| Internet-Draft | Hierarchical RR RT-Constraints | October 2023 |
| Mohanty, et al. | Expires 25 April 2024 | [Page] |
Route Target Constraints (RTC) is used to build a VPN route distribution graph such that routers only receive VPN routes corresponding to specified route-targets (RT) that they are interested in. This is done by exchanging the route-targets as routes in the RTC address-family and a corresponding "RT filter" is installed that influences the VPN route advertisement. In networks employing hierarchical Route Reflectors (RR) the use of RTC can lead to incorrect VPN route distribution and loss in connectivity as detailed in an earlier draft . Two solutions were provided to overcome the problem.¶
This draft presents a method with suggested modifications to the RTC RFC in order to solve the hierarchical RR RTC problem in an efficient manner.¶
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
Hierarchical RR [RFC4456] deployments with VPN [RFC4364] working in conjunction with RTC [RFC4684] may result in sub-optimal and incorrect VPN route distribution that is nicely described in [I-D.ietf-idr-rtc-hierarchical-rr]. The root reason for this is the way the RR rules for RTC are defined in [RFC4684]. The authors of [I-D.ietf-idr-rtc-hierarchical-rr] furnish two solutions for the problem, one based on add-paths and the other based on diverse-paths constructs. In this memo, we present another another solution to the very same problem.¶
When advertising RT membership NLRI to a route-reflector client, Section 3.2 of [RFC4684] advocates the advertising RR to set the ORIGINATOR_ID attribute [RFC4456] to its own router-id, and the Next-hop attribute to be set to the local address for that session. However, this creates the issue in hierarchical RR setups as explained in [I-D.ietf-idr-rtc-hierarchical-rr]. Fig. 1 represents the same Figure as in [I-D.ietf-idr-rtc-hierarchical-rr]. When RR-2 and RR-3 advertise RT-1 to RR-1, the latter will choose one of the routes to be best and will advertise the same to RR-2 and RR-3 respectively after setting the ORIGINATOR_ID and next-hop to itself. Note that RR-1 will also add its own CLUSTER_ID [RFC4456]to the CLUSTER_LIST but importantly not overwrite the CLUSTER_ID of the sender. This leads to the issue explained in [I-D.ietf-idr-rtc-hierarchical-rr].¶
In the Fig 1, when RR-1 chooses the route from RR-2 as the best route, and formats the next-hop and ORIGINATOR_ID as explained above and then advertises the route to RR-2, RR-2 will drop the route reflected from RR-1 because of the CLUSTER_ID check.¶
+---------------------------------+
| +----+ |
| Clu-1 |RR-1| |
| /+----+\ |
| / \ |
| +----+ +----+ |
| Clu-2 |RR-2| |RR-3| Clu-3 |
| +-/--+ +/--\+ |
| / / \ |
| +----+ +----+ +----+ |
| |PE-1| |PE-2| |PE-3| |
| +----+ +----+ +----+ |
| | | | |
+-------|----------|---------|----+
RT-1 | RT-1 | | RT-1
+--------+ +--------+ +--------+
| VPN-1 | | VPN-1 | | VPN-1 |
+--------+ +--------+ +--------+
Figure 1 Hierarchical RR Setup with RTC
RR-2 will therefore not form the outbound filter of RT-1 towards RR-1 which means that after convergence RR-2 will not advertise VPN routes to RR-1 anymore. This leads to an incorrect VPN route distribution across the network.¶
In the scenario of Fig 2. CE-1 is multi-homed to PE-1 and PE-2 and wants to communicate with CE-2 which is behind PE-4. As explained earlier, because RR-1 chooses RR-2 path as best in the RTC family, RR-1 is only receiving the VPN route from RR-3 (and not RR-2) in the steady state.¶
+---------------------------------+
| +----+ +-----+ | +------------+
| Clu-1 |RR-1| ---|PE-4 |- - -| VPN-1 (CE2)|
| /+----+\ +-----+ | +------------+
| / \ |
| +----+ +----+ |
| Clu-2 |RR-2| |RR-3| Clu-3 |
| +-/--+ +--\-+ |
| / \ |
| +----+ +----+ |
| |PE-1| |PE-2| |
| +----+ +--/-+ |
| \ / |
+----------\-----------/----------+
\ RT-1 /
+--------+ ----|
| VPN-1 (CE1) |
--------------|
Figure 2 Hierarchical RR Setup with RTC with dual-homed CE
Notice that even though the link between between RR-3 and RR-1 comes down, The RR-2 PATH still remains as best in the RTC address-family at RR-1 and the VPN route advertisements to RR-1 from RR-2 still continue to be blocked. Thus even though there is an alternative connectivity from CE-1 to PE-4 via PE-1, RR-2 and RR-1, the BGP VPN routes cannot be sent. In fact CE-1 is completely cut-off from rest of the network. Generalizing, it means that in a hierarchical RR with only a single first-level RR as its client, the solution is completely broken. Notice that without RTC, RR-1 would have both VPN paths and the loss of connectivity to RR-3 would just result in local convergence at RR-1 subject to the time when the path from RR-2 becomes best.¶
The solutions presented in [I-D.ietf-idr-rtc-hierarchical-rr] are based on¶
Addpath, RR-1 will advertise both the paths from RR-2 and RR-3 to RR-2 and RR-3 so that each of the first level RRS will accept at least one of the routes and install the filter¶
When RR-1 will advertise the best-path to a client or non-client speaker, and that speaker is the one whose path is the best, the advertising router will use the most "diverse" path (different next-hop and ORIGINATOR_ID than the best-path) to accomplish the same goal, i.e. the path will be accepted at the receiving speaker¶
In the next section, we provide a solution, that does not require add-path and also improves upon [RFC4684] while solving this hierarchical RR issue in RTC.¶
A problem that [RFC4684] does not address is limiting the number for VPN routes advertised to AN RR when only one client advertises RTC routes. Consider in Figure 1 that PE-1 is the only one advertising a RTC route to RR-2. RR-2 will advertise back the route towards PE-1 (with next-hop/ORIGINATOR_ID rewriting to avoid PE-1 discarding the route). PE-1 will advertise VPN routes towards RR-1, which is unnecessary and wastes resources on RR-1.¶
Receiver Acceptance Rule need to change. We need to take care that when reflecting back a RTC route to the advertising client, this client does not discard the update.¶
The following two solutions are proposed, one for the sender of RTC routes, one for the receiver of RTC route. Only one need to be implemented. Implementing both, one at the receiver and one at the sender, allows easier interoperability with non-compliant implementations. If sender option is implemented, it will have preference over receiver option.¶
This rule is to be used by the sender of RTC routes.¶
When reflecting a RTC route from RR client to RR client, some attributes of the route may be overwritten. This overwrite is particular for RTC address family and will not happen for other Afs. It allows propagation of RTC routes back to its originators (needed for RTC filtering information to be useful), but it makes the propagation more prone to loops. Thus, RTC is expected to be used only in standard route-reflectors topology where loops are not possible.¶
When reflecting the (best-path) RTC route from RR client to RR client, the following rules will apply:¶
When RTC route has CLUSTER_LIST, overwrite all CLUSTER_ID of CLUSTER_LIST to local CLUSTER_ID. Note that when advertising that RTC route, the local CLUSTER_LIST will still be prepending per usual rules.¶
ORIGINATOR_ID is set or overwritten with local router-id.¶
NEXT_HOP is overwritten with local peering address (next-hop-self).¶
A RTC route will be always advertised to the client we received it from.¶
Note that the rules above only exposes RTC routes to routing loops (by overwriting attributes) in the client to client top to down direction (i.e. from client to client). Thus, this draft restricts RFC4684 into disallowing attribute overwrite into non-client to client direction.¶
RFC4684 is not explicit about it, but the underlying assumption is that a route received from a route-reflector-client MUST be reflected back to that client. Hereby, this is made explicit.¶
RFC4684 mandates to overwrite next-hop to self and the ORIGINATOR_ID to our local router-id when advertising RTC route a route reflector client (section 3.2, rule (i) ). The proposed rules are an extension of this concept to take care of the case where the reflection happens at a higher level RR.¶
The rule above will apply as well to a top level RR, guaranteeing that top level RR does not have unnecessary routes.¶
This rule is to be used by the receiver of RTC routes.¶
When receiving a RTC route, the following rules will apply:¶
CLUSTER_ID, ORIGINATOR_ID and NEXT_HOP checks will be considered, but instead of discarding the routes, the route will be kept in Adj-RIB-IN as a Received-only route.¶
A route in Received-only state will not be considered for best -path nor advertised to any peer¶
A route in Received-only state will be considered to install a VPN filter.¶
The rules above apply also to just one level of RR, and it’s a solution not contemplated in RFC4684.¶
The rules above will allow propagation of RTC routes in a different way than using the sender option rules (with sender option, non-client to client propagation will not be stopped). But the creation of VPN filters will be the same in a standard RR topology.¶
With the procedures it is not necessary for the RR to know in which level it is operating. The above rules are compatible. We always advertise best-path for any rule and it is easily seen that RR-2 will accept the RT Constraint path advertised from RR-1 . Since the path is accepted, the RT Filter at RR-2 will pass the VPN routes, and the problem scenarios are resolved accordingly.¶
With this specification in the RT-Constraint address-family, we solve both the incorrect and sub-optimal issues as mentioned above. There is no need for add-paths. We can also optimize over [RFC4684] on RTC advertisements based on diversity of ORIGINATOR_ID and CLUSTER_ID so that a higher level RR does not have to be populated with VPN routes with a specific RT if that RT is not present in other clusters.¶
None.¶
This document raises no new security issues for RT Constraints.¶
The authors would like to thank Swadesh Agrawal and M. Mirza for useful discussions related to hierarchical RR RTC.¶