Network Working Group A. Bashandy
Internet Draft M. Konstantynowicz
Intended status: Standards Track N. Kumar
Expires: April 2013 Cisco Systems
October 8, 2012
BGP FRR Protection against Edge Node Failure Using Table Mirroring
with Context Labels
draft-bashandy-bgp-frr-mirror-table-00.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. This draft proposes
a BGP FRR scheme that relies on having the repairing edge router
mirror the protected edge router forwarding table. The repairing edge
router uses a locally allocated context label to identify the correct
mirrored table.
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................5
1.2. Terminology...............................................5
1.3. Problem definition........................................7
2. Overview of BGP FRR using Mirrored Forwarding Table in an MPLS
Core..............................................................8
2.1. Control Plane operation...................................8
2.2. Forwarding behavior at Steady State (while pPE is reachable)
..............................................................10
2.3. Forwarding behavior when pPE Fails.......................10
3. Overview of the BGP FRR using Mirrored Forwarding Table in IP Core
.................................................................12
3.1. Control plane modification for IP core...................12
3.2. Forwarding behavior at Steady State (while pPE is reachable)
..............................................................12
3.3. Forwarding plane at Failure (when pPE is unreachable)....12
4. Rules for Choosing and Managing the Repair path...............13
5. Inter-operability with Existing IP FRR Mechanisms.............14
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6. Example.......................................................15
6.1. Control Plane............................................16
6.2. Forwarding Plane at Steady State (When PE0 is reachable).17
6.3. Forwarding Plane at Failure (When PE0 is not reachable)..17
7. Security Considerations.......................................19
8. IANA Considerations...........................................19
9. Conclusions...................................................19
10. References...................................................19
10.1. Normative References....................................19
10.2. Informative References..................................20
11. Acknowledgments..............................................21
Appendix A. Auto-determination of Operating Parameters on rPE and pPE
.................................................................21
A.1. How rPE determines the Protected PE......................22
A.2. How pPE Determines its rPEs and Assigns pNH for each rPE.22
A.3. Detecting Mis-configuration..............................23
Appendix B. Ensuring correct forwarding at the edge routers......24
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 but not to core routers. 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 [11], 6PE [12], and
Softwire [10]. Suppose that a given edge router is chosen as the
best next-hop for a prefix P/m by an ingress router. The 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
[9]. Another more common and widely deployed scenario is L3VPN [11]
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. On the failure of the edge
router PE0, it is highly desirable for the core router P to re-
route traffic for VPN 1 and VPN 2 to PE1 and PE2, respectively,
without waiting for IGP or BGP to re-converge. This document
proposes a scheme by which the egress and core routers participate
to enable a core router to re-route traffic to the correct backup
edge router when the primary edge router fails while keeping the
core BGP-free
It is noteworthy to mention that the behavior specified in this
draft requires supporting more than one BGP path. Methods, such as
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[9], [17], and [18], may be needed to satisfy the multi-path
requirement in certain scenarios such as the case were MED [2] or
local preference [2] is used to determine the best path. The
mechanism(s) by which a router supports BGP multi-path is beyond
the scope of this document.
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 [11]
o BGP-Free core: A network where BGP prefixes are only known to
the edge routers and traffic is tunneled between edge routers
o External prefix: It is a prefix P/m (of any AFI/SAFI) that a BGP
speaker has an external path for. The BGP speaker may learn
about the prefix from an external peer through BGP, some other
protocol, or manual configuration. The external prefix is
advertised to some or all of the internal peers.
o Protectable prefix: It is an external prefix P/m of any
AFI/SAFI) that a BGP speaker has an external path to and is
eligible to have a repair path.
o Protected prefix: It is an external prefix P/m (of any AFI/SAFI)
that a BGP speaker has an external path to and also has a repair
path to.
o Primary Egress PE, "ePE": It is an IBGP peer that can reach the
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 an egress PE.
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o Protected egress PE, "pPE" (Protected PE for simplicity): It is
an egress PE for which there exists a repair path for some or
all of the prefixes to which it has an external path. Referring
to Figure 1, PE0 is a protected egress PE.
o Protected edge router: Any protected egress PE.
o Protected next-hop (pNH): It is an IPv4 or IPv6 host address
belonging to the protected egress PE. Traffic tunneled to this
IP address will be protected via the mechanism proposed in this
document.
o CE: It is an external router through which an egress PE can
reach a prefix P/m. The routers "CE1" and "CE2" in Figure 1 are
examples of such CEs.
o Ingress PE, "iPE": 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 "rP" (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.
o Repair egress PE "rPE" (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 Repair next-hop (rNH): It is an IPv4 or IPv6 address belonging
to the repair egress PE. If the protected prefix is advertised
via BGP, then the repair next-hop SHOULD be the next-hop
attribute in the BGP update message [2][3].
o BGP nexthop (bgpNH): This is the usual next-hop attribute for
route advertisements as specified in [2] and [3].
o Context Label (cL): It is an MPLS label allocated by the
repairing PE (rPE) to identify the mirrored forwarding table of
the protected PE (pPE). An rPE must allocate a locally distinct
context label for each mirrored forwarding table. Context labels
on different rPEs may overlap
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o Repair path (Also Repair Egress Path): It is the repair next-
hop.
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. On losing connection to
the edge router, a core router "P" detecting the loss of
connection MUST reroute traffic towards the *correct* repair
edge router that can reach prefixes that were reachable via the
failed edge router without waiting for IGP or BGP to re-converge
and update the routing tables.
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 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 introduce loops.
o For labeled prefixes, when a packet gets re-routed to the repair
PE, the label stack on the packet MUST ensure correct
forwarding.
o At steady state, when pPE is reachable, paths taken by traffic
before deploying the solution proposed in this document MUST NOT
be impacted after deploying the solution proposed in this
document unless desired by the operator.
o The solution MUST be incrementally deployable
o Minimize the number of nodes that need to be upgraded. Hence
only egress PE's that participate in the solution (namely pPE's
and rPE's) and protecting core routers (namely rP's) need to be
upgraded.
Applying the problem to the topology in Figure 1 above, both
10.0.0.0/8 and 20.0.0.0/8 share the same primary egress router PE0,
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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. On the failure of PE0, the core router P must
reroute traffic for 10.0.0.0/8 towards PE1 and traffic for 11.0.0.0/8
towards PE2 without requiring the core router P to know about any BGP
prefix.
2. Overview of BGP FRR using Mirrored Forwarding Table in an MPLS Core
The solution proposed in this document relies on the collaboration of
egress PEs, and the repairing core router. This section gives an
overview of how to the solution works for 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 in a core where the tunnels between edge
routers are LDP LSPs [7]. Specifications of the solution in IP core
are provided in Section 3.
2.1. Control Plane operation
Control plan requires certain operating parameters to be assigned.
This section explains how the parameters are assigned through
configuration. Automatic determination of the operating parameters is
explained in Appendix A.
1. Setting the Operating parameters on pPE
a. Suppose the protectable prefixes on a given pPE are protected by
the repair edge routers rPE1, rPE2,...
b. For the set of prefixes protected by a given rPE, assign a
distinct local next-hop pNH. The pNH is also advertised as the
bgpNH when the pPE advertises the prefixes to other iBGP peers.
This section assumes that pNH is assigned via configuration. pNH
can be automatically calculated as described in Appendix A.
c. pNH MUST be unique within a routing domain
d. Because pNH is also used as bgpNH, then pNH MUST be advertised
into IGP as usual
2. Setting the Operating parameters on the rPE
a. Suppose the rPE can protect prefixes whose bgpNH is pNH1,
pNH2,...
b. The operator informs rPE about the bgp next-hops that it can
protect. This task can be carried out through configuration.
Appendix A outlines how rPE can automatically determine the BGP
next-hops it can protect.
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c. rPE performs the following tasks for each pNH
i. rPE allocates a "locally" distinct context label "cL" for
each pNH that the rPE can protect
ii. rPE advertises "pNH" as its own prefix into IGP but with
(maximum metric - 1) so as not to affect the path taken by
the traffic flowing from iPE's to pPE's
iii. rPE advertises "cL" for pNH instead of implicit NULL to its
neighboring LSRs. As explained in Appendix B, this behavior
is necessary to ensure correct forwarding during the period
starting from complete disconnect of pPE till all iPE stop
using pPE as an exit point for BGP traffic.
iv. rPE allocates a separate "mirror" forwarding table for each
pNH. The mirror forwarding table consists of a mirror IP
table and a corresponding label table. The mirror table is
identified by the context label "cL"
v. rPE assigns a local IP address rNH as the repair next-hop.
rNH may be any local IP address on the rPE. "rNH" SHOULD be
any next-hop attribute advertised by rPE when it announces
reachability to the protected prefix P/m to minimize the
number of prefixes advertised into IGP.
vi. rPE advertises the triplet (pNH,rNH,cL) to candidate
repairing core routers. The syntax is TBD. For example, an
LDP optional TLV can be used for this purpose
d. Remember that pNH1, pNH2,... are advertised as the BGP next-hop
by pPE's. When rPE receives a prefix advertisement from an iBGP
peer with bgpNH equal to one of the pNHs it can protect AND rPE
has at least one "external" path for the received prefix:
i. If the prefix is labeled ((AFI/SAFI 1/4, 2/4, 1/128, and
2/128), insert the received label into the mirror label
table corresponding to the pNH
ii. If the prefix is unlabeled, (AFI/SAFI 1/1, 2/1, 1/2, and
2/2), insert the prefix into the mirror IP table
corresponding to the pNH
iii. The forwarding entry of the prefix or the label in the
mirror table is to either send the packet to (one of) the
external path(s) or drop the packet
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iv. Remember that the external path MAY or MAY NOT be the best
path. For example, if MED is used to decide the best path
and the best path happened to be the internal path, then
techniques, such as [9], [17], [18], and [20] are needed to
calculate and advertise (an) alternative external path(s).
3. Determining the Operating Parameters on Protecting Core router
"rP"
a. rP receives the triplet (pNH,rNH,cL) from rPE
b. rP installs the following entry for pNH in its forwarding table
i. if pNH is reachable, forward the packet as usual
ii. If pNH is not reachable
1. Swap the label bound to pNH with "cL"
2. tunnel the traffic towards rNH
4. Operating parameters on the rest of the routers
a. Other than pPE, rPE, and rP, the rest of the routers can remain
totally agnostic to the BGP FRR scheme proposed in this document
b. Because rPE advertises pNH with (maximum-metric - 1), all the
routers will prefer pPE when sending traffic to the IP address
pNH. Hence as long as pPE is reachable, there is no change in
traffic patterns
2.2. Forwarding behavior at Steady State (while pPE is
reachable)
When pPE is reachable, there is no change in behavior due to
deploying the scheme proposed in this document
2.3. Forwarding behavior when pPE Fails
The repairing router "rP" directly connected to a failure detects
that pNH is no longer reachable. The following steps are applied.
1. Repairing router "rP"
a. Receives packet with top label bound to pNH
b. pNH is not reachable
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c. Pop the label of pNH and swap it with the context label cL
received in the triplet (pNH,rNH,cL) from rPE
d. Push the label corresponding to rNH
e. Send the packet towards rNH
2. Penultimate hop of rPE performs the usual penultimate hop popping
3. Repair PE (rPE)
a. Because its penultimate hop performed penultimate hop popping,
rPE receives a packet with the top label being the context label
"cL"
b. rPE uses "cL" to identify the correct mirror forwarding table
c. rPE pops the context label "cl"
d. if the packet underneath "cL" is labeled, lookup the top label in
the mirror label table corresponding to cL
e. If the packet underneath "cL" is unlabeled, lookup the
destination address of the packet in the mirror IP table
corresponding to cL
f. Forward the packet to an external neighbor or drop it based on
the mirror table lookup
4. Ingress PEs (iPEs)
a. An ingress PE that has not yet learnt about the disappearance of
pPE will continue to send traffic towards pNH and hence will be
re-routed towards rPE by rP and forwarded correctly
b. An ingress PE that learns about the disappearance of pPE will
calculate a new best path for traffic previously destined to pNH
5. The rest of the core routers
a. A core router that has not yet learnt that pPE is no longer
reachable will continue send traffic destined to pNH towards pPE.
This traffic will be intercepted by rP and re-routed towards rPE
b. A core router that has learnt that pPE is no longer reachable
will send traffic towards rPE because rPE advertises pNH with
(maximum-metric - 1).
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i. Because rPE advertises the label "cL" for rNH instead of the
usual implicit NULL, a packet originally destined towards
pPE that gets re-routed towards rPE will arrive at rPE with
"cL" at the top
ii. Hence rPE will process it as described in step 3.
c. Eventually all iPEs learn that pPE is unreachable and hence no
traffic will be sent to any of the pNHs advertised by pPE that
has just disappeared
The next section presents the solution in an IP core.
3. Overview of the BGP FRR using Mirrored Forwarding Table in IP Core
This section describes the BGP FRR using mirrored tables solution in
an IP core for 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 primary difference between a MPLS core and an IP core is that the
tunnels between edge routers are IP based such as [5][6][7]. We
assume that rP is capable of handling MPLS labels
3.1. Control plane modification for IP core
When using IP tunnels instead of MPLS tunnels between edge routers,
there is one small modification at the repair edge router rPE. For
the MPLS core, the correct mirror table at rPE is identified by the
context label "cL". For the IP core, the correct mirror table must be
indentified by either the context label "cL" or the protected next-
hop "pNH". As explained in Appendix B, this behavior is necessary to
ensure correct forwarding during the period starting from complete
disconnect of pPE till all iPE stop using pPE as an exit point for
BGP traffic.
3.2. Forwarding behavior at Steady State (while pPE is
reachable)
When pPE is reachable, there is no change in behavior due to
deploying the scheme proposed in this document
3.3. Forwarding plane at Failure (when pPE is unreachable)
1. iPE is not yet aware of the failure so its behavior remains the
same
2. rP
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a. Decapsulates the tunnel header towards pNH
b. Pushes the context label "cL"
c. Encapsulates the packet into a tunnel header with destination
address rNH and forwards the packet towards rPE
3. rPE
a. If the tunnel packet arrives with desitination address "rNH"
i. Decapsulates the tunnel header. This exposes the context
label "cL"
b. Otherwise (i.e. the destination address is "pNH")
i. Decapsulate the tunnel header and associate the exposed
packet with the mirror table based on "pNH"
c. The rest of the behavior is identical to the MPLS core outlined
in Section 2.3.
4. Rules for Choosing and Managing the Repair path
This section specifies rules governing how the repair path is
chosen and installed in the forwarding plan. Other than the rules
in this section, the method of choosing the repair path is beyond
the scope of this document.
1. A repair PE MUST be another edge router that advertises the same
prefix to the protected edge router pPE via IBGP peering.
2. If a repairing core router "rP" determines that the path taken by
the repair tunnel to a repair edge router rPE passes through the
protected edge router pPE, then the repairing router "rP" MUST NOT
install this repair path in its forwarding plane. Instead, the
repairing "p" router MAY use other paths that do not pass through
pPE or use existing core FRR mechanisms such as [13], [14], and
[15].
3. If the repair PE "rPE" advertises one or more protected next-hops,
then the repair next-hop "rNH" MUST be different from any
protected next-hop "pNH" advertised by rPE
If rules (1) and (2) are not applied, then the tunnel to the repair
edge router rPE does not provide protection against the failure of
the edge node pPE. Rule (5. ) ensures that there is no ambiguity
about the primary and repair next-hops
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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 [13], [14], and [15], 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 [16] 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.
+-----------------------------------+
| |
| LDP Core |
| |
| PE1 Lo = 9.9.9.1
| |\
| | \
| | \
| | \
| | CE1.......VRF "Blue"
| | / (10.0.0.0/8)
| | / (11.0.0.0/8)
| | /
| |/
PE11 P--------PE0 Lo1 = 1.1.1.1/32
| |\ Lo2 = 1.1.1.2/32
| | \
| | \
| | \
| | CE2.......VRF "Red"
| | / (20.0.0.0/8)
| | / (21.0.0.0/8)
| | /
| |/
| PE2 Lo = 9.9.9.2
| |
| |
+-----------------------------------+
Figure 2 : Edge node BGP FRR in LDP core
o In Figure 2, PE0 is the pPE for VRFs "Blue" and "Red". PE1 and PE2
are the rPEs for VRFs "Blue" and "Red", respectively. VRF Blue has
10.0.0.0/8 and 11.0.0.0/8 and VRF Red has 20.0.0.0/8 and
21.0.0.0/8
o Assuming PE0 uses per prefix label allocation, PE0 assigns the VPN
labels 4100, 4200, 4300, and 4400 to 10.0.0.0/8, 11.0.0.0/8,
20.0.0.0/8, and 21.0.0.0/8, respectively. PE0 advertises the
prefixes 10.0.0.0/8, 11.0.0.0/8, 20.0.0.0/8, and 21.0.0.0/8 using
MP/BGP as usual
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6.1. Control Plane
1. Configuring the pNHs on PE0
The operator assigns 1.1.1.1 (the IP address of Loopback0) as the
bgpNH for prefixes belonging to vrf "Blue" and 1.1.1.2 (The IP
address of Loopback1) as the bgpNH for prefixes belonging to vrf
"Red"
2. Configuring protection parameters on rPEs
a. The operator informs PE1 that it can protect all traffic with
bgpNH=1.1.1.1. Accordingly
i. PE1 advertises 1.1.1.1 with (maximum-metric - 1) into IGP
ii. PE1 allocates a distinct mirror table for prefixes with
bgpNH=1.1.1.1
iii. PE1 allocates the context label cL=1100 for the mirror
table of bgpNH=1.1.1.1
iv. When advertising the FEC 1.1.1.1 to its neighboring LSRs,
PE1 associates the label 1100
v. PE2 advertises the mapping (1.1.1.1, 9.9.9.1, 1100) to
candidate repair router
vi. When PE1 receives a prefix advertisement from any peer
with bgpNH=1.1.1.1, PE1 inserts the VPN labels in the
mirror table identified by cL=1100. Hence PE1 inserts the
VPN labels 4100 and 4200 in the mirror table. The
forwarding entries for both labels is to either pop the
label and send the packet to an external neighbor or drop
the packet
b. The operator informs PE2 that it can protect all traffic with
bgpNH=1.1.1.2. Accordingly
i. PE2 advertises 1.1.1.2 with (maximum-metric - 1) into IGP
ii. PE2 allocates a distinct mirror table for prefixes with
bgpNH=1.1.1.2
iii. PE2 allocates the context label cl=1200 for the mirror
table of bgpNH=1.1.1.2
iv. When advertising the FEC 1.1.1.2 to its neighboring LSRs,
PE2 associates the label 1200
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v. PE2 advertises the mapping (1.1.1.2, 9.9.9.2, 1200) to
candidate repair router
vi. When PE2 receives a prefix advertisement from any peer
with bgpNH=1.1.1.2, PE2 inserts the labels into the mirror
table identified by cL=1200. Hence PE inserts the VPN
labels 4300 and 4400 in the mirror table. The forwarding
entries for both labels is to either pop the label and
send the packet to an external neighbor or drop the packet
3. Enabling BGP FRR on the penultimate hop router "P"
a. If not enabled by default, the operator enables edge node
protection on the router "P"
b. Acting as a rP, the core router "P" receives the advertisements
(bgpNH,rNH,cL)=(1.1.1.1, 9.9.9.1,1100) and
(bgpNH,rNH,cL)=(1.1.1.2, 9.9.9.2,1200) from PE1 and PE2,
respectively.
c. "rP" creates the following forwarding state for 1.1.1.1 and
1.1.1.2
i. If 1.1.1.1 is not reachable
1. Push the context label 1100
2. Send the packet through the LSP terminating on
9.9.9.1
ii. If 1.1.1.2 is not reachable
1. Push the context label 1200
2. Send the packet through the LSP terminating on
9.9.9.2
6.2. Forwarding Plane at Steady State (When PE0 is reachable)
No change in forwarding behavior when PE0 is reachable.
6.3. Forwarding Plane at Failure (When PE0 is not reachable)
1. Repairing core router "P"
a. Traffic for VRF "Blue"
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i. Receives a packet with the top label being the LDP label for
1.1.1.1
ii. 1.1.1.1 is not reachable
iii. Pop the LDP label of 1.1.1.1.
iv. Push the context label 1100
v. Push the LDP label for 9.9.9.1 and forward the packet
towards PE1
b. Traffic for VRF "Red"
i. Receives a packet with the top label being the LDP label for
1.1.1.2
ii. 1.1.1.2 is not reachable
iii. Pop the LDP label of 1.1.1.2.
iv. Push the context label 1200
v. Push the LDP label for 9.9.9.2 and forward the packet
towards PE2
2. The repair Router "PE1"
a. The penultimate hop of PE1 performs the usual penultimate hop
popping
b. PE1 receives a packet with the top label equals the context label
1100
c. PE1 makes a lookup for 1100 in its label table. The lookup yields
the mirror table of the bgpNH=1.1.1.1
d. Pop the cL=1100. This exposes the VPN label 4100 or 4200.
e. Lookup VPN label 4100 or 4200 in the mirror table corresponding
to cL=1100. The lookup results in popping the VPN label 4100 or
4200 and forwarding the packet natively to CE2
3. The repair Router "PE2"
a. The penultimate hop of PE2 performs the usual penultimate hop
popping
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b. PE2 receives a packet with the top label equals the context label
1200
c. PE2 makes a lookup for 1200 in its label table. The lookup yields
the mirror table of the bgpNH=1.1.1.2
d. Pop the cL=1200. This exposes the VPN label 4300 or 4400
e. Lookup the VPN label 4300 or 4400 in the mirror table. The lookup
results in popping the VPN label 4300 or 4400 and forwarding the
packet natively to CE2
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. The method does not require support of
LFA FRR [13][14][15] and most of the provisioning effort can be
automated at the expense of the possible need to re-advertise
prefixes as described in Appendix A.
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
[4] Malhotra, P. and Rosen, E., " The BGP Encapsulation Subsequent
Address Family Identifier (SAFI) and the BGP Tunnel
Encapsulation Attribute", RFC 5512, April 2009
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[5] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[6] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[7] L. Andersson, I. Minei, B. Thomas, "LDP Specifications", RFC
5036' October 2007
[8] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.
10.2. Informative References
[9] 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.
[10] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[11] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC 4364, February 2006.
[12] 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
[13] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[14] Shand, S., and Bryant, S., "IP Fast Reroute", RFC5714, January
2010
[15] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[16] 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
[17] D. Walton, E. Chen, A. Retana, J. Scudder, "Advertisement of
Multiple Paths in BGP", draft-ietf-idr-add-paths-07.txt, June
2012
[18] R. Raszuk, R. Fernando, K. Patel, D. McPherson, K. Kumaki,
"Distribution of diverse BGP paths", draft-ietf-grow-diverse-
bgp-path-dist-08.txt, July 2012
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[19] T. Bates, E. Chen, and R. Chandra, "BGP Route Reflection: An
Alternative to Full Mesh Internal BGP (IBGP)", RFC4456, Apr
2006
[20] P. Mohapatra, R. Fernando, C. Filsfils, and R. Raszuk, "Fast
Connectivity Restoration Using BGP Add-path", draft-pmohapat-
idr-fast-conn-restore-02, October 2011
11. Acknowledgments
Special thanks to Clarence Filsfils, Eric Rosen, Stewart Bryant,
and Pradosh Mohapatra for the valuable comments
This document was prepared using 2-Word-v2.0.template.dot.
Appendix A. Auto-determination of Operating Parameters on rPE and pPE
The main provisioning effort as outlined in Section 2 is the
assignment of a domain-wide distinct pNH for each rPE-pPE pair and
configuring the pNH on the correct pPE and rPE. This section outlines
a method by which the assignment of pNH to rPE on a given pPE is
automated thereby eliminating the need for any operator intervention
except for configuring the range of IP addresses from which pNHs are
taken. The automation comes at the expense of the need to re-
advertise BGP prefixes under certain conditions as outlined below in
this Section.
The objective of the automation is to
o Let the rPE determine which pPEs the rPE can protect and hence
assign a local context label "cL" for each pPE and mirror the
portion of the pPE routing table that rPE can protect (Remember
that rPE can protect a prefix advertised by pPE if rPE has an
external path for that prefix)
o Let the pPE determine which PEs can act as rPE's for some or all
of its prefixes and hence automatically assign a pNH for each
distinct rPE out of a preconfigured range of IP addresses
When PEs peer directly with each other, it is easy to determine the
router ID of the advertising router. In the presence of a router
reflector [19], it is not possible to directly determine the router
ID of the advertising PE. Hence we introduce the "RID-attr" optional
non-transitive attribute. The actual format of the "RID-attr"
attribute is TBD. It contains the router ID of the advertising PE.
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Each PE MUST have a distinct router ID within a routing domain. "RID-
attr" MUST be advertised with each protectable prefix.
A.1. How rPE determines the Protected PE
Assuming that the "RID-attr" is advertised as an optional attribute
with all protectable prefixes, the rPE applies the following steps to
determine the pPE
1. rPE receives route advertisements from another peer and the
advertisement includes the peer's RID in the optional attribute
"RID-Attr"
2. If rPE has an external path for some or all of the received route
advertisements and rPE advertises some or all these route
advertisements (as best paths or otherwise such as [9], [17], and
[18]), then it considers the peer as a pPE
a. rPE allocates a distinct context "cL" label for the pPE
b. rPE advertises the mapping cL-->RID all the time to all peers.
The syntax is TBD for the time being but a method similar to
advertising tunnel information [4] can be used
3. If rPE loses all external paths for all prefixes from the peer
identified by "RID", then rPE withdraws the mapping "cL-->RID"
4. If rPE cannot protect all routes advertised by the pPE but can
protect some of them, then rPE re-advertises the protectable
prefixes it previously advertise but attaches the context label
"cL" as a non-transitive optional attribute. The syntax of "cL" is
TBD. This is one of the cases where prefixes previously advertised
need to be re-advertised
5. rPE creates a mirror table for pPE. If rPE can protect a route
received from pPE, then rPE mirrors that route into the mirror
table for pPE
A.2. How pPE Determines its rPEs and Assigns pNH for each rPE
1. When pPE receives the mapping cL-->RID where RID is the router ID
of the pPE, pPE assumes the router that advertised the mapping cL-
->RID is an rPE
2. pPE allocates a distinct pNH for the rPE
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3. The next step is for pPE to re-advertise some or all of its
prefixes but use the pNH assigned to rPE as bgpNH. Let
{P1/m1,...,Pk/mk} be the set prefix that rPE advertises to its
peers (as best paths or otherwise such as [9], [17], and [18])
and, at the same time, pPE advertises as reachable prefixes in the
the NLRI field. There are two cases
a. Case 1: rPE advertises the mapping cL-->RID but rPE does not
associate the context label "cL" as an optional attribute with
any prefix {P1/m1,...,Pk/mk}
b. Case 2: rPE advertises the mapping cL-->RID and rPE associates
"cL" as an optional attribute with a *subset* of the prefixes
{P1/m1,...,Pk/mk}
4. Case 1: rPE does not associate the context label "cL" with
advertised prefixes. In that case, pPE assumes that rPE can
protect all of the prefixes {P1/m1,...,Pk/mk}. Hence pPE re-
advertises {P1/m1,...,Pk/mk} uses the pNH assigned for the rPE as
bgpNH.
5. Case 2: rPE associates "cL" with a *subset* of {P1/m1,...,Pk/mk}.
In that case, pPE assumes the rPE can only protect the subset of
{P1/m1,...,Pk/mk} that has "cL". Hence rPE re-advertises this
subset but uses the pNH assigned for the rPE as bgpNH.
6. Cases 1 and 2 are the second case where prefixes previously
advertised are re-advertised without any topology changes
A.3. Detecting Mis-configuration
The auto assignment of pNH described in this appendix still requires
the operator to configure a range of IP addresses from which a pPE
allocates the protected next-hops "pNH". Because the pNH allocated by
two different pPEs MUST NOT be identical, then the range of IP
addresses on two different pPEs MUST NOT overlap. Hence the only
possible misconfiguration is configuring overlapping IP ranges on two
different pPE. This section describes how such misconfiguration can
be detected. Suppose pPE1 and pPE2 where configured with overlapping
IP ranges. Such misconfiguration can be detected as follows:
1. Because in case of misconfiguration the IP ranges on pPE1 and pPE2
overlap, then at one point in time, pPE1 will allocate a pNH that
falls within the IP range configured on pPE2
2. As described in Section A.2 pPE1 re-advertises some or all of its
prefixes and use the allocated pNH as the bgpNH attribute
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3. When pPE2 receives an advertisement from another peer containing a
bgpNH within pPE2's configured IP range, then pPE2 detects the
misconfiguration
Appendix B. Ensuring correct forwarding at the edge routers
As mentioned in Section 2 both rPE and pPE advertise the protected
next-hop "pNH" in the core. To ensure no impact on traffic
engineering, rPE advertises "pNH" with (max-metric - 1). When the
primary edge router pPE becomes totally disconnected from the core,
some core routers may start to forward traffic originally destined to
pPE to rPE. Thus it is possible that traffic originally destined to
pPE arrives at rPE without "cL" appearing at the top of the label
stack. The behavior explained in Section 2 for MPLS core and Section
3 for IP core ensures that traffic is forwarded correctly when
arriving at rPE.
In an MPLS core, the rPE advertises the label "cL" for pNH. Hence
traffic originally destined for pNH and re-routed by a core router
towards rPE will arrive at rPE with "cL" at the top. Hence rPE can
identify the correct mirror table and be able forward the packet
correctly
In an IP core, rPE associates the IP address "pNH" with the mirror
table. Hence if a core router re-routes traffic originally tunneled
towards pPE to rPE, the tunnel packets arrive at rPE with the
destination address "pNH". This allows rPE to identify the correct
mirror table and be able to forward the packet correctly
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Authors' Addresses
Ahmed Bashandy
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: bashandy@cisco.com
Maciek Konstantynowicz
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: mkonstan@cisco.com
Nagendra Kumar
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: naikumar@cisco.com
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