| Internet-Draft | Legacy PE RT Filtering | October 2024 |
| Mohapatra, et al. | Expires 10 April 2025 | [Page] |
This document describes a simple procedure that allows "legacy" BGP speakers to exchange route target membership information in BGP without using mechanisms specified in [RFC4684]. The intention of the proposed technique is to help in partial deployment scenarios and is not meant to replace [RFC4684].¶
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[RFC4684] provides a powerful and general means for BGP speakers to exchange and propagate Route Target reachability information and constrain VPN route distribution to achieve high scale. However, it requires that all the BGP speakers in the network are upgraded to support this functionality. For example, in a network with route reflectors (RR), if one PE client in the cluster doesn't support constrained distribution, the cluster degenerates into storing and processing all the VPN routes. The route reflectors need to request and store all the network routes since they do not receive route target membership information from the legacy PEs. The RR will also generate all those routes to the legacy PEs and the legacy PEs will end up filtering the routes and store the subset of VPN routes that are of interest.¶
This document specifies a mechanism for such legacy PE devices using existing configuration and toolset to provide similar benefits as [RFC4684]. At the same time, it is backward-compatible with the procedures defined in [RFC4684]. It also allows graceful upgrade of the legacy router to be [RFC4684] capable.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].¶
The basic idea is to make use of VPN unicast route exchange from the legacy PEs to a new BGP speaker (e.g. an RR) to signal RT membership. The legacy PEs announce a set of "special" routes with mapped RTs to the RR along with a standard community (defined in this document). The presence of the community triggers the RR to extract the RTs and build RT membership information.¶
The following simple steps are performed on the legacy PE device:¶
Collect the "import route targets" of all the configured customer VRFs. Let's call this set 'IRTS'.¶
Create a special "route-filter VRF" with a route distinguisher(RD) that's configured with the same value across the network for all legacy PE devices. Note: the equivalence of the RD value is for optimization - the operator may choose to use different values.¶
Originate one or more routes in this VRF and attach a subset of 'IRTS' as "translated route-target extended communities" with each route so as to evenly distribute the RTs (and to make sure they can fit into one BGP UPDATE message). Collectively, the union of the "translated route-target extended communities" of all these routes is equal to the set 'IRTS'. The translated RTs are attached as export route-targets for the routes originated in the route-filter VRF.¶
The translation of the IRTs is necessary in order to refrain from importing "route-filter" VRF routes into VPN VRFs that would import the same route-targets. The translation of the IRTS is done as follows. For a given IRT, the equivalent translated RT (TRT) is constructed by means of swapping the value of the high- order octet of the Type field for the IRT (as defined in [RFC4360]).¶
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 | 0x02 | | 0x01 | 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|2B AS | |2B AS => IP(high) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<=>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Local Admin(high) | |Local Admin(high) => IP(low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Local Admin(low) | |Local Admin(low) => Local Admin|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0x02 | | 0x02 | 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IP(high) | |IP(high) => 4B AS(high) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<=>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IP(low) | |IP(low) => 4B AS(low) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Local Admin | |Local Admin => Local Admin |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x02 | 0x02 | | 0x00 | 0x02 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|4B AS(high) | |4B AS(high) => 2B AS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<=>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|4B AS(low) | |4B AS(low) => Local Admin(high)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Local Admin | |Local Admin => Local Admin(low)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
As an example, if IRT R= 65500:12244(hex: 0x0002ffdc00002fd4), equivalent route-filter TRT: 255.220.0.0:12244(hex: 0x0102ffdc00002fd4). One shortcoming of the translation mechanism is a possible collision between IRTs and TRTs if the network has been configured with RTs of multiple higher order octet types (2-byte AS, IP address, and 4-byte AS). It is expected that such a configuration is rare in practice.¶
As an alternative to the translation of the IRTS, the subset of the 'IRTS' can be attached as-is (without swapping the type field as described earlier) as "export route-target extended communities" with each route so as to evenly distribute the RTs (and to make sure they can fit into one BGP UPDATE message). In this case, the IRT subsets can be attached in outbound policy to avoid the route-filter VRFs from being imported into VPN VRFs. Also in this case, the route-filter VRF routes must be tagged with a different special community (from that associated with the translated RTs) as described in Section 4 so that the receiving BGP speaker can distinguish the two cases.¶
The routes are marked with NO_ADVERTISE and NO_EXPORT well-known communities as well as the appropriate new community that's defined in this document Section 4. Note that there is no specific provision made to disallow configuration of subsequent route policies that can potentially alter the set of communities attached to "route-filter" VRF routes. The protocol behavior in such a case is undefined and the use of those policy statements is discouraged.¶
Upon receiving the "route-filter" routes, the BGP speaker does its usual processing to store them in its local RIB. It recognizes them as route-filter routes based on the association of the new standard community as defined in this document. If required (as indicated by the community value), it translates the attached route-target extended communities (TRT) to equivalent import route-targets (IRT). Finally it creates the route-target filter list for each legacy client by collecting the entire set of route targets. From this point onwards, the behavior is similar to that defined in [RFC4684]. The RR does not propagate the routes further because of their association with NO_ADVERTISE community. Also the VPN EoR that is sent by the legacy PE should also be used as an indication that the legacy PE is done sending the route-filter information as per the procedures defined in [RFC4684] for implementing a EoR mechanism to signal the completion of initial RT membership exchange.¶
The RR MAY also translate the received extended communities from legacy clients into route target membership NLRIs as if it had received those NLRIs from the client itself. This is useful for further propagation of the NLRIs to rest of the network to create RT membership flooding graph. When the route_filter routes are received with same RD (from all legacy PE speakers), processing of the paths to generate equivalent NLRIs becomes fairly easy.¶
This memo defines four BGP communities that are attached to BGP UPDATE messages at the legacy PE devices and processed by the route reflectors as defined above. They are as follows:¶
+----------------------------+--------------------------------------+ | Community | Meaning | +----------------------------+--------------------------------------+ | ROUTE_FILTER_v4 | RTs are attached as-is for VPNv4 | | | route filtering | | ... | ... | | ROUTE_FILTER_v6 | RTs are attached as-is for VPNv6 | | | route filtering | | ... | ... | | ROUTE_FILTER_TRANSLATED_v4 | Translated RTs are attached for | | | VPNv4 route filtering | | ... | ... | | ROUTE_FILTER_TRANSLATED_v6 | Translated RTs are attached for | | | VPNv6 route filtering | +----------------------------+--------------------------------------+¶
In the absence of (or lack of support of) AF specific communities (ROUTE_FILTER_v6, ROUTE_FILTER_TRANSLATED_v6), the ROUTE_FILTER_v4 or ROUTE_FILTER_TRANSLATED_v4 MAY be treated by an implementation as a default VPN route-filter community to build a combination VPN filter for all VPN AFs (VPNv4, VPNv6) present on the RR. This is in accordance with the procedures in [RFC4684] to build combination route-filters for VPN AFs and AF specific route-filters defined in [I-D.keyur-bgp-af-specific-rt-constrain]. If this is the case, then subsequent receipt of any "route-filter" routes with AF specific communities (ROUTE_FILTER_v6, ROUTE_FILTER_TRANSLATED_v6) will override the default filters sent with ROUTE_FILTER_v4 or ROUTE_FILTER_TRANSLATED_v4 for the VPNv6 AFI when support for the AF specific communities exists. Suggested values for ROUTE_FILTER_v4 and ROUTE_FILTER_TRANSLATED_v4 are 0xFFFF0002 (65535:2) and 0xFFFF0003 (65535:3) respectively¶
When both the legacy PE and the RR support extended community based Outbound Route Filtering as in [I-D.chen-bgp-ext-community-orf] this may be used as a alternate solution for the legacy PE to signal RT membership information, in order to realize the same benefits as [RFC4684]. Also extended community ORF can be used amongst the RRs in lieu of [RFC4684] to realize similar benefits.¶
Significant contributions were made by Stephane Litkowski, Luis M Tomotaki and James Uttaro which the authors would like to acknowledge.¶
The authors would like to thank Rob Shakir for his review and comments.¶
IANA shall assign new code points from BGP first-come first-serve communities for the four communities as listed in Section 4.¶
There are no additional security risks introduced by this design.¶