Network Working Group D. McPherson
Request for Comments: 4451 Arbor Networks, Inc.
Category: Informational V. Gill
AOL
March 2006
BGP MULTI_EXIT_DISC (MED) Considerations
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The BGP MULTI_EXIT_DISC (MED) attribute provides a mechanism for BGP
speakers to convey to an adjacent AS the optimal entry point into the
local AS. While BGP MEDs function correctly in many scenarios, a
number of issues may arise when utilizing MEDs in dynamic or complex
topologies.
This document discusses implementation and deployment considerations
regarding BGP MEDs and provides information with which implementers
and network operators should be familiar.
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Table of Contents
1. Introduction ....................................................3
2. Specification of Requirements ...................................3
2.1. About the MULTI_EXIT_DISC (MED) Attribute ..................3
2.2. MEDs and Potatoes ..........................................5
3. Implementation and Protocol Considerations ......................6
3.1. MULTI_EXIT_DISC Is an Optional Non-Transitive Attribute ....6
3.2. MED Values and Preferences .................................6
3.3. Comparing MEDs between Different Autonomous Systems ........7
3.4. MEDs, Route Reflection, and AS Confederations for BGP ......7
3.5. Route Flap Damping and MED Churn ...........................8
3.6. Effects of MEDs on Update Packing Efficiency ...............9
3.7. Temporal Route Selection ...................................9
4. Deployment Considerations ......................................10
4.1. Comparing MEDs between Different Autonomous Systems .......10
4.2. Effects of Aggregation on MEDs ............................11
5. Security Considerations ........................................11
6. Acknowledgements ...............................................11
7. References .....................................................12
7.1. Normative References ......................................12
7.2. Informative References ....................................12
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1. Introduction
The BGP MED attribute provides a mechanism for BGP speakers to convey
to an adjacent AS the optimal entry point into the local AS. While
BGP MEDs function correctly in many scenarios, a number of issues may
arise when utilizing MEDs in dynamic or complex topologies.
While reading this document, note that the goal is to discuss both
implementation and deployment considerations regarding BGP MEDs. In
addition, the intention is to provide guidance that both implementors
and network operators should be familiar with. In some instances,
implementation advice varies from deployment advice.
2. Specification of Requirements
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 [RFC2119].
2.1. About the MULTI_EXIT_DISC (MED) Attribute
The BGP MULTI_EXIT_DISC (MED) attribute, formerly known as the
INTER_AS_METRIC, is currently defined in section 5.1.4 of [BGP4], as
follows:
The MULTI_EXIT_DISC is an optional non-transitive attribute that
is intended to be used on external (inter-AS) links to
discriminate among multiple exit or entry points to the same
neighboring AS. The value of the MULTI_EXIT_DISC attribute is a
four-octet unsigned number, called a metric. All other factors
being equal, the exit point with the lower metric SHOULD be
preferred. If received over External BGP (EBGP), the
MULTI_EXIT_DISC attribute MAY be propagated over Internal BGP
(IBGP) to other BGP speakers within the same AS (see also
9.1.2.2). The MULTI_EXIT_DISC attribute received from a
neighboring AS MUST NOT be propagated to other neighboring ASes.
A BGP speaker MUST implement a mechanism (based on local
configuration) that allows the MULTI_EXIT_DISC attribute to be
removed from a route. If a BGP speaker is configured to remove
the MULTI_EXIT_DISC attribute from a route, then this removal MUST
be done prior to determining the degree of preference of the route
and prior to performing route selection (Decision Process phases 1
and 2).
An implementation MAY also (based on local configuration) alter
the value of the MULTI_EXIT_DISC attribute received over EBGP. If
a BGP speaker is configured to alter the value of the
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MULTI_EXIT_DISC attribute received over EBGP, then altering the
value MUST be done prior to determining the degree of preference
of the route and prior to performing route selection (Decision
Process phases 1 and 2). See Section 9.1.2.2 for necessary
restrictions on this.
Section 9.1.2.2 (c) of [BGP4] defines the following route selection
criteria regarding MEDs:
c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS (the
neighboring AS is determined from the AS_PATH attribute).
Routes that do not have the MULTI_EXIT_DISC attribute are
considered to have the lowest possible MULTI_EXIT_DISC value.
This is also described in the following procedure:
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function that returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has
no MULTI_EXIT_DISC attribute, the function returns the lowest
possible MULTI_EXIT_DISC value (i.e., 0).
Similarly, neighborAS(n) is a function that returns the
neighbor AS from which the route was received. If the route is
learned via IBGP, and the other IBGP speaker didn't originate
the route, it is the neighbor AS from which the other IBGP
speaker learned the route. If the route is learned via IBGP,
and the other IBGP speaker either (a) originated the route, or
(b) created the route by aggregation and the AS_PATH attribute
of the aggregate route is either empty or begins with an
AS_SET, it is the local AS.
If a MULTI_EXIT_DISC attribute is removed before re-advertising
a route into IBGP, then comparison based on the received EBGP
MULTI_EXIT_DISC attribute MAY still be performed. If an
implementation chooses to remove MULTI_EXIT_DISC, then the
optional comparison on MULTI_EXIT_DISC, if performed, MUST be
performed only among EBGP-learned routes. The best EBGP-
learned route may then be compared with IBGP-learned routes
after the removal of the MULTI_EXIT_DISC attribute. If
MULTI_EXIT_DISC is removed from a subset of EBGP-learned
routes, and the selected "best" EBGP-learned route will not
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have MULTI_EXIT_DISC removed, then the MULTI_EXIT_DISC must be
used in the comparison with IBGP-learned routes. For IBGP-
learned routes, the MULTI_EXIT_DISC MUST be used in route
comparisons that reach this step in the Decision Process.
Including the MULTI_EXIT_DISC of an EBGP-learned route in the
comparison with an IBGP-learned route, then removing the
MULTI_EXIT_DISC attribute, and advertising the route has been
proven to cause route loops.
2.2. MEDs and Potatoes
Let's consider a situation where traffic flows between a pair of
hosts, each connected to a different transit network, which is in
itself interconnected at two or more locations. Each transit network
has the choice of either sending traffic to the closest peering to
the adjacent transit network or passing traffic to the
interconnection location that advertises the least-cost path to the
destination host.
The former method is called "hot potato routing" (or closest-exit)
because like a hot potato held in bare hands, whoever has it tries to
get rid of it quickly. Hot potato routing is accomplished by not
passing the EBGP-learned MED into IBGP. This minimizes transit
traffic for the provider routing the traffic. Far less common is
"cold potato routing" (or best-exit) where the transit provider uses
its own transit capacity to get the traffic to the point that
adjacent transit provider advertised as being closest to the
destination. Cold potato routing is accomplished by passing the
EBGP-learned MED into IBGP.
If one transit provider uses hot potato routing and another uses cold
potato, traffic between the two tends to be more symmetric. However,
if both providers employ cold potato routing or hot potato routing
between their networks, it's likely that a larger amount of asymmetry
would exist.
Depending on the business relationships, if one provider has more
capacity or a significantly less congested backbone network, then
that provider may use cold potato routing. An example of widespread
use of cold potato routing was the NSF-funded NSFNET backbone and
NSF-funded regional networks in the mid-1990s.
In some cases, a provider may use hot potato routing for some
destinations for a given peer AS and cold potato routing for others.
An example of this is the different treatment of commercial and
research traffic in the NSFNET in the mid-1990s. Today, many
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commercial networks exchange MEDs with customers but not with
bilateral peers. However, commercial use of MEDs varies widely, from
ubiquitous use to none at all.
In addition, many deployments of MEDs today are likely behaving
differently (e.g., resulting in sub-optimal routing) than the network
operator intended, which results not in hot or cold potatoes, but
mashed potatoes! More information on unintended behavior resulting
from MEDs is provided throughout this document.
3. Implementation and Protocol Considerations
There are a number of implementation and protocol peculiarities
relating to MEDs that have been discovered that may affect network
behavior. The following sections provide information on these
issues.
3.1. MULTI_EXIT_DISC Is an Optional Non-Transitive Attribute
MULTI_EXIT_DISC is a non-transitive optional attribute whose
advertisement to both IBGP and EBGP peers is discretionary. As a
result, some implementations enable sending of MEDs to IBGP peers by
default, while others do not. This behavior may result in sub-
optimal route selection within an AS. In addition, some
implementations send MEDs to EBGP peers by default, while others do
not. This behavior may result in sub-optimal inter-domain route
selection.
3.2. MED Values and Preferences
Some implementations consider an MED value of zero less preferable
than no MED value. This behavior resulted in path selection
inconsistencies within an AS. The current version of the BGP
specification [BGP4] removes ambiguities that existed in [RFC1771] by
stating that if route n has no MULTI_EXIT_DISC attribute, the lowest
possible MULTI_EXIT_DISC value (i.e., 0) should be assigned to the
attribute.
It is apparent that different implementations and different versions
of the BGP specification have been all over the map with
interpretation of missing-MED. For example, earlier versions of the
specification called for a missing MED to be assigned the highest
possible MED value (i.e., 2^32-1).
In addition, some implementations have been shown to internally
employ a maximum possible MED value (2^32-1) as an "infinity" metric
(i.e., the MED value is used to tag routes as unfeasible); upon
receiving an update with an MED value of 2^32-1, they would rewrite
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the value to 2^32-2. Subsequently, the new MED value would be
propagated and could result in routing inconsistencies or unintended
path selections.
As a result of implementation inconsistencies and protocol revision
variances, many network operators today explicitly reset (i.e., set
to zero or some other 'fixed' value) all MED values on ingress to
conform to their internal routing policies (i.e., to include policy
that requires that MED values of 0 and 2^32-1 not be used in
configurations, whether the MEDs are directly computed or
configured), so as not to have to rely on all their routers having
the same missing-MED behavior.
Because implementations don't normally provide a mechanism to disable
MED comparisons in the decision algorithm, "not using MEDs" usually
entails explicitly setting all MEDs to some fixed value upon ingress
to the routing domain. By assigning a fixed MED value consistently
to all routes across the network, MEDs are a effectively a non-issue
in the decision algorithm.
3.3. Comparing MEDs between Different Autonomous Systems
The MED was intended to be used on external (inter-AS) links to
discriminate among multiple exit or entry points to the same
neighboring AS. However, a large number of MED applications now
employ MEDs for the purpose of determining route preference between
like routes received from different autonomous systems.
A large number of implementations provide the capability to enable
comparison of MEDs between routes received from different neighboring
autonomous systems. While this capability has demonstrated some
benefit (e.g., that described in [RFC3345]), operators should be wary
of the potential side effects of enabling such a function. The
deployment section below provides some examples as to why this may
result in undesirable behavior.
3.4. MEDs, Route Reflection, and AS Confederations for BGP
In particular configurations, the BGP scaling mechanisms defined in
"BGP Route Reflection - An Alternative to Full Mesh IBGP" [RFC2796]
and "Autonomous System Confederations for BGP" [RFC3065] will
introduce persistent BGP route oscillation [RFC3345]. The problem is
inherent in the way BGP works: a conflict exists between information
hiding/hierarchy and the non-hierarchical selection process imposed
by lack of total ordering caused by the MED rules. Given current
practices, we see the problem manifest itself most frequently in the
context of MED + route reflectors or confederations.
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One potential way to avoid this is by configuring inter-Member-AS or
inter-cluster IGP metrics higher than intra-Member-AS IGP metrics
and/or using other tie-breaking policies to avoid BGP route selection
based on incomparable MEDs. Of course, IGP metric constraints may be
unreasonably onerous for some applications.
Not comparing MEDs between multiple paths for a prefix learned from
different adjacent autonomous systems, as discussed in section 2.3,
or not utilizing MEDs at all, significantly decreases the probability
of introducing potential route oscillation conditions into the
network.
Although perhaps "legal" as far as current specifications are
concerned, modifying MED attributes received on any type of IBGP
session (e.g., standard IBGP, EBGP sessions between Member-ASes of a
BGP confederation, route reflection, etc.) is not recommended.
3.5. Route Flap Damping and MED Churn
MEDs are often derived dynamically from IGP metrics or additive costs
associated with an IGP metric to a given BGP NEXT_HOP. This
typically provides an efficient model for ensuring that the BGP MED
advertised to peers, used to represent the best path to a given
destination within the network, is aligned with that of the IGP
within a given AS.
The consequence with dynamically derived IGP-based MEDs is that
instability within an AS, or even on a single given link within the
AS, can result in widespread BGP instability or BGP route
advertisement churn that propagates across multiple domains. In
short, if your MED "flaps" every time your IGP metric flaps, your
routes are likely going to be suppressed as a result of BGP Route
Flap Damping [RFC2439].
Employment of MEDs may compound the adverse effects of BGP flap-
dampening behavior because it may cause routes to be re-advertised
solely to reflect an internal topology change.
Many implementations don't have a practical problem with IGP
flapping; they either latch their IGP metric upon first advertisement
or employ some internal suppression mechanism. Some implementations
regard BGP attribute changes as less significant than route
withdrawals and announcements to attempt to mitigate the impact of
this type of event.
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3.6. Effects of MEDs on Update Packing Efficiency
Multiple unfeasible routes can be advertised in a single BGP Update
message. The BGP4 protocol also permits advertisement of multiple
prefixes with a common set of path attributes to be advertised in a
single update message. This is commonly referred to as "update
packing". When possible, update packing is recommended as it
provides a mechanism for more efficient behavior in a number of
areas, including the following:
o Reduction in system overhead due to generation or receipt of
fewer Update messages.
o Reduction in network overhead as a result of fewer packets and
lower bandwidth consumption.
o Less frequent processing of path attributes and searches for
matching sets in your AS_PATH database (if you have one).
Consistent ordering of the path attributes allows for ease of
matching in the database as you don't have different
representations of the same data.
Update packing requires that all feasible routes within a single
update message share a common attribute set, to include a common
MULTI_EXIT_DISC value. As such, potential wide-scale variance in MED
values introduces another variable and may result in a marked
decrease in update packing efficiency.
3.7. Temporal Route Selection
Some implementations had bugs that led to temporal behavior in
MED-based best path selection. These usually involved methods to
store the oldest route and to order routes for MED, which caused
non-deterministic behavior as to whether or not the oldest route
would truly be selected.
The reasoning for this is that older paths are presumably more
stable, and thus preferable. However, temporal behavior in route
selection results in non-deterministic behavior and, as such, is
often undesirable.
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4. Deployment Considerations
It has been discussed that accepting MEDs from other autonomous
systems has the potential to cause traffic flow churns in the
network. Some implementations only ratchet down the MED and never
move it back up to prevent excessive churn.
However, if a session is reset, the MEDs being advertised have the
potential of changing. If a network is relying on received MEDs to
route traffic properly, the traffic patterns have the potential for
changing dramatically, potentially resulting in congestion on the
network. Essentially, accepting and routing traffic based on MEDs
allows other people to traffic engineer your network. This may or
may not be acceptable to you.
As previously discussed, many network operators choose to reset MED
values on ingress. In addition, many operators explicitly do not
employ MED values of 0 or 2^32-1 in order to avoid inconsistencies
with implementations and various revisions of the BGP specification.
4.1. Comparing MEDs between Different Autonomous Systems
Although the MED was meant to be used only when comparing paths
received from different external peers in the same AS, many
implementations provide the capability to compare MEDs between
different autonomous systems as well. AS operators often use
LOCAL_PREF to select the external preferences (primary, secondary
upstreams, peers, customers, etc.), using MED instead of LOCAL_PREF
would possibly lead to an inconsistent distribution of best routes,
as MED is compared only after the AS_PATH length.
Though this may seem like a fine idea for some configurations, care
must be taken when comparing MEDs between different autonomous
systems. BGP speakers often derive MED values by obtaining the IGP
metric associated with reaching a given BGP NEXT_HOP within the local
AS. This allows MEDs to reasonably reflect IGP topologies when
advertising routes to peers. While this is fine when comparing MEDs
between multiple paths learned from a single AS, it can result in
potentially "weighted" decisions when comparing MEDs between
different autonomous systems. This is most typically the case when
the autonomous systems use different mechanisms to derive IGP metrics
or BGP MEDs, or when they perhaps even use different IGP protocols
with vastly contrasting metric spaces (e.g., OSPF vs. traditional
metric space in IS-IS).
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4.2. Effects of Aggregation on MEDs
Another MED deployment consideration involves the impact that
aggregation of BGP routing information has on MEDs. Aggregates are
often generated from multiple locations in an AS in order to
accommodate stability, redundancy, and other network design goals.
When MEDs are derived from IGP metrics associated with said
aggregates, the MED value advertised to peers can result in very
suboptimal routing.
5. Security Considerations
The MED was purposely designed to be a "weak" metric that would only
be used late in the best-path decision process. The BGP working
group was concerned that any metric specified by a remote operator
would only affect routing in a local AS if no other preference was
specified. A paramount goal of the design of the MED was to ensure
that peers could not "shed" or "absorb" traffic for networks that
they advertise. As such, accepting MEDs from peers may in some sense
increase a network's susceptibility to exploitation by peers.
6. Acknowledgements
Thanks to John Scudder for applying his usual keen eye and
constructive insight. Also, thanks to Curtis Villamizar, JR
Mitchell, and Pekka Savola for their valuable feedback.
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7. References
7.1. Normative References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-
4)", RFC 1771, March 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2796] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection
- An Alternative to Full Mesh IBGP", RFC 2796, April 2000.
[RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 3065, February 2001.
[BGP4] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
7.2. Informative References
[RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
Flap Damping", RFC 2439, November 1998.
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, August 2002.
Authors' Addresses
Danny McPherson
Arbor Networks
EMail: danny@arbor.net
Vijay Gill
AOL
EMail: VijayGill9@aol.com
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