Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field
contains the incorrect attribute (type, length and value). Syntactic
correctness means that the NEXT_HOP attribute represents a valid IP
host address. Semantic correctness applies only to the external BGP
links. It means that the interface associated with the IP address, as
specified in the NEXT_HOP attribute, shares a common subnet with the
receiving BGP speaker and is not the IP address of the receiving BGP
speaker. If the NEXT_HOP attribute is semantically incorrect, the
error should be logged, and the the route should be ignored. In this
case, no NOTIFICATION message should be sent.
The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode is set to
Malformed AS_PATH.
If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is
discarded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic
validity. If the field is syntactically incorrect, then the Error
Subcode is set to Invalid Network Field.
6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an
unrecognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the
peer. The means to do this, however, lies outside the scope of this
document.
6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the BGP connection
closed.
6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.
6.7 Cease.
In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when a fatal error
indicated by this section does exist.
6.8 Connection collision detection.
If a pair of BGP speakers try simultaneously to establish a TCP
connection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be
closed.
Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a
collision does occur. The convention is to compare the BGP
Identifiers of the peers involved in the collision and to retain only
the connection initiated by the BGP speaker with the higher-valued
BGP Identifier.
Upon receipt of an OPEN message, the local system must examine all of
its connections that are in the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, then the local
system performs the following collision resolution procedure:
1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN
message).
2. If the value of the local BGP Identifier is less than the
remote one, the local system closes BGP connection that already
exists (the one that is already in the OpenConfirm state), and
accepts BGP connection initiated by the remote system.
3. Otherwise, the local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is already in the
OpenConfirm state).
Comparing BGP Identifiers is done by treating them as (4-octet
long) unsigned integers.
A connection collision with an existing BGP connection that is in
Established states causes unconditional closing of the newly
created connection. Note that a connection collision cannot be
detected with connections that are in Idle, or Connect, or Active
states.
Closing the BGP connection (that results from the collision
resolution procedure) is accomplished by sending the NOTIFICATION
message with the Error Code Cease.
7. BGP Version Negotiation.
BGP speakers may negotiate the version of the protocol by making
multiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common
versions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION
messages.
8. BGP Finite State machine.
This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1.
Initially BGP is in the Idle state.
Idle state:
In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. In response to the Start
event (initiated by either system or operator) the local system
initializes all BGP resources, starts the ConnectRetry timer,
initiates a transport connection to other BGP peer, while
listening for connection that may be initiated by the remote
BGP peer, and changes its state to Connect. The exact value of
the ConnectRetry timer is a local matter, but should be
sufficiently large to allow TCP initialization.
If a BGP speaker detects an error, it shuts down the connection
and changes its state to Idle. Getting out of the Idle state
requires generation of the Start event. If such an event is
generated automatically, then persistent BGP errors may result
in persistent flapping of the speaker. To avoid such a
condition it is recommended that Start events should not be
generated immediately for a peer that was previously
transitioned to Idle due to an error. For a peer that was
previously transitioned to Idle due to an error, the time
between consecutive generation of Start events, if such events
are generated automatically, shall exponentially increase. The
value of the initial timer shall be 60 seconds. The time shall
be doubled for each consecutive retry.
Any other event received in the Idle state is ignored.
Connect state:
In this state BGP is waiting for the transport protocol
connection to be completed.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent.
If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
stays in the Connect state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
Active state:
In this state BGP is trying to acquire a peer by initiating a
transport protocol connection.
If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its Hold Timer to a large
value, and changes its state to OpenSent. A Hold Timer value
of 4 minutes is suggested.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a
connection that may be initiated by the remote BGP peer, and
changes its state to Connect.
If the local system detects that a remote peer is trying to
establish BGP connection to it, and the IP address of the
remote peer is not an expected one, the local system restarts
the ConnectRetry timer, rejects the attempted connection,
continues to listen for a connection that may be initiated by
the remote BGP peer, and stays in the Active state.
Start event is ignored in the Active state.
In response to any other event (initiated by either system or
operator), the local system releases all BGP resources
associated with this connection and changes its state to Idle.
OpenSent state:
In this state BGP waits for an OPEN message from its peer.
When an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking or OPEN
message checking detects an error (see Section 6.2), or a
connection collision (see Section 6.8) the local system sends a
NOTIFICATION message and changes its state to Idle.
If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The Hold Timer,
which was originally set to a large value (see above), is
replaced with the negotiated Hold Time value (see section 4.2).
If the negotiated Hold Time value is zero, then the Hold Time
timer and KeepAlive timers are not started. If the value of
the Autonomous System field is the same as the local Autonomous
System number, then the connection is an "internal" connection;
otherwise, it is "external". (This will effect UPDATE
processing as described below.) Finally, the state is changed
to OpenConfirm.
If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and
goes into the Active state.
If the Hold Timer expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its
state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenSent state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenSent to Idle, it closes
the BGP (and transport-level) connection and releases all
resources associated with that connection.
OpenConfirm state:
In this state BGP waits for a KEEPALIVE or NOTIFICATION
message.
If the local system receives a KEEPALIVE message, it changes
its state to Established.
If the Hold Timer expires before a KEEPALIVE message is
received, the local system sends NOTIFICATION message with
error code Hold Timer Expired and changes its state to Idle.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenConfirm state.
In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from OpenConfirm to Idle, it
closes the BGP (and transport-level) connection and releases
all resources associated with that connection.
Established state:
In the Established state BGP can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer.
If the local system receives an UPDATE or KEEPALIVE message, it
restarts its Hold Timer, if the negotiated Hold Time value is
non-zero.
If the local system receives a NOTIFICATION message, it changes
its state to Idle.
If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and
changes its state to Idle.
If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle.
If the Hold Timer expires, the local system sends a
NOTIFICATION message with Error Code Hold Timer Expired and
changes its state to Idle.
If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer, unless the negotiated Hold
Time value is zero.
In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state.
In response to any other event, the local system sends
NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle.
Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all
resources associated with that connection, and deletes all
routes derived from that connection.
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is
quietly ignored. If an optional transitive attribute is
unrecognized, the Partial bit (the third high-order bit) in the
attribute flags octet is set to 1, and the attribute is retained for
propagation to other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible
propagation to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the
previously advertised route is not longer available for use.
If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions
shall be taken:
i) If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus
implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer
available for use.
ii) If the new route is an overlapping route that is included (see
9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP
speaker shall run its Decision Process since the more specific route
has implicitly made a portion of the less specific route unavailable
for use.
iii) If the new route has identical path attributes to an earlier
route contained in the Adj-RIB-In, and is more specific (see 9.1.4)
than the earlier route, no further actions are necessary.
iv) If the new route has NLRI that is not present in any of the
routes currently stored in the Adj-RIB-In, then the new route shall
be placed in the Adj-RIB-In. The BGP speaker shall run its Decision
Process.
v) If the new route is an overlapping route that is less specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
BGP speaker shall run its Decision Process on the set of destinations
described only by the less specific route.
9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIB-In. The output of the Decision
Process is the set of routes that will be advertised to all peers;
the selected routes will be stored in the local speaker's Adj-RIB-
Out.
The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the existence
of other routes, the non-existence of other routes, or the path
attributes of other routes. Route selection then consists of
individual application of the degree of preference function to each
feasible route, followed by the choice of the one with the highest
degree of preference.
The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for:
- selection of routes to be advertised to BGP speakers located in
the local speaker's autonomous system
- selection of routes to be advertised to BGP speakers located in
neighboring autonomous systems
- route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each
triggered by a different event:
a) Phase 1 is responsible for calculating the degree of preference
for each route received from a BGP speaker located in a
neighboring autonomous system, and for advertising to the other
BGP speakers in the local autonomous system the routes that have
the highest degree of preference for each distinct destination.
b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the appropriate Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer
located in a neighboring autonomous system, according to the
policies contained in the PIB. Route aggregation and information
reduction can optionally be performed within this phase.
9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local BGP
speaker receives an UPDATE message from a peer located in a
neighboring autonomous system that advertises a new route, a
replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process which completes
when it has no further work to do.
The Phase 1 decision function shall lock an Adj-RIB-In prior to
operating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP
speaker shall determine a degree of preference. If the route is
learned from a BGP speaker in the local autonomous system, either the
value of the LOCAL_PREF attribute shall be taken as the degree of
preference, or the local system shall compute the degree of
preference of the route based on preconfigured policy information. If
the route is learned from a BGP speaker in a neighboring autonomous
system, then the degree of preference shall be computed based on
preconfigured policy information. The exact nature of this policy
information and the computation involved is a local matter. The
local speaker shall then run the internal update process of 9.2.1 to
select and advertise the most preferable route.
9.1.2 Phase 2: Route Selection
The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process which completes when
it has no further work to do. The Phase 2 process shall consider all
routes that are present in the Adj-RIBs-In, including those received
from BGP speakers located in its own autonomous system and those
received from BGP speakers located in neighboring autonomous systems.
The Phase 2 decision function shall be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function shall
lock all Adj-RIBs-In prior to commencing its function, and shall
unlock them on completion.
If the NEXT_HOP attribute of a BGP route depicts an address to which
the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
route SHOULD be excluded from the Phase 2 decision function.
For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker shall identify the route that has:
a) the highest degree of preference of any route to the same set
of destinations, or
b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules
specified in 9.1.2.1.
The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB. The local speaker MUST determine the immediate
next hop to the address depicted by the NEXT_HOP attribute of the
selected route by performing a lookup in the IGP and selecting one of
the possible paths in the IGP. This immediate next hop MUST be used
when installing the selected route in the Loc-RIB. If the route to
the address depicted by the NEXT_HOP attribute changes such that the
immediate next hop changes, route selection should be recalculated as
specified above.
Unfeasible routes shall be removed from the Loc-RIB, and
corresponding unfeasible routes shall then be removed from the Adj-
RIBs-In.
9.1.2.1 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all equally
preferable routes, both those received from BGP speakers located in
neighboring autonomous systems, and those received from other BGP
speakers located in the local speaker's autonomous system.
The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain
the cost of a path (interior distance) to the address depicted by the
NEXT_HOP attribute of the route. Ties shall be broken according to
the following algorithm:
a) If the local system is configured to take into account
MULTI_EXIT_DISC, and the candidate routes differ in their
MULTI_EXIT_DISC attribute, select the route that has the
lowest value of the MULTI_EXIT_DISC attribute.
b) Otherwise, select the route that has the lowest cost
(interior distance) to the entity depicted by the NEXT_HOP
attribute of the route. If there are several routes with the
same cost, then the tie-breaking shall be broken as follows:
- if at least one of the candidate routes was advertised by
the BGP speaker in a neighboring autonomous system, select
the route that was advertised by the BGP speaker in a
neighboring autonomous system whose BGP Identifier has the
lowest value among all other BGP speakers in neighboring
autonomous systems;
- otherwise, select the route that was advertised by the BGP
speaker whose BGP Identifier has the lowest value.
9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur:
a) when routes in a Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP
have changed
c) when a new BGP speaker - BGP speaker connection has been
established
The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function
shall be blocked from running while the Phase 2 decision function is
in process.
All routes in the Loc-RIB shall be processed into a corresponding
entry in the associated Adj-RIBs-Out. Route aggregation and
information reduction techniques (see 9.2.4.1) may optionally be
applied.
For the benefit of future support of inter-AS multicast capabilities,
a BGP speaker that participates in inter-AS multicast routing shall
advertise a route it receives from one of its external peers and if
it installs it in its Loc-RIB, it shall advertise it back to the peer
from which the route was received. For a BGP speaker that does not
participate in inter-AS multicast routing such an advertisement is
optional. When doing such an advertisement, the NEXT_HOP attribute
should be set to the address of the peer. An implementation may also
optimize such an advertisement by truncating information in the
AS_PATH attribute to include only its own AS number and that of the
peer that advertised the route (such truncation requires the ORIGIN
attribute to be set to INCOMPLETE). In addition an implementation is
not required to pass optional or discretionary path attributes with
such an advertisement.
When the updating of the Adj-RIBs-Out and the Forwarding Information
Base (FIB) is complete, the local BGP speaker shall run the external
update process of 9.2.2.
9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching
multiple routes. Since BGP encodes NLRI using IP prefixes, overlap
will always exhibit subset relationships. A route describing a
smaller set of destinations (a longer prefix) is said to be more
specific than a route describing a larger set of destinations (a
shorted prefix); similarly, a route describing a larger set of
destinations (a shorter prefix) is said to be less specific than a
route describing a smaller set of destinations (a longer prefix).
The precedence relationship effectively decomposes less specific
routes into two parts:
- a set of destinations described only by the less specific
route, and
- a set of destinations described by the overlap of the less
specific and the more specific routes
When overlapping routes are present in the same Adj-RIB-In, the more
specific route shall take precedence, in order from more specific to
least specific.
The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of
destinations described by the overlap will still be reachable using
the less specific route.
If a BGP speaker receives overlapping routes, the Decision Process
shall take into account the semantics of the overlapping routes. In
particular, if a BGP speaker accepts the less specific route while
rejecting the more specific route from the same peer, then the
destinations represented by the overlap may not forward along the ASs
listed in the AS_PATH attribute of that route. Therefore, a BGP
speaker has the following choices:
a) Install both the less and the more specific routes
b) Install the more specific route only
c) Install the non-overlapping part of the less specific
route only (that implies de-aggregation)
d) Aggregate the two routes and install the aggregated route
e) Install the less specific route only
f) Install neither route
If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE
attribute to the route. A route that carries ATOMIC_AGGREGATE
attribute can not be de-aggregated. That is, the NLRI of this route
can not be made more specific. Forwarding along such a route does
not guarantee that IP packets will actually traverse only ASs listed
in the AS_PATH attribute of the route. If a BGP speaker chooses a),
it must not advertise the more general route without the more
specific route.
9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE
messages to all peers. For example, it distributes the routes chosen
by the Decision Process to other BGP speakers which may be located in
either the same autonomous system or a neighboring autonomous system.
Rules for information exchange between BGP speakers located in
different autonomous systems are given in 9.2.2; rules for
information exchange between BGP speakers located in the same
autonomous system are given in 9.2.1.
Distribution of routing information between a set of BGP speakers,
all of which are located in the same autonomous system, is referred
to as internal distribution.
9.2.1 Internal Updates
The Internal update process is concerned with the distribution of
routing information to BGP speakers located in the local speaker's
autonomous system.
When a BGP speaker receives an UPDATE message from another BGP
speaker located in its own autonomous system, the receiving BGP
speaker shall not re-distribute the routing information contained in
that UPDATE message to other BGP speakers located in its own
autonomous system.
When a BGP speaker receives a new route from a BGP speaker in a
neighboring autonomous system, it shall advertise that route to all
other BGP speakers in its autonomous system by means of an UPDATE
message if any of the following conditions occur:
1) the degree of preference assigned to the newly received route
by the local BGP speaker is higher than the degree of preference
that the local speaker has assigned to other routes that have been
received from BGP speakers in neighboring autonomous systems, or
2) there are no other routes that have been received from BGP
speakers in neighboring autonomous systems, or
3) the newly received route is selected as a result of breaking a
tie between several routes which have the highest degree of
preference, and the same destination (the tie-breaking procedure
is specified in 9.2.1.1).
When a BGP speaker receives an UPDATE message with a non-empty
WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
routes whose destinations was carried in this field (as IP prefixes).
The speaker shall take the following additional steps:
1) if the corresponding feasible route had not been previously
advertised, then no further action is necessary
2) if the corresponding feasible route had been previously
advertised, then:
i) if a new route is selected for advertisement that has the
same Network Layer Reachability Information as the unfeasible
routes, then the local BGP speaker shall advertise the
replacement route
ii) if a replacement route is not available for advertisement,
then the BGP speaker shall include the destinations of the
unfeasible route (in form of IP prefixes) in the WITHDRAWN
ROUTES field of an UPDATE message, and shall send this message
to each peer to whom it had previously advertised the
corresponding feasible route.
All feasible routes which are advertised shall be placed in the
appropriate Adj-RIBs-Out, and all unfeasible routes which are
advertised shall be removed from the Adj-RIBs-Out.
9.2.1.1 Breaking Ties (Internal Updates)
If a local BGP speaker has connections to several BGP speakers in
neighboring autonomous systems, there will be multiple Adj-RIBs-In
associated with these peers. These Adj-RIBs-In might contain several
equally preferable routes to the same destination, all of which were
advertised by BGP speakers located in neighboring autonomous systems.
The local BGP speaker shall select one of these routes according to
the following rules:
a) If the candidate route differ only in their NEXT_HOP and
MULTI_EXIT_DISC attributes, and the local system is configured to
take into account MULTI_EXIT_DISC attribute, select the routes
that has the lowest value of the MULTI_EXIT_DISC attribute.
b) If the local system can ascertain the cost of a path to the
entity depicted by the NEXT_HOP attribute of the candidate route,
select the route with the lowest cost.
c) In all other cases, select the route that was advertised by the
BGP speaker whose BGP Identifier has the lowest value.
9.2.2 External Updates
The external update process is concerned with the distribution of
routing information to BGP speakers located in neighboring autonomous
systems. As part of Phase 3 route selection process, the BGP speaker
has updated its Adj-RIBs-Out and its Forwarding Table. All newly
installed routes and all newly unfeasible routes for which there is
no replacement route shall be advertised to BGP speakers located in
neighboring autonomous systems by means of UPDATE message.
Any routes in the Loc-RIB marked as unfeasible shall be removed.
Changes to the reachable destinations within its own autonomous
system shall also be advertised in an UPDATE message.
9.2.3 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE
messages.
9.2.3.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement of routes to a
particular destination from a single BGP speaker. This rate limiting
procedure applies on a per-destination basis, although the value of
MinRouteAdvertisementInterval is set on a per BGP peer basis.
Two UPDATE messages sent from a single BGP speaker that advertise
feasible routes to some common set of destinations received from BGP
speakers in neighboring autonomous systems must be separated by at
least MinRouteAdvertisementInterval. Clearly, this can only be
achieved precisely by keeping a separate timer for each common set of
destinations. This would be unwarranted overhead. Any technique which
ensures that the interval between two UPDATE messages sent from a
single BGP speaker that advertise feasible routes to some common set
of destinations received from BGP speakers in neighboring autonomous
systems will be at least MinRouteAdvertisementInterval, and will also
ensure a constant upper bound on the interval is acceptable.
Since fast convergence is needed within an autonomous system, this
procedure does not apply for routes receives from other BGP speakers
in the same autonomous system. To avoid long-lived black holes, the
procedure does not apply to the explicit withdrawal of unfeasible
routes (that is, routes whose destinations (expressed as IP prefixes)
are listed in the WITHDRAWN ROUTES field of an UPDATE message).
This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected shall be advertised at the end of
MinRouteAdvertisementInterval.
9.2.3.2 Frequency of Route Origination
The parameter MinASOriginationInterval determines the minimum amount
of time that must elapse between successive advertisements of UPDATE
messages that report changes within the advertising BGP speaker's own
autonomous systems.
9.2.3.3 Jitter
To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, Keepalive, and
MinRouteAdvertisementInterval. A given BGP speaker shall apply the
same jitter to each of these quantities regardless of the
destinations to which the updates are being sent; that is, jitter
will not be applied on a "per peer" basis.
The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random
factor which is uniformly distributed in the range from 0.75 to 1.0.
9.2.4 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this
information in an efficient manner.
9.2.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following
methods:
a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address
prefixes. In cases where there is a correspondence between the
address structure and the systems under control of an autonomous
system administrator, it will be possible to reduce the size of
the NLRI carried in the UPDATE messages.
b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or
unordered AS_SETs. AS_SETs are used in the route aggregation
algorithm described in 9.2.4.2. They reduce the size of the
AS_PATH information by listing each AS number only once,
regardless of how many times it may have appeared in multiple
AS_PATHs that were aggregated.
An AS_SET implies that the destinations listed in the NLRI can be
reached through paths that traverse at least some of the
constituent autonomous systems. AS_SETs provide sufficient
information to avoid routing information looping; however their
use may prune potentially feasible paths, since such paths are no
longer listed individually as in the form of AS_SEQUENCEs. In
practice this is not likely to be a problem, since once an IP
packet arrives at the edge of a group of autonomous systems, the
BGP speaker at that point is likely to have more detailed path
information and can distinguish individual paths to destinations.
9.2.4.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of
several different routes in such a way that a single route can be
advertised. Aggregation can occur as part of the decision process
to reduce the amount of routing information that will be placed in
the Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information.
Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical:
MULTI_EXIT_DISC, NEXT_HOP.
Path attributes that have different type codes can not be aggregated
together. Path of the same type code may be aggregated, according to
the following rules:
ORIGIN attribute: If at least one route among routes that are
aggregated has ORIGIN with the value INCOMPLETE, then the
aggregated route must have the ORIGIN attribute with the value
INCOMPLETE. Otherwise, if at least one route among routes that are
aggregated has ORIGIN with the value EGP, then the aggregated
route must have the origin attribute with the value EGP. In all
other case the value of the ORIGIN attribute of the aggregated
route is INTERNAL.
AS_PATH attribute: If routes to be aggregated have identical
AS_PATH attributes, then the aggregated route has the same AS_PATH
attribute as each individual route.
For the purpose of aggregating AS_PATH attributes we model each AS
within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to
(e.g., AS_SEQUENCE, AS_SET), and "value" is the AS number. If the
routes to be aggregated have different AS_PATH attributes, then
the aggregated AS_PATH attribute shall satisfy all of the
following conditions:
- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of routes
to be aggregated.
- all tuples of the type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set (they
may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of the type AS_SEQUENCE in the aggregated
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains Y,
regardless of the type of Y.
- No tuple with the same value shall appear more than once in
the aggregated AS_PATH, regardless of the tuple's type.
An implementation may choose any algorithm which conforms to these
rules. At a minimum a conformant implementation shall be able to
perform the following algorithm that meets all of the above
conditions:
- determine the longest leading sequence of tuples (as defined
above) common to all the AS_PATH attributes of the routes to be
aggregated. Make this sequence the leading sequence of the
aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and append
them to the aggregated AS_PATH attribute.
- if the aggregated AS_PATH has more than one tuple with the
same value (regardless of tuple's type), eliminate all, but one
such tuple by deleting tuples of the type AS_SET from the
aggregated AS_PATH attribute.
Appendix 6, section 6.8 presents another algorithm that satisfies
the conditions and allows for more complex policy configurations.
ATOMIC_AGGREGATE: If at least one of the routes to be aggregated
has ATOMIC_AGGREGATE path attribute, then the aggregated route
shall have this attribute as well.
AGGREGATOR: All AGGREGATOR attributes of all routes to be
aggregated should be ignored.
9.3 Route Selection Criteria
Generally speaking, additional rules for comparing routes among
several alternatives are outside the scope of this document. There
are two exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than
any other route. If such a route were ever used, a routing loop
would result.
- In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid
spontaneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require
experience, but the principle is clear.
9.4 Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing
information acquired by some other means (e.g., via an IGP) into BGP.
A BGP speaker that originates BGP routes shall assign the degree of
preference to these routes by passing them through the Decision
Process (see Section 9.1). These routes may also be distributed to
other BGP speakers within the local AS as part of the Internal update
process (see Section 9.2.1). The decision whether to distribute non-
BGP acquired routes within an AS via BGP or not depends on the
environment within the AS (e.g., type of IGP) and should be
controlled via configuration.
Appendix 1. BGP FSM State Transitions and Actions.
This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states
and events when the negotiated Hold Time value is non-zero.
BGP States:
1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established
BGP Events:
1 - BGP Start
2 - BGP Stop
3 - BGP Transport connection open
4 - BGP Transport connection closed
5 - BGP Transport connection open failed
6 - BGP Transport fatal error
7 - ConnectRetry timer expired
8 - Hold Timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION message
The following table describes the state transitions of the BGP FSM
and the actions triggered by these transitions.
Event Actions Message Sent Next State
--------------------------------------------------------------------
Idle (1)
1 Initialize resources none 2
Start ConnectRetry timer
Initiate a transport connection
others none none 1
Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources
OpenConfirm (5)
1 none none 5
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 5
11 Complete initialization none 6
Restart Hold Timer
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Hold Timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
---------------------------------------------------------------------
The following is a condensed version of the above state transition
table.
Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6)
|---------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
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