VPN Prefix Outbound Route Filter (VPN Prefix ORF) for BGP-4

VPN Prefix Outbound Route Filter (VPN Prefix ORF) for BGP-4 China Telecom

Beiqijia Town, Changping District Beijing Beijing 102209 China weiwang94@foxmail.com

China Telecom

Beiqijia Town, Changping District Beijing Beijing 102209 China wangaj3@chinatelecom.cn

Huawei Technologies

Huawei Building, No.156 Beiqing Rd. Beijing Beijing 100095 China rainsword.wang@huawei.com

Verizon Inc.

13101 Columbia Pike Silver Spring MD 20904 United States of America gyan.s.mishra@verizon.com

Huawei Technologies

Huawei Building, No.156 Beiqing Rd. Beijing Beijing 100095 China jie.dong@huawei.com

RTG Area IDR Working Group RFC This draft defines a new type of Outbound Route Filter (ORF), known as the VPN Prefix ORF. The VPN Prefix ORF mechanism is applicable when VPN routes from different VRFs are exchanged through a single shared BGP session.

BGP Maximum Prefix feature is often used at the network boundary to control the number of prefixes to be injected into the network. But for some scenarios when the VPN routes from several VRFs are advertised via one shared BGP session, there is lack of appropriate methods to control the flooding of VPN routes within one VRF to avoid overwhelming the process of VPN routes in other VRFs, which consequently affects the route processing performance of other normal VRFs (such as route dropping, processing delays, and abnormal customer services). That is to say, the excessive VPN routes advertisement should be controlled individually for each VRF in such shared BGP session. There are several solutions can be used to alleviate this problem: Route Target Constraint (RTC) as defined in Address Prefix ORF as defined in CP-ORF mechanism as defined in PE-CE edge peer Maximum Prefix Configure the Maximum Prefix for each VRF on edge nodes However, there are limitations to existing solutions: 1) Route Target Constraint RTC can only filter the VPN routes from any uninterested VRFs, if the “offending routes (prefixes)” come from an interested VRF, RTC mechanism can't filter them. 2) Address Prefix ORF Using Address Prefix ORF to filter VPN routes requires a pre-configuration, but it is impossible to know in advance which prefix may exceed the predefined threshold. 3) CP-ORF Mechanism defines the Covering Prefixes ORF (CP-ORF). A BGP speaker sends a CP-ORF to a peer in order to pull routes that cover a specified host address. A prefix covers a host address if it can be used to forward traffic towards that host address. CP-ORF is applicable in Virtual Hub-and-Spoke VPN and also the BGP/MPLS Ethernet VPN (EVPN) networks, but its main aim is get any interested VPN prefixes and can't be used to filter the overwhelmed VPN prefixes dynamically. 4) PE-CE edge peer Maximum Prefix The BGP Maximum-Prefix feature is used to control how many prefixes can be received from a neighbor. By default, this feature allows a router to bring down a peer when the number of received prefixes from that peer exceeds the configured Maximum-Prefix limit. This feature is commonly used for external BGP peers. If it is applied to internal BGP peers, for example the VPN scenarios, all the VPN routes from different VRFs will share the common fate. If the number of VPN routes of a certain VPN exceeds the configured Maximum-Prefix limit, the BGP session will be shut down, which will effect the operation of other VPN routes transmitted via this BGP session. 5) Configure the Maximum Prefix for each VRF on edge nodes When a VRF overflows, it stops the import of routes. Any additional VPN routes are held into its RIB. However, PEs still need to parse the incoming BGP. This will cost CPU cycles and further burden the overflowed PE. This draft defines a new type of Outbound Route Filter (ORF), called the VPN Prefix ORF. This ORF mechanism is event-driven and does not require pre-configuration. When the number of VPN routes in a VRF exceeds the prefix limit, the router will identify the VPN prefix (RD, RT, source PE, etc.) of the offending VPN routes in this VRF and send a VPN Prefix ORF message to its BGP peer, who announced these offending routes. The BGP speaker upon receiving a VPN Prefix ORF entry from its BGP peer will filter and withdraw any offending VPN routes that was announced to its peer. The purpose of this mechanism is to control the outage within the minimum range and avoid route churn effects when a VRF on a device in the network overflows. VPN Prefix ORF is applicable when the VPN routes from different VRFs are exchanged via one shared BGP session.

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 .

The following terms are used in this draft: RD: Route Distinguisher, defined in ORF: Outbound Route Filter, defined in AFI: Address Family Identifier, defined in SAFI: Subsequent Address Family Identifier, defined in EVPN: BGP/MPLS Ethernet VPN, defined in RR: Route Reflector, provides a simple solution to the problem of IBGP full mesh connection in large-scale IBGP implementation . VRF: Virtual Routing Forwarding, a virtual routing table based on VPN instance.

The operation of VPN Prefix ORF mechanism on each device is independent, each of them makes a local judgment to determine whether it needs to send a VPN Prefix ORF message to its upstream peer. Operators can configure the algorithms in the devices according to their own circumstances. This section describes the procedures for the receiving BGP peer to receive VPN route information from the sending BGP peer. The VPN information includes the updated VPN routes and their corresponding VPN instance identification information. Based on the VPN instance identification information, the receiving BGP peer determines the newly added VPN routes. It then checks whether the number of the newly added VPN routes has caused the number of total VPN routes to exceed the maximum route limit for the associated VPN instance. If the route limit of the VPN instance, which is identified by the VPN instance identification information, is reached or is exceeded, it will send a VPN Prefix ORF message to the sending BGP peer, indicating that the sending BGP peer stop sending the corresponding VPN routes which are identified by the VPN instance identification information. Before sending a VPN Prefix ORF entry, A sender SHOULD send a "default" entry to the VPN Prefix ORF receiver, to allow other allowed VPN prefixes pass the filter. The "default" entry should be installed in advance in the VPN Pefixes ORF table, with the offending VPN routes process method equal to 0, sequence equal to 0xFFFFFFFF, length is equal to 8, and Route Distinguisher is equal to 0. The receiving BGP peer and the sending BGP peer are iBGP peers within the same AS. The VPN instance identification information is RD and the instruction information is sent using ORF in the ROUTE-REFRESH message. The instruction information that sends from the receiving BGP peer includes the followings information: The ORF entries that are included in the route-refresh message. Set the Action field in the ORF entries to the value that instructs adding the corresponding filter condition to the outbound route filter of the sending BGP peer. Set the Match field in the ORF entries to the value that instructs denying the VPN routes updates that match the corresponding ORF entries. The RD value that identifies the above mentioned VPN instance is added to the type-specific part of the ORF entries. When multiple VRFs on a PE is receiving VPN routes with a specific RD, if one VRF exceeds its limit upon receiving routes with that RD, then the PE sends a VPN Prefix ORF message, which will prevent other VRFs that have not exceeded their limits from receiving VPN routes containing that RD, thereby avoiding any communication disruptions between these VRFs and the rejected VPN routes. In order to more finely control VPN routing, when not all VRFs on a PE that are interested in VPN routes with a specific RD exceed the limit, the PE MUST not send a VPN Prefix ORF entry. When the VPN Prefix ORF mechanism is triggered, the device SHOULD send an alarm information to network operators. The detail procedures for different scenarios are described below:

For intra-AS VPN deployment, there are three scenarios: RD is allocated per VPN per PE, each VRF only import one RT (see ). RD is allocated per VPN per PE. Multiple RTs are associated with such VPN routes, and are imported into different VRFs in other devices(see ). RD is allocated per VPN, each VRF imports one/multiple RTs(see ). The following sections will describe solutions to the above scenarios in detail.

In this scenario, PE1-PE4 and RR are iBGP peers. RD is allocated per VPN per PE. The offending VPN routes only carry one RT. We assume that the network topology is shown in Figure 1.

When PE3 sends an excessive number of VPN routes with RT1, and both PE1 and PE2 import VPN routes with RT1, the process of offending VPN routes will influence performance of VRFs on PEs. PEs and RR should have appropriate mechanisms to identify and control the advertising of offending VPN routes. a) PE1 If quota value is not set on PE1, and each VRF has a prefix limit on PE1. When the PE1 receives VPN routes from its BGP peer, it does the following:

NOTE: When the prefix limit for VPN1 VRF is exceeded, there are no other VRFs on PE1 that need the VPN routes with RT1. PE1 sends a VPN Prefix ORF message to the RR and a warning message to the operator. If each <RD31, source PE3> tuple imported into a VRF has a quota, and each VRF has a prefix limit. When the PE1 receives VPN routes from its BGP peer, it does the following:

tuple exceed the quota) { S02. If (the prefix limit of VPN1 VRF is not exceeded) { S03. PE1 sends a warning message to the operator, and the VPN Prefix ORF mechanism should not be triggered. S04. } else { S05. PE1 generates a BGP ROUTE-REFRESH message containing a VPN Prefix ORF entry with (RD31, source PE is PE3, RT is RT1), and send the entry to RR. RR handles the offending VPN routes according to the value of "Offending VPN routes process method". S06. } S07. } ]]> b) PE2 If quota value is not set on PE2, and each VRF has a prefix limit on PE2. When the PE2 receives VPN routes from its BGP peer, it does the following:

NOTE: PE2 cannot directly trigger the VPN Prefix ORF mechanism when the prefix limit of VPN1 VRF is exceeded, because VPN2 VRF requires the VPN routes with RT1. PE2 triggers the mechanism only when the prefix limits for both the VPN1 and VPN2 VRFs have been exceeded. If each <RD31, source PE3> tuple imported into a VRF has a quota, and each VRF has a prefix limit. When the PE2 receives VPN routes from its BGP peer, it does the following:

tuple exceed the quota) { S02. If (the prefix limit of VPN1 VRF is not exceeded) { S03. PE2 sends a warning message to the operator, and the VPN Prefix ORF mechanism should not be triggered. S04. } else { S05. If (the prefix limit of VPN2 VRF is not exceeded) { S06. PE2 should not trigger the VPN Prefix ORF mechanism, and only performs VPN route filtering for the target VPN1 VRF, stopping the import of VPN routes with . S07. } else { S08. PE2 generates a BGP ROUTE-REFRESH message containing a VPN Prefix ORF entry with (RD31, source PE is PE3, RTs are RT1 and RT2), and send the entry to RR. RR handles the offending VPN routes according to the value of "Offending VPN routes process method". S09. } S10. } S11. } ]]>

In this scenario, PE1-PE4 and RR are iBGP peers. RDs are allocated per VPN per PE. Multiple RTs are associated with the offending VPN routes and are imported into different VRFs on other devices. We assume the network topology is depicted in Figure 2.

When PE3 sends an excessive number of VPN routes with RT1 and RT2, while both PE1 and PE2 import VPN routes with RT1, and PE1 also imports VPN routes with RT2. a) PE1 If quota value is not set on PE1, and each VRF has a prefix limit on PE1. Since VPN2 VRF requires the VPN routes with RT1, PE1 cannot directly trigger VPN Prefix ORF mechanism when the prefix limit of VPN1 VRF is exceeded. This case is similar to PE2 without quota in , which is modified as follows:

If each <RD31, source PE3> tuple imported into a VRF has a quota, and each VRF has a prefix limit. This case is similar to PE2 with quota in , which is modified as follows:

b) PE2 If quota value is not set on PE2, and each VRF has a prefix limit on PE2. Since only VPN1 VRF needs to import VPN routes with RT1, this case is similar to PE1 without quota in , which is modified as follows:

If each <RD31, source PE3> tuple imported into a VRF has a quota, and each VRF has a prefix limit. This case is similar to PE1 with quota in , which is modified as follows:

In this scenario, PE1-PE4 and RR are iBGP peers. RD is allocated per VPN. One/Multiple RTs are associated with the offending VPN routes and are imported into different VRFs on other devices. We assume the network topology is shown in Figure 3.

When PE3 sends an excessive number of VPN routes associated with RD1, RT1 and RT2, and both PE1 and PE2 import VPN routes with RT1, the process of offending VPN routes can affect the performance of the VRFs on PEs. a) PE1 If quota value is not set on PE1, and each VRF has a prefix limit on PE1. Since VPN2 VRF requires the VPN routes with RT2, PE1 cannot trigger VPN Prefix ORF mechanism directly when the prefix limit of VPN1 VRF is exceeded. This case is similar to PE2 without quota in , which is modified as follows:

If each <RD1, source PE3> tuple imported into a VRF has a quota, and each VRF has a prefix limit. This case is similar to PE2 with quota in , which is modified as follows:

If each <RD31, source PE3> tuple imported into a VRF has a quota, and each VRF has a prefix limit. This case is similar to PE1 with quota in , which is modified as follows:

We usually use next hop to identify the source, but it may not be useful in the following scenarios: a PE may have multiple addresses so that its BGP peer may receive several different next hop addresses from the same source. In Option B inter-domain scenario, the ASBR will change the next hop. ORIGINATOR_ID is a non-transitive attribute generated by RR to identify the source, but ORIGINATOR_ID cannot be advertised outside the local AS. To address the above scenarios, we have defined a new Extended Community: Source PE Extended Community (SPE EC), which is designed to transmit the identifier of source. The value of SPE EC can be set by source PE, RR or ASBR. Once set and attached to the BGP UPDATE message, its value should not be altered along the advertisement path. The AS number of source PE can be conveyed by Source AS Extended Community, as defined in The format of SPE EC is shown as Figure 4.

Where: Type: specifies the type value assigned by IANA, now it is TBD. ORIGINATOR_ID: specifies the identifier of source. Reserved: MUST be zero on transmit. For the RR/ASBR, it should perform as following: Check the existence of the SPE EC. If it exists, does not change it. If SPE EC does not exist, check the existence of ORIGINATOR_ID. If it exists, put it into SPE EC. If ORIGINATOR_ID does not exist, put the router-id of source PE into SPE EC. The SPE EC SHOULD be attached by source PE, or else the RR SHOULD attach it, with the value set as the router-id of source PE. When none of them attach the SPE EC, the ASBR SHOULD attach it when the packet leaves the source AS, with the value set as the ORIGINATOR_ID. This section updates route reflection procedures, which means need to be updated.

In this section, we defined a new ORF type called VPN Prefix Outbound Route Filter (VPN Prefix ORF). The ORF entries are carried in the BGP ROUTE-REFRESH message as defined in . A BGP ROUTE-REFRESH message can carry one or more ORF entries. The ROUTE-REFRESH message which carries ORF entries contains the following fields: AFI (2 octets) SAFI (1 octet) When-to-refresh (1 octet): the value is IMMEDIATE or DEFER ORF Type (1 octet): The type of VPN Prefix ORF is 66. Length of ORF entries (2 octets) A VPN Prefix ORF entry contains a common part and type-specific part. The common part is encoded as follows: Action (2 bits): the value is ADD, REMOVE or REMOVE-ALL Match (1 bit): the value is PERMIT or DENY Offending VPN routes process method (1 bit): if the value is set to 0, it means all offending VPN routes on the sender of VPN Prefix ORF message should be withdrawn; if the value is set to 1, it means the sender of VPN Prefix ORF message refuse to receive new offending VPN routes. The default value is 0. Reserved (4 bits) VPN Prefix ORF also contains type-specific part. The encoding of the type-specific part is shown in Figure 5.

Sequence: identifying the order in which VPN Prefix ORF is generated and evaluated. It can uniquely identify a VPN Prefix ORF entry together with AFI/SAFI, ORF-Type, and Route Distinguisher. The sequence numbers SHOULD be discontinuous to facilitate the insertion of new rules at a later stage. Length: identifying the length of this VPN Prefix ORF entry. Route Distinguisher: distinguish the different user routes. The VPN Prefix ORF filters the VPN routes it tends to send based on Route Distinguisher. If RD is equal to 0, it means all VPN prefixes. Optional TLVs: carry the potential additional information to give the extensibility of the VPN Prefix ORF mechanism. Its format is shown in Figure 6. If one or more TLV(s) are not well formed, they should be ignored.

Note that if the Action component of an ORF entry specifies REMOVE-ALL, the ORF entry does not include the type-specific part. When the BGP ROUTE-REFRESH message carries VPN Prefix ORF entries, it must be set as follows: The ORF-Type MUST be set to 66 (VPN Prefix ORF). The AFI MUST be set to IPv4, IPv6, or Layer 2 VPN (L2VPN). If the AFI is set to IPv4 or IPv6, the SAFI MUST be set to MPLS-labeled VPN address. If the AFI is set to L2VPN, the SAFI MUST be set to BGP EVPN. The purpose of VPN Prefix ORF is to block unwanted VPN prefixes, then the "action" of one valid entry should be set to "DENY". In order to allow other allowed VPN prefixes pass the filter, one default, last resort entry should be installed in advance in the VPN Pefixes ORF table, with the RD is set to 0 and the corresponding Sequence are set to 0xFFFFFFFF. According to , if any of the fields of a VPN Prefix ORF entry in the message contains an unrecognized value, the whole specified ORF previously received is removed. A BGP speaker that is willing to receive ORF entries from its peer, or a BGP speaker that would like to send ORF entries to its peer, advertises this to the peer by using the Outbound Route Filtering Capability defined in .

Source PE TLV is defined to identify the source of the VPN routes. For the sender of VPN Prefix ORF, it will check the existence of SPE EC. If it exists, the sender will put it into Source PE TLV. Otherwise, the value of Source PE TLV should be set to next hop address. The Source PE TLV SHOULD only appear once within an individual ORF entry. If one ORF entry contains multiple Source PE TLVs, it SHOULD be ignored. The source PE TLV contains the following types: IPv4 Source PE TLV: Type = 1 (suggested), Length = 4 octets, value = next hop address in IPv4 format. IPv6 Source PE TLV: Type = 2 (suggested), Length = 16 octets, value = next hop address in IPv6 format. Source PE identifier TLV: Type = 3 (suggested), Length = 4 octets, value = the value of ORIGINATOR_ID in Source PE Extended Community.

Source AS TLV is defined to identify the source AS number of source PE. The Source AS TLV SHOULD only appear once within an individual ORF entry. If one ORF entry contains multiple Source AS TLVs, it SHOULD be ignored. The encoding of Source AS TLV is as follow: Type = 4 (suggested), Length = 4 octets, value = the value of Source AS in Source AS Extended Community as defined in .

Route Target TLV is defined to identify the RT of the offending VPN routes. RT and RD can be used together to filter VPN routes when the source VRF contains multiple RTs, and the VPN routes with different RTs may be assigned to different VRFs on the receiver. The Route Target TLV contains the following types: Type = 5 (suggested), Length = 8*n (n is the number of RTs that the offending VPN routes attached) octets, value = the RT of the offending VPN routes. If multiple RTs are included, there MUST be an exact match.

The VPN Prefix ORF is used mainly to block the unwanted BGP updates. When the receiver receives VPN Prefix ORF entry, it should check first whether the "Match" bit is "DENY" or not. If the "Match" bit is "PERMIT", and is the "default" entry (the offending VPN routes process method equal to 0, sequence equal to 0xFFFFFFFF, length is equal to 8, and Route Distinguisher is equal to 0), the entry SHOULD be installed. Otherwise, if the "Match" bit is "PERMIT", the entry should be discarded and a warning should be sent to the operator. The following procedures will only be evaluated when the "Match" bit is "DENY". The receiver of VPN Prefix ORF entries, which may be a RR, ASBR or PE, when receives VPN Prefix ORF entry from its BGP peer, it does the following:

of the received VPN Prefix ORF entry. S02. If (the combination does not already exist in the ORF-Policy table) { S03. The receiver adds the VPN Prefix ORF entry to the ORF-Policy table. S04. } else { S05. If (Action is ADD) { S06. Overwrite the old VPN Prefix ORF entry with the new one. S07. else { Remove the corresponding VPN Prefix ORF entry. S06. } ]]> The filtering conditions for the stored VPN Prefix ORF entries contain the RD and RT of the source PE. If the SPE EC is not attached to the BGP Update message of the VPN prefixes, the receiver should use NEXT_HOP or ORIGINATOR_ID as the orignator of VPN Prefix to match against the VPN Prefix ORF entry. After installing the filter entries for the outbound VPN prefixes, the RR or ASBR does the following before sending VPN routes:

When the VPN Prefix ORF mechanism is triggered, a warning message will be generated and sent to the network operators. Operators should manually configure the network to resume normal operation. Since devices can record the VPN Prefix ORF entries sent by each VRF, operators can identify the entries that are needed to be withdrawn and manually trigger the withdraw process as described in .

Using the scenario in , we demonstrate how to determine each field when the sender generates a VPN Prefix ORF entry. Assuming it is an IPv4 network, after PE1-PE4 and RR have advertised the Outbound Route Filtering Capability, each of PE1-PE4 should send a VPN Prefix ORF entry that means "PERMIT-ALL" as follows: AFI is equal to IPv4 SAFI is equal to MPLS-labeled VPN address When-to-refresh is equal to IMMEDIATE ORF Type is equal to VPN Prefix ORF Length of ORF entries is equal to 22 Action is equal to ADD Match is equal to PERMIT Offending VPN routes process method is equal to 0 Sequence is equal to 0xFFFFFFFF Length is equal to 8 Route Distinguisher is equal to 0 When the VPN Prefix ORF mechanism is triggered on PE1, PE1 generates a VPN Prefix ORF entry contains the following information: AFI is equal to IPv4 SAFI is equal to MPLS-labeled VPN address When-to-refresh is equal to IMMEDIATE ORF Type is equal to VPN Prefix ORF Length of ORF entries is equal to 45 Action is equal to ADD Match is equal to DENY Offending VPN routes process method is equal to 0 Sequence is equal to 1 Length is equal to 31 Route Distinguisher is equal to RD31 Optional TLV: Type is equal to 1 (Source PE TLV) Length is equal to 4 value is equal to PE3's IPv4 address Type is equal to 4 (Source AS TLV) Length is equal to 4 value is equal to PE3's source AS number Type is equal to 5 (Route Target TLV) Length is equal to 8 value is equal to RT1

This draft is experimental to determine whether the proposed mechanism can block the offending routes as expected and whether it could cause potential network failures. The first subsection describes implementation considerations for the mechanism. The second subsection gives a brief description of implementation status. The third subsection provides a short summary of the experimental topology.

Before originating a VPN Prefix ORF message, the device should compare the list of RDs and RTs carried by VPN routes to those are imported by other VRFs on the device. If there is an intersection, the VPN Prefix ORF message MUST NOT be originated. In deployment, the quota value can be set with different granularity, such as by <PE>, <RD, Source AS>, etc. If the quota value is set to (VRF prefix limit/the number of PEs), whenever a new PE access to the network, the quota value should be re-evaluated or adjusted accordingly. To avoid frequent changes to the quota value, the value SHOULD be set based on the following formula: Quota=MIN[(Margins coefficient)*<PE,CE limit>*<Number of PEs within the VPN, includes the possibility expansion in futures>, VRF Prefixes Limit] It should be noted that the above formula is only an example, the operators can use different formulas based on actual needs in management plane.

Currently, H3C has implemented VPN Prefix ORF mechanism related functions as follows: By configuring quota, achieve the use of RD and Source PE to control VPN routing. Generating, transmitting and processing Type 1 and Type 2 Source PE TLV. Using the Offending VPN routes process method to revoke all routes. Besides, we also implemented the following functions based on the open-source BGP implementation (FRR): VPN Prefix ORF mechanism triggered based on VRF limit in intra-domain and inter-domain scenarios. RD based VPN routing filtering in intra-domain and inter-domain scenarios.

The experiments will test whether the VPN Prefix ORF blocks the offending routes in the following scenarios: Intra-domain as a standalone mechanism, Inter-domain as a standalone mechanisms, Adding the VPN Prefix ORF to existing mechanisms for intra-domain VPNs, Adding the VPN Prefix ORF to existing mechanisms for intra-domain VPNs.

This draft does build upon . A BGP speaker will maintain the VPN Prefix ORF entries in an ORF-Policy table, this behavior consumes its memory and compute resources. To avoid the excessive consumption of resources, specifies that a BGP speaker can only accept ORF entries transmitted by its interested peers.

This document defines a new Outbound Route Filter type - VPN Prefix Outbound Route Filter (VPN Prefix ORF). We would want to refer to the text from : This new ORF is exchanged using outbound route filtering capability defined in (for the sake of completeness).

This document also define a VPN Prefix ORF TLV type under "Border Gateway Protocol (BGP) Parameters", four TLV types are defined:

This document also requests a new Transitive Extended Community Type. The new Transitive Extended Community Type name shall be "Source PE Extended Community".

Shunwan Zhuang Huawei Technologies Huawei Building, No.156 Beiqing Rd. Beijing Beijing, 100095 China

Thanks Jeffrey Haas, Robert Raszuk, Jim Uttaro, Jakob Heitz, Jeff Tantsura, Rajiv Asati, John E Drake, Gert Doering, Shuanglong Chen, Enke Chen, Srihari Sangli and Igor Malyushkin for their valuable comments on this draft.

The experimental topology is shown in Figure 6.

This topology can be used to verify as follows: whether the VPN Prefix ORF mechanism could block the offending routes in intra-domain scenario. whether the VPN Prefix ORF mechanism could block the offending routes in inter-domain scenario. whether the VPN Prefix ORF mechanism conflicts with the existing mechanism and cause failure. whether the quota value leads to flapping.