that the
availability of another BGP path does not mean that all failure
scenarios can be covered by simply forwarding traffic to the
available secondary path. The discussion of how to cover various
failure scenarios is beyond the scope of this document
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7.3. Pre-Computation of a secondary BGP next-hop
[13] describes how a secondary BGP next-hop can be precomputed on a
per BGP destination basis.
8. Security Considerations
The behavior described in this document is internal functionality
to a router that result in significant improvement to convergence
time as well as reduction in CPU and memory used by FIB while not
showing change in basic routing and forwarding functionality. As
such no additional security risk is introduced by using the
mechanisms proposed in this document.
9. IANA Considerations
No requirements for IANA
10. Conclusions
This document proposes a hierarchical and shared forwarding chain
structure that allows achieving BGP prefix independent
convergence, and in the case of locally detected failures, sub-50
msec convergence. A router can construct the forwarding chains in
a completely transparent manner with zero operator intervention
thereby supporting smooth and incremental deployment.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol
4 (BGP-4), RFC 4271, January 2006
[3] Bates, T., Chandra, R., Katz, D., and Rekhter Y.,
"Multiprotocol Extensions for BGP", RFC 4760, January 2007
[4] Y. Rekhter and E. Rosen, " Carrying Label Information in BGP-
4", RFC 3107, May 2001
[5] Andersson, L., Minei, I., and B. Thomas, "LDP Specification",
RFC 5036, October 2007
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11.2. Informative References
[6] Marques,P., Fernando, R., Chen, E, Mohapatra, P., Gredler, H.,
"Advertisement of the best external route in BGP", draft-ietf-
idr-best-external-05.txt, January 2012.
[7] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[8] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[9] De Clercq, J. , Ooms, D., Prevost, S., Le Faucheur, F.,
"Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider
Edge Routers (6PE)", RFC 4798, February 2007
[10] O. Bonaventure, C. Filsfils, and P. Francois. "Achieving sub-
50 milliseconds recovery upon bgp peering link failures, "
IEEE/ACM Transactions on Networking, 15(5):1123-1135, 2007
[11] D. Walton, A. Retana, E. Chen, J. Scudder, "Advertisement of
Multiple Paths in BGP", draft-ietf-idr-add-paths-12.txt,
November 2015
[12] R. Raszuk, R. Fernando, K. Patel, D. McPherson, K. Kumaki,
"Distribution of diverse BGP paths", RFC 6774, November 2012
[13] P. Mohapatra, R. Fernando, C. Filsfils, and R. Raszuk, "Fast
Connectivity Restoration Using BGP Add-path", draft-pmohapat-
idr-fast-conn-restore-03, Jan 2013
[14] C. Filsfils, S. Previdi, A. Bashandy, B. Decraene, S.
Litkowski, M. Horneffer, R. Shakir, J. Tansura, E. Crabbe
"Segment Routing with MPLS data plane", draft-ietf-spring-
segment-routing-mpls-02 (work in progress), October 2015
[15] C. Filsfils, S. Previdi, A. Bashandy, B. Decraene, " Topology
Independent Fast Reroute using Segment Routing", draft-
francois-spring-segment-routing-ti-lfa-02 (work in progress),
August 2015
[16] M. Shand and S. Bryant, "IP Fast Reroute Framework", RFC 5714,
January 2010
[17] S. Bryant, C. Filsfils, S. Previdi, M. Shand, N So, " Remote
Loop-Free Alternate (LFA) Fast Reroute (FRR)", RFC 7490 April
2015
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[18] A. Atlas, C. Bowers, G. Enyedi, " An Architecture for IP/LDP
Fast-Reroute Using Maximally Redundant Trees", draft-ietf-
rtgwg-mrt-frr-architecture-10 (work in progress), February
2016
12. Acknowledgments
Special thanks to Neeraj Malhotra, Yuri Tsier for the valuable
help
Special thanks to Bruno Decraene for the valuable comments
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Ahmed Bashandy
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134, USA
Email: bashandy@cisco.com
Clarence Filsfils
Cisco Systems
Brussels, Belgium
Email: cfilsfil@cisco.com
Prodosh Mohapatra
Sproute Networks
Email: mpradosh@yahoo.com
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