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9. UPDATE Message Handling

  1. UPDATE Message Handling

An UPDATE message may be received only in the Established state. Receiving an UPDATE message in any other state is an error. 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If the UPDATE message contains a non-empty WITHDRAWN ROUTES field, the previously advertised routes, whose destinations (expressed as IP prefixes) are contained in this field, SHALL be removed from the Adj-RIB-In. This BGP speaker SHALL run its Decision Process because the previously advertised route is no longer available for use.

If the UPDATE message contains a feasible route, the Adj-RIB-In will be updated with this route as follows: if the NLRI of the new route is identical to the one the route currently has 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. Otherwise, if the Adj-RIB-In has no route with NLRI identical to the new route, the new route SHALL be placed in the Adj-RIB-In.

Once the BGP speaker updates the Adj-RIB-In, the speaker SHALL run its Decision Process.

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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-RIBs-In. The output of the Decision Process is the set of routes that will be advertised to peers; the selected routes will be stored in the local speaker's Adj-RIBs-Out, according to policy.

The BGP Decision Process described here is conceptual, and does not have to be implemented precisely as described, as long as the implementations support the described functionality and they exhibit the same externally visible behavior.

The selection process is formalized by defining a function that takes the attribute of a given route as an argument and returns either (a) a non-negative integer denoting the degree of preference for the route, or (b) a value denoting that this route is ineligible to be installed in Loc-RIB and will be excluded from the next phase of route selection.

The function that calculates the degree of preference for a given route SHALL NOT use any of the following as its inputs: the existence of other routes, the non-existence of other routes, or the path attributes of other routes. Route selection then consists of the 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 the Adj-RIBs-In, and is responsible for:

  • selection of routes to be used locally by the speaker

  • selection of routes to be advertised to other BGP peers

  • 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 peer.

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 Loc-RIB.

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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, 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 is invoked whenever the local BGP speaker receives, from a peer, an UPDATE message that advertises a new route, a replacement route, or withdrawn routes.

The Phase 1 decision function is a separate process,f which completes when it has no further work to do.

The Phase 1 decision function locks an Adj-RIB-In prior to operating on any route contained within it, and unlocks it after operating on all new or unfeasible routes contained within it.

For each newly received or replacement feasible route, the local BGP speaker determines a degree of preference as follows:

If the route is learned from an internal peer, either the value of the LOCAL_PREF attribute is taken as the degree of preference, or the local system computes the degree of preference of the route based on preconfigured policy information. Note that the latter may result in formation of persistent routing loops.

If the route is learned from an external peer, then the local BGP speaker computes the degree of preference based on preconfigured policy information. If the return value indicates the route is ineligible, the route MAY NOT serve as an input to the next phase of route selection; otherwise, the return value MUST be used as the LOCAL_PREF value in any IBGP readvertisement.

The exact nature of this policy information, and the computation involved, is a local matter.

9.1.2. Phase 2: Route Selection

The Phase 2 decision function is 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 considers all routes that are eligible in the Adj-RIBs-In.

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The Phase 2 decision function is blocked from running while the Phase 3 decision function is in process. The Phase 2 function locks all Adj-RIBs-In prior to commencing its function, and unlocks them on completion.

If the NEXT_HOP attribute of a BGP route depicts an address that is not resolvable, or if it would become unresolvable if the route was installed in the routing table, the BGP route MUST be excluded from the Phase 2 decision function.

If the AS_PATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the Phase 2 decision function. AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path. Operations of a BGP speaker that is configured to accept routes with its own autonomous system number in the AS path are outside the scope of this document.

It is critical that BGP speakers within an AS do not make conflicting decisions regarding route selection that would cause forwarding loops to occur.

For each set of destinations for which a feasible route exists in the Adj-RIBs-In, the local BGP speaker identifies 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 Section 9.1.2.2.

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. When the new BGP route is installed in the Routing Table, care must be taken to ensure that existing routes to the same destination that are now considered invalid are removed from the Routing Table. Whether the new BGP route replaces an existing non-BGP route in the Routing Table depends on the policy configured on the BGP speaker.

The local speaker MUST determine the immediate next-hop address from the NEXT_HOP attribute of the selected route (see Section 5.1.3). If either the immediate next-hop or the IGP cost to the NEXT_HOP (where the NEXT_HOP is resolved through an IGP route) changes, Phase 2 Route Selection MUST be performed again.

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Notice that even though BGP routes do not have to be installed in the Routing Table with the immediate next-hop(s), implementations MUST take care that, before any packets are forwarded along a BGP route, its associated NEXT_HOP address is resolved to the immediate (directly connected) next-hop address, and that this address (or multiple addresses) is finally used for actual packet forwarding.

Unresolvable routes SHALL be removed from the Loc-RIB and the routing table. However, corresponding unresolvable routes SHOULD be kept in the Adj-RIBs-In (in case they become resolvable).

9.1.2.1. Route Resolvability Condition

As indicated in Section 9.1.2, BGP speakers SHOULD exclude unresolvable routes from the Phase 2 decision. This ensures that only valid routes are installed in Loc-RIB and the Routing Table.

The route resolvability condition is defined as follows:

  1. A route Rte1, referencing only the intermediate network address, is considered resolvable if the Routing Table contains at least one resolvable route Rte2 that matches Rte1's intermediate network address and is not recursively resolved (directly or indirectly) through Rte1. If multiple matching routes are available, only the longest matching route SHOULD be considered.

  2. Routes referencing interfaces (with or without intermediate addresses) are considered resolvable if the state of the referenced interface is up and if IP processing is enabled on this interface.

BGP routes do not refer to interfaces, but can be resolved through the routes in the Routing Table that can be of both types (those that specify interfaces or those that do not). IGP routes and routes to directly connected networks are expected to specify the outbound interface. Static routes can specify the outbound interface, the intermediate address, or both.

Note that a BGP route is considered unresolvable in a situation where the BGP speaker's Routing Table contains no route matching the BGP route's NEXT_HOP. Mutually recursive routes (routes resolving each other or themselves) also fail the resolvability check.

It is also important that implementations do not consider feasible routes that would become unresolvable if they were installed in the Routing Table, even if their NEXT_HOPs are resolvable using the current contents of the Routing Table (an example of such routes

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would be mutually recursive routes). This check ensures that a BGP speaker does not install routes in the Routing Table that will be removed and not used by the speaker. Therefore, in addition to local Routing Table stability, this check also improves behavior of the protocol in the network.

Whenever a BGP speaker identifies a route that fails the resolvability check because of mutual recursion, an error message SHOULD be logged.

9.1.2.2. 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 routes with the same degrees of preference, both those received from internal peers, and those received from external peers.

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, and follow the same route selection algorithm.

The tie-breaking algorithm begins by considering all equally preferable routes to the same destination, and then selects routes to be removed from consideration. The algorithm terminates as soon as only one route remains in consideration. The criteria MUST be applied in the order specified.

Several of the criteria are described using pseudo-code. Note that the pseudo-code shown was chosen for clarity, not efficiency. It is not intended to specify any particular implementation. BGP implementations MAY use any algorithm that produces the same results as those described here.

a) Remove from consideration all routes that are not tied for having the smallest number of AS numbers present in their AS_PATH attributes. Note that when counting this number, an AS_SET counts as 1, no matter how many ASes are in the set.

b) Remove from consideration all routes that are not tied for having the lowest Origin number in their Origin attribute.

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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 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.

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d) If at least one of the candidate routes was received via EBGP, remove from consideration all routes that were received via IBGP.

e) Remove from consideration any routes with less-preferred interior cost. The interior cost of a route is determined by calculating the metric to the NEXT_HOP for the route using the Routing Table. If the NEXT_HOP hop for a route is reachable, but no cost can be determined, then this step should be skipped (equivalently, consider all routes to have equal costs).

This is also described in the following procedure.

for m = all routes still under consideration for n = all routes in still under consideration if (cost(n) is lower than cost(m)) remove m from consideration

In the pseudo-code above, cost(n) is a function that returns the cost of the path (interior distance) to the address given in the NEXT_HOP attribute of the route.

f) Remove from consideration all routes other than the route that was advertised by the BGP speaker with the lowest BGP Identifier value.

g) Prefer the route received from the lowest peer address.

9.1.3. Phase 3: Route Dissemination

The Phase 3 decision function is invoked on completion of Phase 2, or when any of the following events occur:

a) when routes in the 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 connection has been established

The Phase 3 function is a separate process that completes when it has no further work to do. The Phase 3 Routing Decision function is blocked from running while the Phase 2 decision function is in process.

All routes in the Loc-RIB are processed into Adj-RIBs-Out according to configured policy. This policy MAY exclude a route in the Loc-RIB from being installed in a particular Adj-RIB-Out. A route SHALL NOT

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be installed in the Adj-Rib-Out unless the destination, and NEXT_HOP described by this route, may be forwarded appropriately by the Routing Table. If a route in Loc-RIB is excluded from a particular Adj-RIB-Out, the previously advertised route in that Adj-RIB-Out MUST be withdrawn from service by means of an UPDATE message (see 9.2).

Route aggregation and information reduction techniques (see Section 9.2.2.1) may optionally be applied.

Any local policy that results in routes being added to an Adj-RIB-Out without also being added to the local BGP speaker's forwarding table is outside the scope of this document.

When the updating of the Adj-RIBs-Out and the Routing Table is complete, the local BGP speaker runs the Update-Send process of 9.2.

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. Because 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 shorter prefix); similarly, a route describing a larger set of destinations is said to be less specific than a route describing a smaller set of destinations.

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

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 MUST consider both routes based on the configured acceptance policy. If both a less and a more specific route are accepted, then the Decision Process MUST install, in Loc-RIB, either both the less and

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the more specific routes or aggregate the two routes and install, in Loc-RIB, the aggregated route, provided that both routes have the same value of the NEXT_HOP attribute.

If a BGP speaker chooses to aggregate, then it SHOULD either include all ASes used to form the aggregate in an AS_SET, or add the ATOMIC_AGGREGATE attribute to the route. This attribute is now primarily informational. With the elimination of IP routing protocols that do not support classless routing, and the elimination of router and host implementations that do not support classless routing, there is no longer a need to de-aggregate. Routes SHOULD NOT be de-aggregated. In particular, a route that carries the ATOMIC_AGGREGATE attribute MUST NOT be de-aggregated. That is, the NLRI of this route cannot be more specific. Forwarding along such a route does not guarantee that IP packets will actually traverse only ASes listed in the AS_PATH attribute of the 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.

When a BGP speaker receives an UPDATE message from an internal peer, the receiving BGP speaker SHALL NOT re-distribute the routing information contained in that UPDATE message to other internal peers (unless the speaker acts as a BGP Route Reflector [RFC2796]).

As part of Phase 3 of the route selection process, the BGP speaker has updated its Adj-RIBs-Out. All newly installed routes and all newly unfeasible routes for which there is no replacement route SHALL be advertised to its peers by means of an UPDATE message.

A BGP speaker SHOULD NOT advertise a given feasible BGP route from its Adj-RIB-Out if it would produce an UPDATE message containing the same BGP route as was previously advertised.

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.

If, due to the limits on the maximum size of an UPDATE message (see Section 4), a single route doesn't fit into the message, the BGP speaker MUST not advertise the route to its peers and MAY choose to log an error locally.

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9.2.1. 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.

9.2.1.1. Frequency of Route Advertisement

The parameter MinRouteAdvertisementIntervalTimer determines the minimum amount of time that must elapse between an advertisement and/or withdrawal of routes to a particular destination by a BGP speaker to a peer. This rate limiting procedure applies on a per- destination basis, although the value of MinRouteAdvertisementIntervalTimer is set on a per BGP peer basis.

Two UPDATE messages sent by a BGP speaker to a peer that advertise feasible routes and/or withdrawal of unfeasible routes to some common set of destinations MUST be separated by at least MinRouteAdvertisementIntervalTimer. This can only be achieved by keeping a separate timer for each common set of destinations. This would be unwarranted overhead. Any technique that ensures that the interval between two UPDATE messages sent from a BGP speaker to a peer that advertise feasible routes and/or withdrawal of unfeasible routes to some common set of destinations will be at least MinRouteAdvertisementIntervalTimer, and will also ensure that a constant upper bound on the interval is acceptable.

Since fast convergence is needed within an autonomous system, either (a) the MinRouteAdvertisementIntervalTimer used for internal peers SHOULD be shorter than the MinRouteAdvertisementIntervalTimer used for external peers, or (b) the procedure describe in this section SHOULD NOT apply to routes sent to internal peers.

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 MinRouteAdvertisementIntervalTimer, the last route selected SHALL be advertised at the end of MinRouteAdvertisementIntervalTimer.

9.2.1.2. Frequency of Route Origination

The parameter MinASOriginationIntervalTimer 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.

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9.2.2. Efficient Organization of Routing Information

Having selected the routing information it will advertise, a BGP speaker may avail itself of several methods to organize this information in an efficient manner.

9.2.2.1. 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 Section 9.2.2.2. 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 because such paths are no longer listed individually in the form of AS_SEQUENCEs. In practice, this is not likely to be a problem because once an IP packet arrives at the edge of a group of autonomous systems, the BGP speaker is likely to have more detailed path information and can distinguish individual paths from destinations.

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9.2.2.2. 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 the same type and to the Network Layer Reachability Information.

Routes that have different MULTI_EXIT_DISC attributes SHALL NOT be aggregated.

If the aggregated route has an AS_SET as the first element in its AS_PATH attribute, then the router that originates the route SHOULD NOT advertise the MULTI_EXIT_DISC attribute with this route.

Path attributes that have different type codes cannot be aggregated together. Path attributes of the same type code may be aggregated, according to the following rules:

NEXT_HOP: When aggregating routes that have different NEXT_HOP attributes, the NEXT_HOP attribute of the aggregated route SHALL identify an interface on the BGP speaker that performs the aggregation.

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 cases,, the value of the ORIGIN attribute of the aggregated route is IGP.

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

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belongs to (e.g., AS_SEQUENCE, AS_SET), and "value" identifies 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 type AS_SEQUENCE in the aggregated AS_PATH SHALL appear in all of the AS_PATHs in the initial set of routes to be aggregated.

  • all tuples of type AS_SET in the aggregated AS_PATH SHALL appear in at least one of the AS_PATHs in the initial set (they may appear as either AS_SET or AS_SEQUENCE types).

  • for any tuple X of 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 of type AS_SET with the same value SHALL appear more than once in the aggregated AS_PATH.

  • Multiple tuples of type AS_SEQUENCE with the same value may appear in the aggregated AS_PATH only when adjacent to another tuple of the same type and value.

An implementation may choose any algorithm that 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.

  • for each pair of adjacent tuples in the aggregated AS_PATH, if both tuples have the same type, merge them together, as long as doing so will not cause a segment with a length greater than 255 to be generated.

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Appendix F, Section F.6 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: Any AGGREGATOR attributes from the routes to be aggregated MUST NOT be included in the aggregated route. The BGP speaker performing the route aggregation MAY attach a new AGGREGATOR attribute (see Section 5.1.7).

9.3. Route Selection Criteria

Generally, 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 (provided that the speaker is configured to accept such routes). If such a route were ever used, a routing loop could result.

  • In order to achieve a successful distributed operation, only routes with a likelihood of stability can be chosen. Thus, an AS SHOULD avoid using unstable routes, and it SHOULD NOT make rapid, spontaneous changes to its choice of route. Quantifying the terms "unstable" and "rapid" (from the previous sentence) will require experience, but the principle is clear. Routes that are unstable can be "penalized" (e.g., by using the procedures described in [RFC2439]).

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 assigns the degree of preference (e.g., according to local configuration) 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 update process (see Section 9.2). The decision of whether to distribute non-BGP acquired routes within an AS via BGP depends on the environment within the AS (e.g., type of IGP) and SHOULD be controlled via configuration.

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