Network Working Group M. McBride
Request for Comments: 4611 J. Meylor
BCP: 121 D. Meyer
Category: Best Current Practice August 2006
Multicast Source Discovery Protocol (MSDP) Deployment Scenarios
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes best current practices for intra-domain and
inter-domain deployment of the Multicast Source Discovery Protocol
(MSDP) in conjunction with Protocol Independent Multicast Sparse Mode
(PIM-SM).
Table of Contents
1. Introduction ....................................................2
1.1. BCP, Experimental Protocols, and Normative References ......3
2. Inter-domain MSDP Peering Scenarios .............................4
2.1. Peering between PIM Border Routers .........................4
2.2. Peering between Non-Border Routers .........................5
2.3. MSDP Peering without BGP ...................................7
2.4. MSDP Peering at a Multicast Exchange .......................7
3. Intra-domain MSDP Peering Scenarios .............................7
3.1. Peering between MSDP- and MBGP-Configured Routers ..........8
3.2. MSDP Peer Is Not BGP Peer (or No BGP Peer) .................8
3.3. Hierarchical Mesh Groups ...................................9
3.4. MSDP and Route Reflectors .................................10
3.5. MSDP and Anycast RPs ......................................11
4. Security Considerations ........................................11
4.1. Filtering SA Messages .....................................11
4.2. SA Message State Limits ...................................12
5. Acknowledgements ...............................................12
6. References .....................................................12
6.1. Normative References ......................................12
6.2. Informative References ....................................13
McBride, et al. Best Current Practice [Page 1]
RFC 4611 MSDP Deployment Scenarios August 2006
1. Introduction
MSDP [RFC3618] is used primarily in two deployment scenarios:
o Between PIM Domains
MSDP can be used between Protocol Independent Multicast Sparse
Mode (PIM-SM) [RFC4601] domains to convey information about active
sources available in other domains. MSDP peering used in such
cases is generally one-to-one peering, and utilizes the
deterministic peer-RPF (Reverse Path Forwarding) rules described
in the MSDP specification (i.e., it does not use mesh-groups).
Peerings can be aggregated on a single MSDP peer. Such a peer can
typically have from one to hundreds of peerings, which is similar
in scale to BGP peerings.
o Within a PIM Domain
MSDP is often used between Anycast Rendezvous Points (Anycast-RPs)
[RFC3446] within a PIM domain to synchronize information about the
active sources being served by each Anycast-RP peer (by virtue of
IGP reachability). MSDP peering used in this scenario is
typically based on MSDP mesh groups, where anywhere from two to
tens of peers can comprise a given mesh group, although more than
ten is not typical. One or more of these mesh-group peers may
also have additional one-to-one peerings with MSDP peers outside
that PIM domain for discovery of external sources. MSDP for
anycast-RP without external MSDP peering is a valid deployment
option and common.
Current best practice for MSDP deployment utilizes PIM-SM and the
Border Gateway Protocol with multi-protocol extensions (MBGP)
[RFC4271, RFC2858]. This document outlines how these protocols work
together to provide an intra-domain and inter-domain Any Source
Multicast (ASM) service.
The PIM-SM specification assumes that SM operates only in one PIM
domain. MSDP is used to enable the use of multiple PIM domains by
distributing the required information about active multicast sources
to other PIM domains. Due to breaking the Internet multicast
infrastructure down to multiple PIM domains, MSDP also enables the
possibility of setting policy on the visibility of the groups and
sources.
Transit IP providers typically deploy MSDP to be part of the global
multicast infrastructure by connecting to their upstream and peer
multicast networks using MSDP.
McBride, et al. Best Current Practice [Page 2]
RFC 4611 MSDP Deployment Scenarios August 2006
Edge multicast networks typically have two choices: to use their
Internet providers' RP, or to have their own RP and connect it to
their ISP using MSDP. By deploying their own RP and MSDP, they can
use internal multicast groups that are not visible to the provider's
RP. This helps internal multicast be able to continue to work in the
event that there is a problem with connectivity to the provider or
that the provider's RP/MSDP is experiencing difficulties. In the
simplest cases, where no internal multicast groups are necessary,
there is often no need to deploy MSDP.
1.1. BCP, Experimental Protocols, and Normative References
This document describes the best current practice for a widely
deployed Experimental protocol, MSDP. There is no plan to advance
the MSDP's status (for example, to Proposed Standard). The reasons
for this include:
o MSDP was originally envisioned as a temporary protocol to be
supplanted by whatever the IDMR working group produced as an
inter-domain protocol. However, the IDMR WG (or subsequently, the
BGMP WG) never produced a protocol that could be deployed to
replace MSDP.
o One of the primary reasons given for MSDP to be classified as
Experimental was that the MSDP Working Group came up with
modifications to the protocol that the WG thought made it better
but that implementors didn't see any reasons to deploy. Without
these modifications (e.g., UDP or GRE encapsulation), MSDP can
have negative consequences to initial packets in datagram streams.
o Scalability: Although we don't know what the hard limits might be,
readvertising everything you know every 60 seconds clearly limits
the amount of state you can advertise.
o MSDP reached nearly ubiquitous deployment as the de facto standard
inter-domain multicast protocol in the IPv4 Internet.
o No consensus could be reached regarding the reworking of MSDP to
address the many concerns of various constituencies within the
IETF. As a result, a decision was taken to document what is
(ubiquitously) deployed and to move that document to Experimental.
While advancement of MSDP to Proposed Standard was considered, for
the reasons mentioned above, it was immediately discarded.
o The advent of protocols such as source-specific multicast and bi-
directional PIM, as well as embedded RP techniques for IPv6, have
further reduced consensus that a replacement protocol for MSDP for
the IPv4 Internet is required.
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RFC 4611 MSDP Deployment Scenarios August 2006
The RFC Editor's policy regarding references is that they be split
into two categories known as "normative" and "informative".
Normative references specify those documents that must be read for
one to understand or implement the technology in an RFC (or whose
technology must be present for the technology in the new RFC to work)
[RFCED]. In order to understand this document, one must also
understand both the PIM and MSDP documents. As a result, references
to these documents are normative.
The IETF has adopted the policy that BCPs must not have normative
references to Experimental protocols. However, this document is a
special case in that the underlying Experimental document (MSDP) is
not planned to be advanced to Proposed Standard.
The MBONED Working Group has requested approval under the Variance
Procedure as documented in RFC 2026 [RFC2026]. The IESG followed the
Variance Procedure and, after an additional 4 week IETF Last Call,
evaluated the comments and status, and has approved 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 [RFC2119].
2. Inter-domain MSDP Peering Scenarios
The following sections describe the most common inter-domain MSDP
peering possibilities and their deployment options.
2.1. Peering between PIM Border Routers
In this case, the MSDP peers within the domain have their own RP
located within a bounded PIM domain. In addition, the domain will
typically have its own Autonomous System (AS) number and one or more
MBGP speakers. The domain may also have multiple MSDP speakers.
Each border router has an MSDP and MBGP peering with its peer
routers. These external MSDP peering deployments typically configure
the MBGP peering and MSDP peering using the same directly connected
next hop peer IP address or other IP address from the same router.
Typical deployments of this type are providers who have a direct
peering with other providers, providers peering at an exchange, or
providers who use their edge router to MSDP/MBGP peer with customers.
For a direct peering inter-domain environment to be successful, the
first AS in the MBGP best path to the originating RP should be the
same as the AS of the MSDP peer. As an example, consider the
following topology:
McBride, et al. Best Current Practice [Page 4]
RFC 4611 MSDP Deployment Scenarios August 2006
AS1----AS2----AS4
| /
| /
| /
AS3
In this case, AS4 receives a Source Active (SA) message, originated
by AS1, from AS2. AS2 also has an MBGP peering with AS4. The MBGP
first hop AS from AS4, in the best path to the originating RP, is
AS2. The AS of the sending MSDP peer is also AS2. In this case, the
peer-Reverse Path Forwarding check (peer-RPF check) passes, and the
SA message is forwarded.
A peer-RPF failure would occur in this topology when the MBGP first
hop AS, in the best path to the originating RP, is AS2 and the origin
AS of the sending MSDP peer is AS3. This reliance upon BGP AS PATH
information prevents endless looping of SA packets.
Router code, which has adopted the latest rules in the MSDP document,
will relax the rules between AS's a bit. In the following topology,
we have an MSDP peering between AS1<->AS3 and AS3<->AS4:
RP
AS1----AS2----AS3----AS4
If the first AS in best path to the RP does not equal the MSDP peer,
MSDP peer-RPF fails. So AS1 cannot MSDP peer with AS3, since AS2 is
the first AS in the MBGP best path to AS4 RP. With the latest MSDP
document compliant code, AS1 will choose the peer in the closest AS
along best AS path to the RP. AS1 will then accept SA's coming from
AS3. If there are multiple MSDP peers to routers within the same AS,
the peer with the highest IP address is chosen as the RPF peer.
2.2. Peering between Non-Border Routers
For MSDP peering between border routers, intra-domain MSDP
scalability is restricted because it is necessary to also maintain
MBGP and MSDP peerings internally towards their border routers.
Within the intra-domain, the border router becomes the announcer of
the next hop towards the originating RP. This requires that all
intra-domain MSDP peerings mirror the MBGP path back towards the
border router. External MSDP (eMSDP) peerings rely upon AS path for
peer RPF checking, while internal MSDP (iMSDP) peerings rely upon the
announcer of the next hop.
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RFC 4611 MSDP Deployment Scenarios August 2006
While the eMBGP peer is typically directly connected between border
routers, it is common for the eMSDP peer to be located deeper into
the transit provider's AS. Providers, which desire more flexibility
in MSDP peering placement, commonly choose a few dedicated routers
within their core networks for the inter-domain MSDP peerings to
their customers. These core MSDP routers will also typically be in
the provider's intra-domain MSDP mesh group and be configured for
Anycast RP. All multicast routers in the provider's AS should
statically point to the Anycast RP address. Static RP assignment is
the most commonly used method for group-to-RP mapping due to its
deterministic nature. Auto-RP [RFC4601] and/or the Bootstrap Router
(BSR) [BSR] dynamic RP mapping mechanisms could also be used to
disseminate RP information within the provider's network
For an SA message to be accepted in this (multi-hop peering)
environment, we rely upon the next (or closest, with latest MSDP
spec) AS in the best path towards the originating RP for the RPF
check. The MSDP peer address should be in the same AS as the AS of
the border router's MBGP peer. The MSDP peer address should be
advertised via MBGP.
For example, in the diagram below, if customer R1 router is MBGP
peering with the R2 router and if R1 is MSDP peering with the R3
router, then R2 and R3 must be in the same AS (or must appear, to
AS1, to be from the same AS in the event that private AS numbers are
deployed). The MSDP peer with the highest IP address will be chosen
as the MSDP RPF peer. R1 must also have the MSDP peer address of R3
in its MBGP table.
+--+ +--+ +--+
|R1|----|R2|----|R3|
+--+ +--+ +--+
AS1 AS2 AS2
From R3's perspective, AS1 (R1) is the MBGP next AS in the best path
towards the originating RP. As long as AS1 is the next AS (or
closest) in the best path towards the originating RP, RPF will
succeed on SAs arriving from R1.
In contrast, with the single hop scenario, with R2 (instead of R3)
border MSDP peering with R1 border, R2's MBGP address becomes the
announcer of the next hop for R3, towards the originating RP, and R3
must peer with that R2 address. Moreover, all AS2 intra-domain MSDP
peers need to follow iMBGP (or other IGP) peerings towards R2 since
iMSDP has a dependence upon peering with the address of the MBGP (or
other IGP) announcer of the next hop.
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2.3. MSDP Peering without BGP
In this case, an enterprise maintains its own RP and has an MSDP
peering with its service provider but does not BGP peer with them.
MSDP relies upon BGP path information to learn the MSDP topology for
the SA peer-RPF check. MSDP can be deployed without BGP, however,
and as a result, there are some special cases where the requirement
to perform a peer-RPF check on the BGP path information is suspended.
These cases are:
o There is only a single MSDP peer connection.
o A default peer (default MSDP route) is configured.
o The originating RP is directly connected.
o A mesh group is used.
o An implementation is used that allows for an MSDP peer-RPF check
using an IGP.
An enterprise will typically configure a unicast default route from
its border router to the provider's border router and then MSDP peer
with the provider's MSDP router. If internal MSDP peerings are also
used within the enterprise, then an MSDP default peer will need to be
configured on the border router that points to the provider. In this
way, all external multicast sources will be learned, and internal
sources can be advertised. If only a single MSDP peering was used
(no internal MSDP peerings) towards the provider, then this stub site
will MSDP default peer towards the provider and skip the peer-RPF
check.
2.4. MSDP Peering at a Multicast Exchange
Multicast exchanges allow multicast providers to peer at a common IP
subnet (or by using point-to-point virtual LANs) and share MSDP SA
updates. Each provider will MSDP and MBGP peer with each others
directly connected exchange IP address. Each exchange router will
send/receive SAs to/from their MSDP peers. They will then be able to
forward SAs throughout their domain to their customers and any direct
provider peerings.
3. Intra-domain MSDP Peering Scenarios
The following sections describe the different intra-domain MSDP
peering possibilities and their deployment options.
McBride, et al. Best Current Practice [Page 7]
RFC 4611 MSDP Deployment Scenarios August 2006
3.1. Peering between MSDP- and MBGP-Configured Routers
The next hop IP address of the iBGP peer is typically used for the
MSDP peer-RPF check (IGP can also be used). This is different from
the inter-domain BGP/MSDP case, where AS path information is used for
the peer-RPF check. For this reason, it is necessary for the IP
address of the MSDP peer connection to be the same as the internal
MBGP peer connection whether or not the MSDP/MBGP peers are directly
connected. A successful deployment would be similar to the
following:
+----+
| Rb | 3.3.3.3
/ +----+
AS1 AS2 / |
+---+ +--+ / |
|RP1|---------|Ra| |
+---+ +--+ |
1.1.1.1 2.2.2.2 |
\ |
\ |
\ +-----+
| RP2 |
+-----+
where RP2 MSDP and MBGP peers with Ra (using 2.2.2.2) and with Rb
(using 3.3.3.3). When the MSDP SA update arrives on RP2 from Ra, the
MSDP RPF check for 1.1.1.1 passes because RP2 receives the SA update
from MSDP peer 2.2.2.2, which is also the correct MBGP next hop for
1.1.1.1.
When RP2 receives the same SA update from MSDP peer 3.3.3.3, the MBGP
lookup for 1.1.1.1 shows a next hop of 2.2.2.2, so RPF correctly
fails, preventing a loop.
This deployment could also fail on an update from Ra to RP2 if RP2
was MBGP peering to an address other than 2.2.2.2 on Ra. Intra-
domain deployments must have MSDP and MBGP (or other IGP) peering
addresses that match, unless a method to skip the peer-RPF check is
deployed.
3.2. MSDP Peer Is Not BGP Peer (or No BGP Peer)
This is a common MSDP intra-domain deployment in environments where
few routers are running MBGP or where the domain is not running MBGP.
The problem here is that the MSDP peer address needs to be the same
as the MBGP peer address. To get around this requirement, the intra-
domain MSDP RPF rules have been relaxed in the following topologies:
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RFC 4611 MSDP Deployment Scenarios August 2006
o By configuring the MSDP peer as a mesh group peer.
o By having the MSDP peer be the only MSDP peer.
o By configuring a default MSDP peer.
o By peering with the originating RP.
o By relying upon an IGP for MSDP peer-RPF.
The common choice around the intra-domain BGP peering requirement,
when more than one MSDP peer is configured, is to deploy MSDP mesh
groups. When an MSDP mesh group is deployed, there is no RPF check
on arriving SA messages when they are received from a mesh group
peer. Subsequently, SA messages are always accepted from mesh group
peers. MSDP mesh groups were developed to reduce the amount of SA
traffic in the network since SAs, which arrive from a mesh group
peer, are not flooded to peers within that same mesh group. Mesh
groups must be fully meshed.
If recent (but not currently widely deployed) router code is running
that is fully compliant with the latest MSDP document, another
option, to work around not having BGP to MSDP RPF peer, is to RPF
using an IGP like OSPF, IS-IS, RIP, etc. This new capability will
allow for enterprise customers, who are not running BGP and who don't
want to run mesh groups, to use their existing IGP to satisfy the
MSDP peer-RPF rules.
3.3. Hierarchical Mesh Groups
Hierarchical mesh groups are occasionally deployed in intra-domain
environments where there are a large number of MSDP peers. Allowing
multiple mesh groups to forward to one another can reduce the number
of MSDP peerings per router (due to the full mesh requirement) and
hence reduce router load. A good hierarchical mesh group
implementation (one that prevents looping) contains a core mesh group
in the backbone, and these core routers serve as mesh group
aggregation routers:
[R2]{A,2}
/ \
/ \
/ \
/ \
/ \
/ \
/ \
{A,1}[R1]-------------[R3]{A,3}
McBride, et al. Best Current Practice [Page 9]
RFC 4611 MSDP Deployment Scenarios August 2006
In this example, R1, R2, and R3 are in MSDP mesh group A (the core
mesh group), and each serves as MSDP aggregation routers for their
leaf (or second tier) mesh groups 1, 2, and 3. Since SA messages
received from a mesh group peer are not forwarded to peers within
that same mesh group, SA messages will not loop. Do not create
topologies that connect mesh groups in a loop. In the above example,
for instance, second-tier mesh groups 1, 2, and 3 must not directly
exchange SA messages with each other or an endless SA loop will
occur.
Redundancy between mesh groups will also cause a loop and is
subsequently not available with hierarchical mesh groups. For
instance, assume that R3 had two routers connecting its leaf mesh
group 3 with the core mesh group A. A loop would be created between
mesh group 3 and mesh group A because each mesh group must be fully
meshed between peers.
3.4. MSDP and Route Reflectors
BGP requires all iBGP speakers that are not route-reflector clients
or confederation members be fully meshed to prevent loops. In the
route reflector environment, MSDP requires that the route reflector
clients peer with the route reflector since the router reflector (RR)
is the BGP announcer of the next hop towards the originating RP. The
RR is not the BGP next hop, but is the announcer of the BGP next hop.
The announcer of the next hop is the address typically used for MSDP
peer-RPF checks. For example, consider the following case:
Ra--------RR
/|\
/ | \
A B C
Ra is forwarding MSDP SAs to the route reflector RR. Routers A, B,
and C also MSDP peer with RR. When RR forwards the SA to A, B, and
C, these RR clients will accept the SA because RR is the announcer of
the next hop to the originating RP address.
An SA will peer-RPF fail if Ra MSDP peers directly with Routers A, B,
or C because the announcer of the next hop is RR but the SA update
came from Ra. Proper deployment is to have RR clients MSDP peer with
the RR. MSDP mesh groups may be used to work around this
requirement. External MSDP peerings will also prevent this
requirement since the next AS is compared between MBGP and MSDP
peerings, rather than the IP address of the announcer of the next
hop.
McBride, et al. Best Current Practice [Page 10]
RFC 4611 MSDP Deployment Scenarios August 2006
Some recent MSDP implementations conform to the latest MSDP document,
which relaxes the requirement of peering with the Advertiser of the
next hop (the Route Reflector). This new rule allows for peering
with the next hop, in addition to the Advertiser of the next hop. In
the example above, for instance, if Ra is the next hop (perhaps due
to using BGP's next hop self attribute), and if routers A, B, and C
are peering with Ra, the SA's received from Ra will now succeed.
3.5. MSDP and Anycast RPs
A network with multiple RPs can achieve RP load sharing and
redundancy by using the Anycast RP mechanism in conjunction with MSDP
mesh groups [RFC3446]. This mechanism is a common deployment
technique used within a domain by service providers and enterprises
that deploy several RPs within their domains. These RPs will each
have the same IP address configured on a Loopback interface (making
this the Anycast address). These RPs will MSDP peer with each other
using a separate loopback interface and are part of the same fully
meshed MSDP mesh group. This loopback interface, used for MSDP
peering, will typically also be used for the MBGP peering. All
routers within the provider's domain will learn of the Anycast RP
address through Auto-RP, BSR, or a static RP assignment. Each
designated router in the domain will send source registers and group
joins to the Anycast RP address. Unicast routing will direct those
registers and joins to the nearest Anycast RP. If a particular
Anycast RP router fails, unicast routing will direct subsequent
registers and joins to the nearest Anycast RP. That RP will then
forward an MSDP update to all peers within the Anycast MSDP mesh
group. Each RP will then forward (or receive) the SAs to (from)
external customers and providers.
4. Security Considerations
An MSDP service should be secured by explicitly controlling the state
that is created by, and passed within, the MSDP service. As with
unicast routing state, MSDP state should be controlled locally, at
the edge origination points. Selective filtering at the multicast
service edge helps ensure that only intended sources result in SA
message creation, and this control helps to reduce the likelihood of
state-aggregation related problems in the core. There are a variety
of points where local policy should be applied to the MSDP service.
4.1. Filtering SA Messages
The process of originating SA messages should be filtered to ensure
that only intended local sources are resulting in SA message
origination. In addition, MSDP speakers should filter which SA
messages get received and forwarded.
McBride, et al. Best Current Practice [Page 11]
RFC 4611 MSDP Deployment Scenarios August 2006
Typically, there is a fair amount of (S,G) state in a PIM-SM domain
that is local to the domain. However, without proper filtering, SA
messages containing these local (S,G) announcements may be advertised
to the global MSDP infrastructure. Examples of this include domain-
local applications that use global IP multicast addresses and sources
that use RFC 1918 addresses [RFC1918]. To improve on the scalability
of MSDP and to avoid global visibility of domain local (S,G)
information, an external SA filter list is recommended to help
prevent unnecessary creation, forwarding, and caching of well-known
domain local sources.
4.2. SA Message State Limits
Proper filtering on SA message origination, receipt, and forwarding
will significantly reduce the likelihood of unintended and unexpected
spikes in MSDP state. However, an SA-cache state limit SHOULD be
configured as a final safeguard to state spikes. When an MSDP
peering has reached a stable state (i.e., when the peering has been
established and the initial SA state has been transferred), it may
also be desirable to configure a rate limiter for the creation of new
SA state entries.
5. Acknowledgements
The authors would like to thank Pekka Savola, John Zwiebel, Swapna
Yelamanchi, Greg Shepherd, and Jay Ford for their feedback on earlier
versions of this document.
6. References
6.1. Normative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
"Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.
McBride, et al. Best Current Practice [Page 12]
RFC 4611 MSDP Deployment Scenarios August 2006
[RFC3446] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci, "Anycast
Rendevous Point (RP) mechanism using Protocol Independent
Multicast (PIM) and Multicast Source Discovery Protocol
(MSDP)", RFC 3446, January 2003.
[RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery
Protocol (MSDP)", RFC 3618, October 2003.
6.2. Informative References
[BSR] Fenner, W., et. al., "Bootstrap Router (BSR) Mechanism for
PIM Sparse Mode", Work in Progress, February 2003.
[RFCED]
http://www.rfc-editor.org/policy.html
Authors' Addresses
Mike McBride
Cisco Systems
EMail:
[email protected]
John Meylor
Cisco Systems
EMail:
[email protected]
David Meyer
EMail:
[email protected]
McBride, et al. Best Current Practice [Page 13]
RFC 4611 MSDP Deployment Scenarios August 2006
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
McBride, et al. Best Current Practice [Page 14]
