| Internet-Draft | EVPN Fast Reroute | March 2022 |
| Burdet, et al. | Expires 8 September 2022 | [Page] |
This document summarises EVPN convergence mechanisms and specifies procedures for EVPN networks to achieve sub‑second and scale‑independant convergence.¶
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EVPN convergence and failure recovery methods from different types of network failures is described in Section 17 of [RFC7432]. Similarly for EVPN‑VPWS, the end of Section 5 of [RFC8214] briefly evokes an egress link protection mechanism.¶
The fundamentals of EVPN convergence rely on a mass‑withdraw technique of the Ethernet A-D per ES route to unresolve all the associated forwarding paths (Section 9.2.2 of [RFC7432] 'Route Resolution'). The mass‑withdraw grouping approach results in suitable EVPN convergence at lower scale, but is not sufficent to meet stricter sub-second requirements. Other control-plane enhancements such as route‑prioritisation ([I-D.ietf-bess-rfc7432bis]) help further but still provide no guarantees.¶
EVPN convergence using only control-plane approaches is constrained by BGP route propagation delays, routes processing times in software and hardware programming. These are additionally often performed sequentially and linearly given the potential large scale of EVPN routes present in control plane.¶
This document presents a mechanism for fast reroute to minimise packet loss in the case of a link failure using EVPN redirect labels (ERLs) with special forwarding attributes. Multiple-failures where loops may occur are addressed, as are cascading failures. A mechanism for distributing redirect labels (ERLs) alongside EVPN service labels (ESLs) is shown.¶
The main objective is to achieve sub-second convergence in EVPN networks without relying on control plane actions. The procedures in this document apply equally to EVPN services (EVPN [RFC7432], EVPN-VPWS [RFC8214] and EVPN-IRB [RFC9135]), and all Ethernet-Segment load-balancing modes.¶
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].¶
Some of the terminology in this document is borrowed from [RFC8679] for consistency across fast reroute frameworks.¶
Sub-second convergence in EVPN networks is achieved using a combined approach to minimising traffic loss:¶
The solution presented in this document addresses the local failure detection and restoration, without impeding on or impacting existing EVPN control plane convergence mechanisms.¶
Consider the following EVPN topology where PE1 and PE2 are multihoming PEs on a shared ES, ESI1. EVPN (known unicast) or EVPN‑VPWS traffic from CE1 to CE2 is sent to PE1 and PE2 using EVPN service labels ESL1 and/or ESL2 (depending on load-balancing mode of the ESI1 interfaces).¶
+------+
| PE1 |
| |
+-------+ | ESL1---BDF--X
| |--------| | \
| | | ERL1--------> \
+-----+ | | +------+ \
| | |IP/MPLS| \
CE1 ---| PE3 |----|Core | ESI1 === CE2
| | |Network| /
+-----+ | | +------+ /
| | | ERL2--------> /
| |--------| | /
+-------+ | ESL2---DF----
| |
| PE2 |
+------+
EVPN Multihoming with service and redirect labels¶
Alongside the service labels ESL1 and ESL2, two redirect labels ERL1 and ERL2 are allocated with special forwarding attributes, as detailed in Section 6. Fast-reroute and use of the ERLs is shown in Section 5.2¶
EVPN DF-Election lends itself well to the selection of a pre-computed path amongst any given number of peering PEs by providing a DF‑Elected and BDF‑Elected node at the <EVI, ESI> granularity ([RFC8584] and [I-D.ietf-bess-rfc7432bis]).¶
In All-active mode, all PEs in the Ethernet Segment are actively forwarding known unicast traffic to the CE. In Single-active mode, only a single PE in the Ethernet Segment is actively forwarding known unicast traffic to the CE: the DF-Elected PE. The BDF-Elected PE is next to be elected in the redundancy group and is already known.¶
For consistency across PEs and load-balancing modes, the backup path selected should be in order of {DF, BDF, NDF1, NDF2, ...}. The DF-Elected PE selects the next-best BDF-Elected as backup and all BDF- and NDF-Elected nodes select the best DF-Elected for the protection of their egress links.¶
The number of peering PEs is not limited by existing DF-Election algorithms. A solution based on DF-Election supports subsequent redirection upon multiple cascading failures, once a new DF-Election has occurred. Pre-selection of a backup path is supported by all current DF-Election algorithms, and more generally by all algorithms supporting BDF-Election, as recommended in ([I-D.ietf-bess-rfc7432bis]).¶
+------+
| PE1 |
| |
+-------+ | ESL1----BDF-X
| |--------| | \
| | | ERL1 * * * * *
+-----+ | | +----*-+ *
| | |IP/MPLS| * *
CE1 ---| PE3 |----|Core | * ESI1 *** CE2
| | |Network| * /
+-----+ | | +------+ * /
| | | ERL2----*---> /
| |--------| | * /
+-------+ | ESL2-----XX--
| |
| PE2 |
+------+
EVPN Multihoming failure scenario¶
The procedures for forwarding known unicast packets received from a remote PE on the local redirect label largely follow Section 13.2.2 of [RFC7432].¶
Consider the EVPN multihoming topology in Figure 1, and a traffic flow from CE1 to CE2 which is currently using EVPN service label ESL2 and forwarded through the core arriving at PE2. When the local AC representing the <EVI,ESI> pair is protected using the fast-reroute solution, the pre-computed backup path's redirect label (i.e. ERL1 from BDF-Elected PE1) is installed against the AC.¶
Under normal conditions, PE2 disposition using ESL2 will result in forwarding the packet to the CE by selecting the local AC associated with the EVPN service label (EVPN-VPWS) or MAC address lookup (EVPN). When this local AC is in failed state, the fast-reroute solution at PE2 will begin rerouting packets using the BDF-Elected peer's nexthop and ERL1. ERL1 is chosen for redirection and not ESL1 for the redirected traffic to prevent loops and overcome DF-Election timing as described in Sections 6.2 and 6.1 respectively.¶
In EVPN multihoming where the CE connects to peering PEs through link aggregation (LAG), a single LAG failure at the CE may manifest as multiple ES failures at all peering PEs simultaneously.¶
As all peering PEs would enable simultaneously the fast-reroute mechanism, redirection would be permanent causing a traffic storm or until TTL expires.¶
Once-redirected traffic may not be redirected again, according to the terminal nature of ERLs described in Section 6.2¶
Trying to support cascading failures by redirecting once-redirected traffic is substantially equivalent to simultaneous failures above.¶
Once-redirected traffic may not be redirected again, according to the terminal nature of ERLs described in Section 6.2 and loss is to be expected until EVPN control plane reconverges for double-failure scenarios.¶
In a scenario with 3 peering PEs (PE1-DF, PE2-BDF, PE3-NDF) where PE1 fails, followed by a PE2 failure before control-plane reconvergence, there is no reroute of traffic towards PE3 because the reroute-label is terminal.¶
In such rapid-succession failures, it is expected that control plane must first correct for the initial failure and DF-Elect PE2 as new‑DF and PE3 as the new‑BDF. PE2 to PE3 redirection would then begin, unless control-plane is rapid enough to correct directly, and elect PE3 new-DF.¶
The EVPN redirect labels MUST be downstream assigned, and it is directly associated with the <EVI,ESI> AC being egress protected. The special forwarding characteristics and use of an EVPN redirect label (ERL) described below, are a matter of local significance only to the advertising PE (which is also the disposition PE).¶
Special-attributes to the ERLs do not affect any other PEs or transit P nodes. There are no extra labels appended to the label stack in the IP/MPLS network and the ERL appears to label-switching transit nodes as would any other EVPN service label.¶
Two special forwarding characteristics of EVPN redirect labels are described below to mitigate these issues.¶
Local detection and restoration at PE2 will begin rapidly redirecting traffic onto the
backup path.
Redirected packets will arrive at the Backup-DF port much faster than control plane
DF-Election at the Backup-DF peer is capable of unblocking its local egress link for the
shared ES (ESI1). All redirected traffic would drop at Backup-DF and
no net reduction in traffic loss achieved.¶
Traffic restoration remains dependant upon ES route or Ethernet A-D per ES routes withdrawal for a DF-Election operation and for PE1 to assume the traffic forwarding role. This is especially important in single-active load-balancing mode where known unicast traffic is blocked.¶
To mitigate this, the redirect labels allocated must carry a special attribute in the local forwarding and decapsulation chain: for traffic received on the ERL when the AC is up, an override to the DF‑Election is applied and traffic from the ERL will bypass the local Backup-DF blocking state. Once EVPN control plane reconverges, traffic from the ERL will cease and the optimal forwarding path based on ESLs will resume.¶
The EVPN redirect label MUST carry a context locally, such that from disposition to egress redirected packets are allowed to bypass the BDF blocking state that would otherwise drop. Similarly, this may open the gate to the traffic in the reverse direction.¶
The reroute scheme is susceptible to loops and persistant redirects between peering PEs which have setup FRR redirection. Consider the scenario where both CE-facing interfaces fail simultaneously, fast reroute will be activated at both PE1 and PE2 effectively bouncing a redirected packet between the two PEs indefinitely (or until the TTL expires) causing a traffic storm.¶
To prevent this, a distinction is made between 'regular' EVPN service labels for disposition (i.e. known unicast EVI label or EVPN-VPWS label) and reroute labels with terminal disposition.¶
At the redirecting PE2, we consider the case of ESL2 vs. ERL2 , where both are locally allocated and provided in EVPN routes (downstream allocation) to BGP peers:¶
EVPN Service label, ESL2:¶
EVPN Reroute label, ERL2:¶
The ERL acts like a local cross-connect by providing a direct channel from disposition to the AC. ERLs are terminal-disposition and prevents once‑redirected packets from being redirected again. With this forwarding attribute on ERLs, known only locally to the downstream-allocating PE, redirection is achieved without growing the label stack with another special purpose label.¶
BUM traffic is treated using EVPN defaults. There is no further extension to exiting procedure as of now, this work is left for future study.¶
Fast reroute mechanisms such as the one described in this document generally provide a way to preserve traffic flows at failure time. Use of fast reroute in EVPN, however, permits setting up a controlled recovery sequence to shorten the period of loss between an interface coming up and the EVPN DF-Election procedures and default timers for peer discovery.¶
The benefit of a controlled recovery sequence is amplified when used in conjunction with [I-D.ietf-bess-evpn-fast-df-recovery] (synchronised DF-Election)>¶
The solution is agnostic to transport underlays, for instance similar behaviour is carried forward for VXLAN and SRv6¶
There are no new BGP extensions required to advertise the redirect label(s) used for EVPN egress link protection. The ESI Label Extended Community defined in Section 7.5 of [RFC7432] may be advertised along with Ethernet A-D routes:¶
Remote PEs SHALL NOT use the ERLs as a substitution for ESLs in route resolution, and is especially not to be confused with the aliasing and backup path ESL as described and used in Section 8.4 of [RFC7432].¶
The mechanisms in this document use the EVPN control plane as defined in [RFC7432] and [RFC8214], and the security considerations described therein are equally applicable. Reroute labels redistributed in EVPN control plane are meant for consumption by the peering PE in a same ES. It is, however, visible in the EVPN control plane to remote peers. Care shall be taken when installing reroute labels, since their use may result in bypassing DF-Election procedures and lead to duplicate traffic at CEs if incorrectly installed.¶
This document makes no specific requests to IANA.¶