Scalable BGP FRR Protection against Edge Node Failure
draft-bashandy-bgp-edge-node-frr-02
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| Authors | Ahmed Bashandy , Burjiz , Keyur Patel | ||
| Last updated | 2012-03-05 | ||
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draft-bashandy-bgp-edge-node-frr-02
Network Working Group A. Bashandy
Internet Draft B. Pithawala
Intended status: Standards Track K. Patel
Expires: September 2012 Cisco Systems
March 2, 2012
Scalable BGP FRR Protection against Edge Node Failure
draft-bashandy-bgp-edge-node-frr-02.txt
Abstract
Consider a BGP free core scenario. Suppose the edge BGP speakers PE1,
PE2,..., PEn know about a prefix P/m via the external routers CE1,
CE2,..., CEm. If the edge router PEi crashes or becomes totally
disconnected from the core, it is desirable for a core router "P"
carrying traffic to the failed edge router PEi to immediately restore
traffic by re-tunneling packets originally tunneled to PEi and
destined to the prefix P/m to one of the other edge routers that
advertised P/m, say PEj, until BGP re-converges. In doing so, it is
highly desirable to keep the core BGP-free while not imposing
restrictions on external connectivity. Thus (1) a core router should
not be required to learn any BGP prefix, (2) the size of the
forwarding and routing tables in the core routers should be
independent of the number of BGP prefixes,(3) there should be no
special router (or group of routers) that handles restoring traffic
or the need for one router to store the forwarding table of another
router, (4) re-routing traffic without waiting for re-convergence
must not cause loops, and (4) there should be no restrictions on what
edge routers advertise what prefixes. For labeled prefixes, (6) the
repairing core router must swap the label stack advertised by the
failed edge router PEi for the prefix P/m with the label stack
advertised for the same prefix by the edge router PEj before re-
tunneling the packet to PEj
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................4
1.2. Terminology...............................................5
1.3. Problem definition........................................6
2. Control Plane Operation........................................7
2.1. Step 1: Calculation of the Repair PE......................8
2.2. Step 2: Assigning and Advertising the BGP Next-hop........8
2.3. Step 3: Informing core routers about the repair path......9
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2.4. Step 4: How a repairing P router (a core router) programs its
forwarding plane..............................................10
3. Rules for Choosing and Managing The Repair path...............11
3.1. General Rules for Managing the Repair Path...............11
3.2. Rules for the "Push" Flag................................12
3.3. Rules for Choosing the Repair Path for Labeled Prefixes..13
4. Forwarding Plane Operation....................................14
5. Inter-operability with Existing IP FRR Mechanisms.............15
6. Example.......................................................16
7. Security Considerations.......................................18
8. IANA Considerations...........................................18
9. Conclusions...................................................18
10. References...................................................18
10.1. Normative References....................................18
10.2. Informative References..................................18
11. Acknowledgments..............................................19
Appendix A. Changes from Version 01..............................20
1. Introduction
In a BGP free core, where traffic is tunneled between edge routers,
BGP speakers advertise reachability information about prefixes to
other edge routers not to core rourers. 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 [6], 6PE [7], and
Softwire [5]. Suppose that a given edge router is chosen as the
best next-hop for a prefix P/m. An ingress router that receives a
packet from an external router and destined to the prefix P/m
"tunnels" the packet across the core to that egress router. If the
prefix P/m is a labeled prefix, the ingress router pushes the label
advertised by the egress router before tunneling the packet to the
egress router. Upon receiving the packet from the core, the egress
router takes the appropriate forwarding decision based on the
content of the packet or the label pushed on the packet.
In modern networks, it is not uncommon to have a prefix reachable
via multiple edge routers. One example is the best external path
[4]. Another more common and widely deployed scenario is L3VPN [6]
with multi-homed VPN sites. As an example, consider the L3VPN
topology depicted in Figure 1.
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PE1 .............+
|
+--------+---------------+
| |
| VPN 1 Network |
| |
| VPN prefix |
| (10.0.0.0/8) |
| |
+---+--------------------+
|
/------CE1
/
/
BGP-free core P--------PE0
\
\
\------CE2
|
+---+--------------------+
| |
| VPN 2 Network |
| |
| VPN prefix |
| (20.0.0.0/8) |
| |
+--------+---------------+
|
PE2 .............+
Figure 1 VPN prefix reachable via multiple PEs
As illustrated in Figure 1, the edge router PE0 is the primary NH
for both 10.0.0.0/8 and 20.0.0.0/8. At the same time, both
10.0.0.0/8 and 20.0.0.0/8 are reachable through the other edge
routers PE1 and PE2, respectively.
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.
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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 [6]
o BGP-Free core: A network where BGP prefixes are only known to
the edge routers and traffic is tunneled between edge routers
o Protected prefix: It is a prefix P/m (of any AFI) that a BGP
speaker has an external path to. The BGP speaker may learn about
the prefix from an external peer through BGP, some other
protocol, or manual configuration. The protected prefix is
advertised to some or all of the internal peers.
o Primary egress PE (Primary PE for simplicity): It is an IBGP
peer that can reach the protected prefix P/m through an external
path and advertised the prefix to the other IBGP peers. The
primary egress PE was chosen as the best path by one or more
internal peers. In other words, the primary egress PE is an
egress PE that will normally be used by some ingress PEs when
there is no failure. Referring to Figure 1, PE0 is a primary
egress PE.
o Primary next-hop: It is an IPv4 or IPv6 host address belonging
to the primary egress PE. If the prefix is advertised via BGP,
then the primary next-hop is the next-hop attribute in the BGP
update message [2][3].
o CE: It is an external router through which an egress PE can
reach a prefix P/m. The routers "CE1" and "E2" in Figure 1 are
examples of such CE.
o Ingress PE: It is a BGP speaker that learns about a prefix
through another IBGP peer and chooses that IBGP peer as the
next-hop for the prefix.
o Repairing P router (Also "Repairing core router" and "repairing
router"): A core router that attempts to restore traffic when
the primary egress PE is no longer reachable without waiting for
IGP or BGP to re-converge. The repairing P router restores the
traffic by rerouting the traffic (through a tunnel) towards the
pre-calculated repair PE when it detects that the primary egress
PE is no longer reachable. Referring to Figure 1, the router "P"
is the repairing P router.
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o Repair egress PE (Repair PE for simplicity): It is an egress PE
other than the primary egress PE that can reach the protected
prefix P/m through an external neighbor. The repair PE is pre-
calculated via other PEs prior to any failure. Referring to
Figure 1, PE1 is the repair PE for 10.0.0.0/8 while PE2 is the
repair PE for 20.0.0.0/8.
o Underlying Repair label stack: The underlying repair label stack
is the label stack that will be pushed or swapped in when the
repairing P router re-tunnels traffic to the repair PE after
detecting that the primary egress PE is no longer reachable.
o Protected egress PE (Protected PE for simplicity): Any primary
egress PE protected by a repairing P router.
o Protected edge router: Any protected egress PE.
o Repair next-hop: It is an IPv4 or IPv6 host address belonging to
the repair egress PE. If the protected prefix is advertised via
BGP, then the repair next-hop MAY be the next-hop attribute in
the BGP update message [2][3].
o Repair path (Also Repair Egress Path): It is the repair next-
hop. If an underlying repair label exists, the repair path is
the repair next-hop together with the underlying repair label
that will be pushed or swapped in when the repairing P router
reroutes traffic to the repair PE.
o Primary tunnel: It is the tunnel from the ingress PE to the
primary egress PE
o Repair tunnel: It is the tunnel from the repairing P router to
the repair egress PE
1.3. Problem definition
The problem that we are trying to solve is as follows
o Even though multiple prefixes may share the same egress router,
they have different repair edge router. In Figure 1 above, both
10.0.0.0/8 and 20.0.0.0/8 share the same primary next hop PE0,
the routing protocol(s) must identify that the node protecting
repair node for 10.0.0.0/8 is PE1 while the node protecting
repair node for 11.0.0.0/8 is PE2
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o On loosing connection to the edge router, the core router "P"
MUST reroute traffic towards the *correct* repair edge router
without waiting for IGP or BGP to re-converge and update the
routing tables. On the failure of PE0 illustrated in Figure 1,
the core router P needs to reroute traffic for 10.0.0.0/8
towards PE1 and traffic for 11.0.0.0/8 towards PE2
o The repairing core router P MUST NOT be forced to learn about
the BGP prefixes on any of the edge router. The same applies for
all core routers.
o There SHOULD NOT be a need for a special router or group of
routers to handle rerouting traffic on edge node failure. Also a
router SHOULD not be forced to learn or mirror part or all of
the routing or forwarding table of other router(s).
o The size of the routing table on any core router MUST be
independent of the number of BGP prefixes in the network.
o Rerouting traffic without waiting for IGP and BGP to re-converge
after a failure MUST NOT cause loops.
o For labeled prefixes, repair router MUST forward the re-routed
traffic correctly to the external neighbor. Thus the core router
MUST either swap the label stack advertised by primary egress
edge router (PE0 in Figure 1) with the underlying repair label
stack advertised by the repair router (PE1 and PE2 in Figure 1)
or push a label on top of the label stack advertised by the
primary edge router to allow the repair edge router to forward
the packet correctly.
2. Control Plane Operation
This section specifies the control plane operation needed to solve
the problem defined in the Section 1.3. The control plane operation
cover both labeled (AFI/SAFI 1/4, 2/4, 1/128, and 2/128) and
unlabeled (AFI/SAFI 1/1, 2/1, 1/2, and 2/2) protected prefixes.
The control plane operation can be summarized in 4 steps:
1. Calculation of the repair PE
2. Assigning and advertising the next-hop for protected prefixes
3. Informing core routers about repair paths
4. How a repairing P router (a core router) programs its forwarding
plane
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2.1. Step 1: Calculation of the Repair PE
1. Consider the prefix P/m learnt by an egress edge router PEi via
an external neighbor. If the prefix P/m is labeled, then PEi
allocates a local label for each prefix it can reach through an
external neighbor CE. A PEi MAY also allocate a repair label
such as specified in [11].
2. Each edge router advertises the unlabeled prefix P/m (belonging
to AFI/SAFI 1/1, 2/1, 1/2, and 2/2) and possibly a repair label
[11] to some or all of its IBGP peers. If P/m is a labeled
prefix (belonging to AFI/SAFI 1/4, 2/4, 1/128, and 2/128), the
edge router also advertises the local label.
3. As a result, an edge router PEi having an external path to the
prefix P/m may learn about the prefix through other IBGP peers.
4. The edge router PEi MAY chooses a repair PE for the external
prefix P/m from among the iBGP peers advertising the prefix P/m.
Rules for choosing the repair PE are specified in Section 3.
5. The edge router PEi chooses the one of labels advertised by the
other edge router PEj for the prefix P/m as the "underlying
repair label". The algorithm for choosing the underlying repair
label is specified in Section 3.
6. In the end, if the edge router PEi can reach the prefix P/m
through and external path and the prefix P/m is advertised by at
least one other PE, the edge router PEi will have
o a primary path towards the CE through which the prefix is
reachable, and
o a repair path consisting of a repair PE and possibly an
underlying repair label advertised by the chosen repair PE.
2.2. Step 2: Assigning and Advertising the BGP Next-hop
1. The edge router PEi groups all BGP prefixes for which PEi has an
external path and a repair path as follows:
a. If the edge router received a repair label from the repair
PE: Two prefixes belong to the same group Gi if they share
the same repair PE and underlying repair label
b. If the edge router did NOT receive a repair label from the
repair PE: Two prefixes belong to the same group if they
share the same repair PE
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2. For each group, the PE assigns a distinct local next-hop. Thus
if a prefix P/m belongs to group Gi, its primary next-hop is
NHi.
3. When advertising the prefix and its primary next-hop to its IBGP
peers, the PE router uses NHi as the next-hop attribute of
prefixes belonging to the group Gi.
2.3. Step 3: Informing core routers about the repair path
1. In step 2 (Section 2.2) the egress PE assigns a primary next-
hop NHi and a repair path (consisting of a repair next-hop and
possibly an underlying repair label) for protected prefixes
belonging to group Gi.
1. The primary next-hop NHi is advertised into IGP as usual
2. The repair next-hop is a host address local or advertised by the
repair PE. The repair next-hop MAY be the BGP next-hop attribute
advertised for the prefix P/m by the repair edge router PEj or
some other, possibly future attribute, in the BGP advertisement.
Denote the repair next-hop for prefixes belonging to group Gi by
"rNHi". Denote the underlying repair label for prefixes
belonging to group Gi by "rLi". Because rNHi is the next-hop
attribute advertised by the repair PE, rNHi will also be known
to all core routers via IGP
3. The repairing egress PE MUST advertise the pair (NHi, rNHi) OR
the quadruple (NHi, rNHi, rLi, Push) to core routers that are
designated as candidate repairing routers. Note that designating
a core router as a candidate repairing router may be subject to
administrative actions and/or policy. For example, an
administrator may limit candidate repairing routers to only core
routers that are directly connected to edge routers. The
mechanism by which a core router is designated as a candidate
repair router is beyond the scope of this document.
4. The pair (NHi, rNHi) or the quadruple (NHi, rNHi, rLi, Push) may
be advertised through various means, such as ISIS optional TLV.
The structure and method of advertising (NHi, rNHi) and/or (NHi,
rNHi, rLi, Push) is beyond the scope of this document.
5. The semantics of the pair (NHi,rNHi) are: If the next-hop NHi
becomes unreachable, then traffic tunneled to the next-hop NHi
SHOULD be re-tunneled to the next-hop rNHi because rNHi can
reach protected prefixes reachable via the next-hop NHi.
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6. The semantics of the quadruple (NHi,r NHi, rLi, Push) are: If
the next-hop NHi becomes unreachable, then traffic destined to
the next-hop NHi should be re-tunneled to the next-hop rNHi and
a. If the PUSH flag is cleared, the label pushed by the ingress
PE MUST be swapped with the label rLi before re-tunneling to
the repair PE, irrespective of the value of the label pushed
by the ingress PE.
b. If the PUSH flag is set, then the label rLi MUST be pushed
on the packet before re-tunneling to the repair PE.
7. Because of the previous steps, candidate repairing core routers
become aware of the repair path for protected BGP prefixes
reachable via the primary egress edge router. Note that all core
routers remain totally unaware of the BGP prefixes.
2.4. Step 4: How a repairing P router (a core router) programs its
forwarding plane
1. Through usual IGP mechanisms, the P router has a prefix matching
every BGP next-hop. Let the primary next-hop NHi match the IGP
route Ri
2. Any next-hop of prefix Ri is on the path towards the protected
egress edge router PEi. A next-hop of the prefix Ri is considered
the primary path for the prefix Ri
3. Thus the FIB entry for Ri is programmed as follows
a. Primary path: All next hop routers on the path towards NHi
b. Repair path when the candidate repair router receives the
pair (NHi,rNHi)
i. Primary next-hop: the next router on the path towards
NHi
ii. Repair next-hop: the next-router on the path towards
rNHi
c. Repair path when the candidate repair router receives the
quadruple (NHi,rNHi, rLi, Push)
i. If the "Push" flag is *cleared*
Pop label in the packet right under the tunnel
header (irrespective of the value of that label)
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endif
ii. Push the underlying repair label rLi
iii. Re-tunnel the packet towards the repair next-hop rNHi
3. Rules for Choosing and Managing The Repair path
This section specifies rules governing how an egress edge router
PEi chooses and advertises the repair path. Other than the rules in
this section, the method of choosing the repair path is beyond the
scope of this document.
3.1. General Rules for Managing the Repair Path
This section specifies general rules for choosing the repair path
for both labeled and unlabeled prefixes.
1. A repair PE MUST be another edge router PEj that advertises the
same prefix to the primary edge router PEi via IBGP peering.
2. A primary PE MAY advertise more than one repair path for the
same primary next-hop NHi. In that case, all advertised repair
next-hop identify valid repair PEs for the primary next-hop NHi.
Thus a core router MAY choose any repair PE as the repair path
for the primary next-hop NHi
3. If a repairing "P" router determines that the path taken by the
repair tunnel to a repair edge router PEj passes through the
protected edge router PEi, then the repairing router "P" SHOULD
NOT install this repair path in its forwarding plane.
4. Let the primary next-hop NHi match the IGP route Ri. If the
repairing "P" router determines that the repair tunnel to a
repair edge router passes through a next-hop of the IGP route
Ri, then the repairing router SHOULD NOT install this repair
path in its forwarding plane.
5. A primary next-hop identifies an egress PE. Thus a primary next-
hop NH MUST NOT be advertised by two different PEs. However a
primary next-hop of one PE MAY be the repair next-hop for
another PE.
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6. At any point in time, for the same primary and repair next-hops
NHi and rNHi, only one advertisement is valid. Thus for the same
value of NHi and rNHi, an advertisement of the pair (NHi,rNHi)
or the quadruple (NHi,rNHi,rLi,Push) for any values of "rLi" and
"Push" MUST override or be preceded by the withdrawal of any
previously advertised pair (NHi,rNHi) or the quadruple
(NHi,rNHi,rLi,Push).
If rules (3) and (4) are not applied, then the tunnel to the repair
edge router PEj does not provide protection against the failure of
the edge node PEi. Instead it provides core protection against the
failure of the path through the core leading to the protected edge
node PEi. Thus existing core FRR protection mechanisms such as those
specified in [8], [9], and [10] can be used instead.
Rules (5) and (6) ensures that there is no ambiguity about the
primary and repair next-hops
3.2. Rules for the "Push" Flag
This section covers basic rules for advertising, setting, and
clearing the "Push" flag
1. If the repairing PE advertises a repair label, then the "Push"
flag MUST be advertised. I.e., the repairing PE MUST either
advertise the pair (NHi, rNHi) or the quadruple (NHi, rNHi, rLi,
Push)
2. If the repair PE advertises a repair label, then the repair PE MAY
advertise the "Push" flag with the repair label. In that case, if
the primary egress PE (PEi) decides to advertise the quadruple
(NHi, rNHi, rLi, Push), then the primary egress PE SHOULD set the
"Push" flag to the same value that is received from the repair PE.
3. If the protected prefix is unlabeled (i.e. belongs to AFI/SAFI
1/1, 2/1, 1/2, or 2/2) and the repairing router advertises the
quadruple (NHi, rNHi, rLi, Push), then the "Push" flag MUST be
set.
4. If the protected prefix is labeled (i.e. belongs to AFI/SAFI 1/4,
2/4, 1/128, and 2/128), then
a. The repairing router MUST advertise the quadruple (NHi, rNHi,
rLi, Push)
b. repairing router MAY set the "Push" flag
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Rule (1) is required because the repairing core router(s) need to
know what to do with the underlying repair label if it exists
Rule (2) is required because the repairing label is allocated by the
repairing PE. Hence the repairing PE should be able to specify what
to do with it. For example, suppose the repair PE has eiBGP paths for
the protected prefix and the protected prefix is unlabeled. In that
case, to make sure that the repair PE does not loop the repaired
packet back to the primary egress PE, the repair PE advertises a
repair label for the unlabeled protected prefix with semantics
defined in [11]
Rule (3) is needed because the protected prefix is unlabeled. Hence
the tunneled packet arriving at the repairing core router "P" has no
label and thus the label swapping operation cannot be performed.
This document defines the minimum set of rules governing the "Push"
flag. Additional rules may be set by other documents.
3.3. Rules for Choosing the Repair Path for Labeled Prefixes
This section specifies rules in additions to those mentioned in
Section 3.1. by which an egress edge router PEi chooses and
advertises the repair path for a protected labeled prefix P/m.
1. A primary edge router PEi SHOULD only choose the edge router PEj
and the underlying repair label rLi as a repair path for the
prefix P/m if the label advertised for the prefix P/m by the
repair edge router PEj is allocated on per-VPN or per-CE/per-
next-hop basis.
The reason for this is as follows. As mentioned in Section 1.3.
the core of the network MUST remain BGP-free and the size of the
routing table on a core router MUST remain independent of the
number of BGP prefixes. BGP prefix grouping in section 2.2.
requires two prefixes to belong to two different groups if the
labels advertised by the repair PE for the two prefixes are
different. Thus if the repair edge router allocates labels on
per-prefix basis, the protected edge router PEi will advertise a
different primary next-hop for each protected prefix. This is
equivalent to having core router "P" knowing about every BGP
prefix. In addition, the size of the routing table of the "P"
router becomes comparable to the number of BGP prefixes.
2. If the repair edge router PEj advertises a repair label as
described in [11] and the protected edge router understands the
repair label attribute described in [11], then the protected
edge router PEi SHOULD choose the repair label advertised by PEj
as the underlying repair label for the prefix P/m.
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Using the repair label specified in [11] has few advantages:
o A repairing edge router PEj need not change the primary
label allocation policy (which may be per-prefix) but can be
chosen as repair PE if the repair labels are allocated on
per-CE or per-VRF basis.
o As mentioned in [11], an edge router does not repair a
packet arriving with a repair label. Hence using the repair
label when re-tunneling the packet towards PEj guarantees
loop freedom in case of PE-CE link failure.
o If the repair PE programs an eiBGP multipath for the
protected prefix, then choosing the repair label as
specified in [11] guarantees that the repaired packet will
not be looped back towards the primary egress PE during
repair
4. Forwarding Plane Operation
This section specifies the forwarding plane operation on the core
router "P" when it detects that the protected edge router PEi is no
longer reachable. We assume that the core router has pre-programmed
its forwarding plane according to Sections 2.4.
As soon as the "P" router detects that the primary next-hop for Ri
is not reachable it does the following for any arriving packet
destined to the protected edge router PEi
1. Decapsulate the tunnel header to expose the tunneled packet
2. If the underlying repair label rLi is programmed in the
forwarding plane
a. If the "Push" flag is set
Push the underlying repair label rLi
b. Else
Swap the label on the top of the packet (irrespective
of the value of that label) with the underlying repair
label rLi
3. Tunnel the packet towards the repair egress PE identified by
rNHj
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5. Inter-operability with Existing IP FRR Mechanisms
Current existing IP FRR mechanisms can be divided into two
categories: core protection and edge protection. Core protection
techniques, such as [8], [9], and [10], provide protection against
internal node and/or link failure. Thus the technique proposed in
this document is not related to existing IP FRR mechanisms. If the
failure of an internal node or link results in completely
disconnecting a protectable edge node, then an administrator MAY
configure the repairing router to prefer the technique proposed in
this document over existing IP FRR mechanisms.
Edge protection techniques, such as [12] and its practical
implementation [11] provide protection against the failure of the
link between PE and CE routers. Thus existing PE-CE link protection
can co-exist with the techniques proposed in this document because
the two techniques are independent of each other.
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6. Example
We will use and LDP core as an example. Consider the diagram
depicted in Figure 2 below. We assume that the PEs advertise repair
labels as specified in [11]
+-----------------------------------+
| |
| LDP Core |
| |
| PE1
| |\
| | \
| | \
| | \
| | CE1....... VPN prefix
| | / (10.0.0.0/8)
| | / (11.0.0.0/8)
| | /
| |/
PEx P--------PE0 Lo1 = 1.1.1.1/32
| |\ Lo2 = 2.2.2.2/32
| | \
| | \
| | \
| | CE2....... VPN prefix
| | / (20.0.0.0/8)
| | / (21.0.0.0/8)
| | /
| |/
| PE2
| |
| |
+-----------------------------------+
Figure 2 : Edge node BGP FRR in LDP core
1. As we can see, PE0 has 4 prefixes: 10.0.0.0/8, 11.0.0.0/8,
20.0.0.0/8, and 21.0.0.0/8. PE0 may assign a separate label to
each prefix. The method and policy of assigning primary labels
to each prefixes is irrelevant to this document.
2. PE1 advertises the repair label rL1 for prefixes 10.0.0.0/8 and
11.0.0.0/8
3. PE2 advertises the repair label rL2 for prefixes 20.0.0.0/8 and
21.0.0.0/8
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4. As such, PE0 divides its prefixes into two groups
G1 = {10.0.0.0/8, 11.0.0.0/8}
G2 = {20.0.0.0/8, 21.0.0.0/8}
5. When advertising the next-hop to its IBGP peer, PE0 advertises
1.1.1.1 as the next-hop for prefixes belonging to group G1 and
2.2.2.2 as the next-hop for prefixes belonging to group G2.
6. PE0 advertises the prefixes 1.1.1.1/32 and 2.2.2.2/32 using the
usual IGP mechanism.
7. When advertising 1.1.1.1/32 into the core, PE0 advertises rL1
and PE1 as a repair path. When advertising 2.2.2.2/32 into the
core, PE0 advertises rL2 and PE2 as a repair path. The mechanism
by which a repair path is advertised is beyond the scope of the
proposal.
8. On the penultimate hop router "P", LDP assigns a different LDP
label to 1.1.1.1/32 and 2.2.2.2/32. Core routers other than
penultimate hop routers may employ some sort of label
aggregation to reduce the number of LDP labels
9. Assume that the penultimate hop router "P" assigns the local LDP
label L1 for prefix 1.1.1.1/32 and L2 for prefix 2.2.2.2/32
10.On the penultimate router P, the forwarding entry for L1 will be
as follows
Primary path:
- nexthop is PE0.
- swap the incoming outer label with the LDP label towards
1.1.1.1
Repair path
- Pop the incoming LDP label
- Swap the internal label with the repair label rL1
- Push the LDP label towards PE1
- Forward the packet
11.On the core router P, the forwarding entry for L2 will be as
follows
Primary path: Same as L1
Repair Path
- Pop the incoming LDP label
- Swap the internal label with the repair label rL2
- Push the LDP label towards PE2
- Forward the packet
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12.If the P router detects that PE0 is no longer reachable, it can
use the repair path already pre-programmed in the forwarding
plane as described above. Because the repair path is pre-
programmed as in the case of TE and IP FRR, the P router can re-
route traffic very fast
7. Security Considerations
No additional security risk is introduced by using the mechanisms
proposed in this document
8. IANA Considerations
No requirements for IANA
9. Conclusions
This document proposes a method that allows fast re-route
protection against edge node failure or complete disconnected from
the core in a BGP-free core
10. References
10.1. 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
10.2. Informative References
[4] Marques,P., Fernando, R., Chen, E, Mohapatra, P., Gredler, H.,
"Advertisement of the best external route in BGP", draft-ietf-
idr-best-external-04.txt, April 2011.
[5] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[6] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC 4364, February 2006.
[7] 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
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[8] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[9] Shand, S., and Bryant, S., "IP Fast Reroute", RFC5714, January
2010
[10] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[11] Bashandy, A., Pithawala, P., and Heitz, J., "Scalable, Loop-
Free BGP FRR using Repair Label", draft-bashandy-idr-bgp-
repair-label-02.txt", July 2011
[12] 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. Acknowledgments
Special thanks to Les Ginsberg and Anton Smirnov for the valuable
comments
This document was prepared using 2-Word-v2.0.template.dot.
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Appendix A. Changes from Version 01
1. Use the term "underlying repair label" instead of just "repair
label" to avoid confusion with the term "repair label" used in
[11].
2. In version 01, it was assumed in many places that the repairing
router is the penultimate hop P router. Although this would
probably be the most common case, it is not always true. Hence in
this version the repairing router may be any core router
3. Merged handling labeled and unlabeled prefixes into a single
algorithm.
4. Allowed sending a repair label for unlabeled prefixes and added
the "Push" flag. This ensures loop-free repair even for unlabeled
prefixes in case that the repair PE has eiBGP paths as mentioned
in Section 3.3.
5. In Section 3.3 discussing the rules governing the choice of the
underlying repair label for labeled prefix, we changed the wording
so that the primary egress PE "SHOULD" instead of "MAY" use the
repair label advertised according to [11] as an underlying repair
label.
6. All occurrences of the term "backup" were replaced by "repair" as
the term "repair" is the commonly used term in the IP FRR context
such as [10][9][8]
7. Added the definition of primary and repair tunnels in Section 1.2.
8. Added a definition of the term "Repair Next-hop" in Section 1.2.
9. Modified the definition of "repair path" in Section 1.2 to being
the repair next-hop plus the underlying repair label instead of
being the repair PE plus the underlying repair label.
10.Outlined inter-operability with existing IP FRR techniques in
Section 5.
11.There were few editorial corrections.
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Authors' Addresses
Ahmed Bashandy
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: bashandy@cisco.com
Burjiz Pithawala
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: bpithaw@cisco.com
Keyur Patel
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: keyupate@cisco.com
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