| Internet-Draft | VRRP BFD | January 2023 |
| Gupta, et al. | Expires 9 July 2023 | [Page] |
This document describes how Point to Point Bidirectional Forwarding Detection (BFD) can be used to support sub-second detection of a Active Router failure in the Virtual Router Redundancy Protocol (VRRP).¶
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The Virtual Router Redundancy Protocol (VRRP) provides redundant Virtual gateways in the Local Area Network (LAN), which is typically the first point of failure for end-hosts sending traffic out of the LAN. Fast failure detection of VRRP Active is critical in supporting high availability of services and improved Quality of Experience to users. In VRRP [RFC5798] specification, Backup routers depend on VRRP packets generated at a regular interval by the Active router, to detect the health of the VRRP Active. Faster failure detection can be achieved within VRRP protocol by reducing the Advertisement and Active Down Interval. However, sub second Advert timers, can put extra load on CPU and the network bandwidth which may not be desirable.¶
Since the VRRP protocol depends on the availability of Layer 3 IPv4 or IPv6 connectivity between redundant peers, the VRRP protocol can interact with the Layer 3 variant of BFD as described in [RFC5881] to achieve a much faster failure detection of the VRRP Active on the LAN. BFD, as specified by the [RFC5880] can provide a much faster failure detection in the range of 150ms, if implemented in the part of a Network device which scales better than VRRP when sub second Advert timers are used.¶
Terminology of this draft has been updated conform to inclusive language guidelines for VRRP Inclusive Terminologies [VRRP-RFC5798bis]. The IETF has designated National Institute of Standards and Technology (NIST) "Guidance for NIST Staff on Using Inclusive Language in Documentary Standards" [NISTIR8366] for its inclusive language guidelines.¶
In this document, several words are used to signify the requirements of the specification. 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]¶
BFD for IPv4 or IPv6 (Single Hop) [RFC5881] requires that in order for a BFD session to be formed both peers participating in a BFD session need to know its peer IPv4 or IPV6 address. This poses a unique problem with the definition of the VRRP protocol, that makes the use of BFD for IPv4 or IPv6 [RFC5881] more challenging. In VRRP it is only the Active router that sends Advert packets. This means that a Active router is not aware of any Backup routers, and Backup routers are only aware of the Active router. This also means that a Backup router is not aware of any other Backup routers in the Network.¶
Since BFD for IPv4 or IPv6 [RFC5881] requires that a session be formed by both peers using a full destination and source address, there needs to be some external means to provide this information to BFD on behalf of VRRP. Once the peer information is made available, VRRP can form BFD sessions with its peer Virtual Router. The BFD session for a given Virtual Router is identified as the Critical Path BFD Session, which is the session that forms between the current VRRP Active router, and the highest priority Backup router. When the Critical Path BFD Session identified by VRRP as having changed state from Up to Down, then this will be interpreted by the VRRP state machine on the highest priority Backup router as a Active Down event. A Active Down event means that the highest priority Backup peer will immediately become the new Active for the Virtual Router.¶
NOTE: At all times, the normal fail-over mechanism defined in the VRRP [RFC5798] will be unaffected, and the BFD fail-over mechanism will always resort to normal VRRP fail-over.¶
This draft defines the mechanism used by the VRRP protocol to build a peer table that will help in forming of BFD session and the detection of Critical Path BFD session. If the Critical Path BFD session were to go down, it will signal a Active Down event and make the most preferred Backup router as the VRRP Active router. This requires an extension to the VRRP protocol.¶
This can be achieved by defining a new type in the VRRP Advert packet, and allowing VRRP peers to build a peer table in any of the operational state, Active or Backup.¶
In this mode of operation VRRP peers learn the adjacent routers, and form BFD session between the learnt routers. In order to build the peer table, all routers send VRRP Advert packets whilst in any of the operational states (Active or Backup). Normally VRRP peers only send Advert packets whilst in the Active state, however in this mode VRRP Backup peers will also send Advert packets with the type field set to BACKUP ADVERTISEMENT type defined in Section 3.3 of this document. The VRRP Active router will still continue to send packets with the Advert type as ADVERTISEMENT as defined in the VRRP protocol. This is to maintain inter-operability with peers complying to VRRP protocol.¶
Additionally, Advert packets sent from Backup Peers must not use the Virtual router MAC address as the source address. Instead it must use the Interface MAC address as the source address from which the packet is sent from. This is because the source MAC override feature is used by the Active to send Advert packets from the Virtual Router MAC address, which is used to keep the bridging cache on LAN switches and bridging devices refreshed with the destination port for the Virtual Router MAC.¶
VRRP peers can now form the peer table by learning the source address in the ADVERTISEMENT or BACKUP ADVERTISEMENT packet sent by VRRP Active or Backup peers. This allows peers to create BFD sessions with other operational peers.¶
A peer entry should be removed from the peer table if Advert is not received from a peer for a period of (3 * the Advert interval).¶
The following figure shows the VRRP packet as defined in VRRP [RFC5798] RFC.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 Fields or IPv6 Fields | ... ... | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| Type | Virtual Rtr ID| Priority |Count IPvX Addr| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |(rsvd) | Max Advert Int | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | IPvX Address(es) | + + + + + + + + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
The type field specifies the type of this VRRP packet. The type field can have two values. Type 1 (ADVERTISEMENT) is used by the VRRP Active Router. Type 2 (BACKUP ADVERTISEMENT) is used by the VRRP Backup router. This is to distinguish the packets sent by the VRRP backup Router. VRRP Backup fills Backup_Advertisement_Interval in the Max Advert Int of BACKUP ADVERTISEMENT packet. Rest of the fields in Advert packet remain the same.¶
1 ADVERTISEMENT 2 BACKUP ADVERTISEMENT¶
A packet with unknown type MUST be discarded.¶
The following figure shows a simple network with three VRRP routers implementing one virtual router.¶
+-----------+ +-----------+ +-----------+
| Rtr1 | | Rtr2 | | Rtr3 |
|(AR VRID=1)| |(BR VRID=1)| |(BR VRID=1)|
| (PR=200) | | (PR=150) | | (PR=100) |
| VRIPVX= A | | VRIPVX= A | | VRIPVX= A |
+-----------+ +-----------+ +-----------+
B C D
| | |
| | |
| | |
---------+--------+--------+---------+--------+---------
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
AR = Active Router
BR = Backup Router
PR = VRRP Router priority
VRID = VRRP Router ID
VRIPVX= IPv4 or IPv6 address protected by
the VRRP Router
B,C,D = Interface IPv4 or IPv6 address of
the Virtual Router
¶
In the above configuration there are three routers on the LAN protecting an IPv4 or IPv6 address associated to a Virtual Router ID 1. Rtr1 is the Active router since it has the highest priority compared to Rtr2 and Rtr3. Now if peer learning extension is enabled on all the peers. Rtr1 will send the Advert packet with type field set to 1. While Rtr2 and Rtr3 will send the Advert packet with type field set to 2. In the above configuration the peer table built at each router is shown below:¶
Rtr1 Peer table¶
+------------------------------------+ | Peer Address | Priority | +------------------------------------+ | C | 150 | +------------------------------------+ | D | 100 | +------------------------------------+¶
Rtr2 Peer table¶
+------------------------------------+ | Peer Address | Priority | +------------------------------------+ | B | 200 | +------------------------------------+ | D | 100 | +------------------------------------+¶
Rtr3 Peer table¶
+------------------------------------+ | Peer Address | Priority | +------------------------------------+ | B | 200 | +------------------------------------+ | C | 150 | +------------------------------------+¶
Once the peer tables are formed, VRRP on each router can form a BFD sessions with the learnt peers.¶
The Critical BFD Session is determined to be the session between the VRRP Active and the next best VRRP Backup. Failure of the Critical BFD session indicates that the Active is no longer available and the most preferred Backup will now become Active.¶
In the above example the Critical BFD session is shared between Rtr1 and Rtr2. If the BFD Session goes from Up to Down state, Rtr2 can treat it as a Active down event and immediately assume the role of VRRP Active router for VRID 1 and Rtr3 will become the critical Backup. If the priorities of two Backup routers are same then the primary IPvX Address of the sender is used to determine the highest priority Backup. Where higher IPvX address has higher priority.¶
Following parameters are added to the VRRP protocol to support this mode of operation.¶
Backup_Advertisement_Interval Time interval between
BACKUP ADVERTISEMENTS
(centiseconds). Default is 100
centiseconds (1 second).
Backup_Adver_Interval Advertisement interval contained in
BACKUP ADVERTISEMENTS received from
the Backup (centiseconds). This
value is saved by virtual routers
used, to compute Backup_Down_Interval.
Backup_Down_Interval Time interval for VRRP instance
to declare Backup down
(centiseconds). Calculated as
(3 * Backup_Adver_Interval) for
each VRRP Backup.
Critical_Backup Procedure outlined in section 3.4
of this document is used to
determine the Critical_Backup at
each VRRP Instance.
Critical_BFD_Session The Critical BFD Session is
the session between
the VRRP Active and Critical_Backup.
¶
Following timers are added to the VRRP protocol to support this mode of operation.¶
Backup_Down_Timer Timer that fires when BACKUP ADVERTISEMENT
has not been heard from a backup peer for
Backup_Down_Interval.
Backup_Adver_Timer Timer that fires to trigger sending of
BACKUP ADVERTISEMENT based on
Backup_Advertisement_Interval.
¶
Following State Machine replaces the state Machine outlined in section 6.4 of the VRRP protocol [RFC5798] to support this mode of operation. Please refer to the section 6.4 of [RFC5798] for State description.¶
Following state machine replaces the state machine outlined in section 6.4.1 of [RFC5798]¶
(100) If a Startup event is received, then:
(105) - If the Priority = 255 (i.e., the router owns the IPvX
address associated with the virtual router), then:
(110) + Send an ADVERTISEMENT
(115) + If the protected IPvX address is an IPv4 address, then:
(120) * Broadcast a gratuitous ARP request containing the
virtual router MAC address for each IP address associated
with the virtual router.
(125) + else // IPv6
(130) * For each IPv6 address associated with the virtual
router, send an unsolicited ND Neighbor Advertisement with
the Router Flag (R) set, the Solicited Flag (S) unset, the
Override flag (O) set, the target address set to the IPv6
address of the virtual router, and the target link-layer
address set to the virtual router MAC address.
(135) +endif // was protected addr IPv4?
(140) + Set the Adver_Timer to Advertisement_Interval
(145) + Transition to the {Active} state
(150) - else // rtr does not own virt addr
(155) + Set Active_Adver_Interval to Advertisement_Interval
(160) + Set the Active_Down_Timer to Active_Down_Interval
(165) + Set Backup_Adver_Timer to Backup_Advertisement_Interval
(170) + Transition to the {Backup} state
(175) -endif // priority was not 255
(180) endif // startup event was recv
¶
Following state machine replaces the state machine outlined in section 6.4.2 of [RFC5798]¶
(300) While in this state, a VRRP router MUST do the following:
(305) - If the protected IPvX address is an IPv4 address, then:
(310) + MUST NOT respond to ARP requests for the IPv4
address(es) associated with the virtual router.
(315) - else // protected addr is IPv6
(320) + MUST NOT respond to ND Neighbor Solicitation messages
for the IPv6 address(es) associated with the virtual router.
(325) + MUST NOT send ND Router Advertisement messages for the
virtual router.
(330) -endif // was protected addr IPv4?
(335) - MUST discard packets with a destination link-layer MAC
address equal to the virtual router MAC address.
(340) - MUST NOT accept packets addressed to the
IPvX address(es) associated with the virtual router.
(345) - If a Shutdown event is received, then:
(350) + Cancel the Active_Down_Timer.
(355) + Cancel the Backup_Adver_Timer.
(360) + Cancel Backup_Down_Timers.
(365) + Remove Peer table.
(370) + If Critical_BFD_Session Exists:
(375) * Tear down the Critical_BFD_Session.
(380) + endif // Critical_BFD_Session Exists?
(385) + Send a BACKUP ADVERTISEMENT with Priority = 0.
(390) + Transition to the {Initialize} state.
(395) -endif // shutdown recv
(400) - If the Active_Down_Timer fires or
If Critical_BFD_Session transitions from UP to DOWN, then:
(405) + Send an ADVERTISEMENT
(415) + If the protected IPvX address is an IPv4 address, then:
(420) * Broadcast a gratuitous ARP request on that interface
containing the virtual router MAC address for each IPv4
address associated with the virtual router.
(425) + else // ipv6
(430) * Compute and join the Solicited-Node multicast
address defined in RFC4291 for the IPv6 address(es)
associated with the virtual router.
(435) * For each IPv6 address associated with the virtual
router, send an unsolicited ND Neighbor Advertisement with
the Router Flag (R) set, the Solicited Flag (S) unset, the
Override flag (O) set, the target address set to the IPv6
address of the virtual router, and the target link-layer
address set to the virtual router MAC address.
(440) +endif // was protected addr ipv4?
(445) + Set the Adver_Timer to Advertisement_Interval.
(450) + If the Critical_BFD_Session exists:
(455) @ Tear Critical_BFD_Session.
(460) + endif // Critical_BFD_Session exists
(465) + Calculate the Critical_Backup.
(470) + If the Critical_Backup exists:
(475) * BootStrap Critical_BFD_Session with the
Critical_Backup.
(480) + endif //Critical_Backup exists?
(485) + Transition to the {Active} state.
(490) -endif // Active_Down_Timer fired
(485) - If an ADVERTISEMENT is received, then:
(490) + If the Priority in the ADVERTISEMENT is zero, then:
(495) * Set the Active_Down_Timer to Skew_Time.
(500) * If the Critical_BFD_Session exists:
(505) * Tear Critical_BFD_Session with the Active.
(510) * endIf // Critical_BFD_Session exists
(515) + else // priority non-zero
(520) * If Preempt_Mode is False, or if the Priority in the
ADVERTISEMENT is greater than or equal to the local
Priority, then:
(525) @ Set Active_Adver_Interval to Adver Interval
contained in the ADVERTISEMENT.
(530) @ Recompute the Active_Down_Interval.
(535) @ Reset the Active_Down_Timer to
Active_Down_Interval.
(540) @ Determine Critical_Backup.
(545) @ If Critical_BFD_Session does not exists and this
instance is the Critical_Backup:
(550) @+ BootStrap Critical_BFD_Session with Active.
(555) @ endif //Critical_BFD_Session exists check
(560) * else // preempt was true or priority was less
(565) @ Discard the ADVERTISEMENT.
(570) *endif // preempt test
(575) +endif // was priority zero?
(580) -endif // was advertisement recv?
(585) - If a BACKUP ADVERTISEMENT is received, then:
(590) + If the Priority in the BACKUP ADVERTISEMENT is zero,
then:
(595) * Cancel Backup_Down_Timer.
(600) * Remove the Peer from Peer table.
(605) + else // priority non-zero
(610) * Update the peer table with peer information.
(615) * Set Backup_Adver_Interval to Adver Interval
contained in the BACKUP ADVERTISEMENT.
(620) * Recompute the Backup_Down_Interval.
(625) * Reset the Backup_Down_Timer to Backup_Down_Interval.
(630) +endif // was priority zero?
(635) + Recalculate Critical_Backup.
(640) + If Critical_BFD_Session exists and this
instance is not the Critical_Backup:
(645) * Tear Down the Critical_BFD_Session.
(650) + else If Critical_BFD_Session does not exists and this
instance is the Critical_Backup:
(655) * BootStrap Critical_BFD_Session with Active.
(660) + endif // Critical_Backup change
(665) -endif // was backup advertisement recv?
(670) - If Backup_Down_Timer fires, then:
(675) + Remove the Peer from Peer table.
(680) + If Critical_BFD_Session does not exist:
(685) @ Recalculate Critical_Backup.
(690) @ If This instance is the Critical_Backup:
(695) +@ BootStrap Critical_BFD_Session with Active.
(700) @ endif // Critical_Backup change
(705) + endif // Critical_BFD_Session does not exist?
(710) -endif // Backup_Down_Timer fires?
(715) - If Backup_Adver_Timer fires, then:
(720) + Send a BACKUP ADVERTISEMENT.
(725) + Reset the Backup_Adver_Timer to
Backup_Advertisement_Interval.
(730) -endif // Backup_Down_Timer fires?
(735) endwhile // Backup state
¶
Following state machine replaces the state machine outlined in section 6.4.3 of [RFC5798]¶
(800) While in this state, a VRRP router MUST do the following:
(805) - If the protected IPvX address is an IPv4 address, then:
(810) + MUST respond to ARP requests for the IPv4 address(es)
associated with the virtual router.
(815) - else // ipv6
(820) + MUST be a member of the Solicited-Node multicast
address for the IPv6 address(es) associated with the virtual
router.
(825) + MUST respond to ND Neighbor Solicitation message for
the IPv6 address(es) associated with the virtual router.
(830) + MUST send ND Router Advertisements for the virtual
router.
(835) + If Accept_Mode is False: MUST NOT drop IPv6
Neighbor Solicitations and Neighbor Advertisements.
(840) -endif // ipv4?
(845) - MUST forward packets with a destination link-layer MAC
address equal to the virtual router MAC address.
(850) - MUST accept packets addressed to the IPvX address(es)
associated with the virtual router if it is the IPvX address
owner or if Accept_Mode is True. Otherwise, MUST NOT accept
these packets.
(855) - If a Shutdown event is received, then:
(860) + Cancel the Adver_Timer.
(865) + Send an ADVERTISEMENT with Priority = 0,
(870) + Cancel Backup_Down_Timers.
(875) + Remove Peer table.
(880) + If Critical_BFD_Session Exists:
(885) * Tear down Critical_BFD_Session
(890) + endif // If Critical_BFD_Session Exists
(895) + Transition to the {Initialize} state.
(900) -endif // shutdown recv
(905) - If the Adver_Timer fires, then:
(910) + Send an ADVERTISEMENT.
(915) + Reset the Adver_Timer to Advertisement_Interval.
(920) -endif // advertisement timer fired
(925) - If an ADVERTISEMENT is received, then:
(930) -+ If the Priority in the ADVERTISEMENT is zero, then:
(935) -* Send an ADVERTISEMENT.
(940) -* Reset the Adver_Timer to Advertisement_Interval.
(945) -+ else // priority was non-zero
(950) -* If the Priority in the ADVERTISEMENT is greater
than the local Priority,
(955) -* or
(960) -* If the Priority in the ADVERTISEMENT is equal to
the local Priority and the primary IPvX Address of the
sender is greater than the local primary IPvX Address, then:
(965) -@ Cancel Adver_Timer
(970) -@ Set Active_Adver_Interval to Adver Interval
contained in the ADVERTISEMENT
(975) -@ Recompute the Skew_Time
(980) @ Recompute the Active_Down_Interval
(985) @ Set Active_Down_Timer to Active_Down_Interval
(990) If Critical_BFD_Session Exists:
(995) @+ Tear Critical_BFD_Session
(960) @ endif //Critical_BFD_Session Exists?
(965) @ Calculate Critical_Backup.
(970) @ If this instance is Critical_Backup:
(975) @+ BootStrap Critical_BFD_Session with new
Active.
(980) @ endif // am i Critical_Backup?
(985) @ Transition to the {Backup} state
(990) * else // new Active logic
(995) @ Discard ADVERTISEMENT
(1000) *endif // new Active detected
(1005) +endif // was priority zero?
(1010) -endif // advert recv
(1015) - If a BACKUP ADVERTISEMENT is received, then:
(1020) + If the Priority in the BACKUP ADVERTISEMENT is
zero, then:
(1025) * Remove the Peer from peer table.
(1030) + else: // priority non-zero
(1035) * Update the Peer info in peer table.
(1040) * Recompute the Backup_Down_Interval
(1045) * Reset the Backup_Down_Timer to
Backup_Down_Interval
(1050) + endif // priority in backup advert zero
(1055) + Calculate the Critical_Backup
(1060) + If Critical_BFD_Session doesnot exist:
(1065) * BootStrap Critical_BFD_Session
(1070) + else if Critical_BFD_Session exist and
Critical_Backup changes:
(1075) + Tear Critical_BFD_Session with old Backup
(1080) + BootStrap Critical_BFD_Session with Critical_Backup
(1085) + endif // Critical_BFD_Session check?
(1090) - endif // backup advert recv
(1095) - If Critical_BFD_Session transitions from UP to DOWN,
then:
(1100) + Cancel Backup_Down_Timer
(1105) + Delete the Peer info from peer table
(1200) + Calculate the Critical_Backup
(1205) + BootStrap Critical_BFD_Session with Critical_Backup
(1210) - endif // BFD session transition
(1215) endwhile // in Active
¶
To reduce the number of packets generated at a regular interval, Backup Advert packets may be sent at a reduced rate as compared to Advert packets sent by the VRRP Active.¶
A VRRP peer that forms a member of this Virtual Router, but does not support this feature or extension must be configured with the lowest priority, and will only operate as the Router of last resort on failure of all other VRRP routers supporting this functionality.¶
It is recommended that mechanism defined by this draft, to interface VRRP with BFD should be used when BFD can support more aggressive monitoring timers than VRRP. Otherwise it is desirable not to interface VRRP with BFD for determining the health of VRRP Active.¶
This Draft does not preclude the possibility of the peer table being populated by means of manual configuration, instead of using the BACKUP ADVERTISEMENT as defined by the Draft.¶
The workings of this Draft can be extended to VRRPv2 defined in RFC3768, with the introduction of BACKUP ADVERTISEMENT and Peer Table as outlined in the Draft.¶
This document requests IANA to create a new name space that is to be managed by IANA. The document defines a new VRRP Packet Type. The VRRP Packet Types are discussed below.¶
a) Type 1 (ADVERTISEMENT) defined in section 5.2.2 of [RFC5798] b) Type 2 (BACKUP ADVERTISEMENT) defined in section 3.3 of this document¶
This document defines in Section 3.3 a "BACKUP ADVERTISEMENT" VRRP Packet Type. The new name space has to be created by the IANA and they will maintain this new name space. The field for this namespace is 4-Bits, and IANA guidelines for assignments for this field are as follows:¶
ADVERTISEMENT 1
BACKUP ADVERTISEMENT 2
¶
Future allocations of values in this name space are to be assigned by IANA using the "Specification Required" policy defined in [IANA-CONS]¶
Security considerations discussed in [RFC5798], [RFC5880], apply to this document. There are no additional security considerations identified by this draft.¶
The authors gratefully acknowledge the contributions of Gerry Meyer, and Mouli Chandramouli, for their contributions to the draft. The authors will also like to thank Jeffrey Haas, Maik Pfeil, Chris Bowers, Vengada Prasad Govindan and Alexander Vainshtein for their comments and suggestions.¶