How Cloud Router works

Cloud Router is an API abstraction implemented by multiple and redundant BGPtasks, a dynamic route control plane, and Virtual Private Cloud (VPC) networkcontrol and data planes. Understanding how these three software components worktogether helps you understand Cloud Router operations and howlearned-route-best-path-selection options work.

Software components of Cloud Router

There are several software components within Cloud Router andVPC:

Cloud Router BGP task

Cloud Router BGP tasks are grouped together within a region. EachBGP task communicates with a dynamic route control plane for its region andgroup. BGP tasks don't handle packet data processing. Instead, BGP tasks manageBGP sessions to send and receive BGP prefixes.

Dynamic route control plane

Each region contains a dynamic route control plane that communicates withBGP tasks for its region and group. In global dynamic routing mode, dynamicroute control planes in one region also communicate with dynamic route controlplanes in other regions. Each dynamic route control plane sends messages to theVPC network control plane.

VPC network control and data planes

Google Cloud uses theAndromeda network virtualization stack (PDFdownload)as the distributed control and data plane for VPC networking, andincludes the following components:

VPC network control plane

Each region contains a VPC network control plane thatreceives information from the groups of dynamic route control planes intheir own region. Each VPC network control plane programsdynamic routes in receiving VPC networks. VPCnetwork control planes also enforce dynamic route quotas.

VPC network data plane

Each region contains a VPC network data plane thatevaluates and implements dynamic routes using information from theVPC network control plane. The VPC networkdata plane performs packet forwarding.

Cloud Router BGP tasks

The following table shows how many BGP tasks a Cloud Router uses forcommon scenarios:

Software maintenance

Cloud Router maintenance events release new features and improvereliability. During maintenance, new BGP tasks take over as BGP speakers andresponders. Before maintenance, the last BGP task notifies its peer router inone of the following ways:

• If the peer router supports graceful restart, Cloud Router sends agraceful restart notification (a TCPFIN packet).

• If the peer routerdoesn't support graceful restart, Cloud Routersends a BGPCEASE notification to the peer router to terminate the BGPsession.

Cloud Router maintenance events aren't announced in advance becausemaintenance events are automatic and not disruptive, as long as the peerrouter supports graceful restart. Maintenance events are designed to complete inless than 120 seconds—thus, Cloud Router uses a120 secondgraceful restarttimer. Forinformation about how to find completed maintenance events, seeIdentify routermaintenance events.

If the peer router supports graceful restart, the peer router logs a gracefulrestart event during Cloud Router maintenance. In accordance withSection 4.2 of RFC4724, the peerrouter must honor the 120 second Cloud Router graceful restart timer,preserving learned routes and continuing to advertise routes, in the event that:

• Cloud Router stops sending BGP keepalive packets.

• Applicable only when BFD is configured: Cloud Router stops sending BFD packets. Consequently, the peer routermust honor the BFD control plane independent bit value of0 because Cloud Router uses a control planedependent BFD implementation. For more information, seegraceful restart and BFD.

If the peer routerdoesn't support graceful restart or if a peer router hasgraceful restart disabled, Cloud Router sends a BGPCEASEnotification followingSection 4.5 of RFC4271. After theCEASE notification, the BGP session remains down until Cloud Routerreplaces the BGP task. Making adjustments to the Cloud Router holdtimer or the peer router hold timer doesn't prevent the BGP session fromterminating.

Planned Cloud Interconnect maintenance

Forplanned Cloud Interconnectmaintenance,Cloud Router sends a BGPCEASE notification that terminates the BGPsession, removing the session's learned and advertised routes. Neither thegraceful restart timer nor the negotiated BGP hold timer apply during plannedmaintenance events.

Unexpected BGP task failures

Cloud Router uses multiple BGP tasks so that HA VPNtunnel pairs, Router appliances, and VLAN attachments that meet aCloud Interconnect SLA don't depend on a single BGP task. For moreinformation, see theCloud Router BGP tasks section ofthis document. If a Cloud Router BGP task fails unexpectedly,Cloud Router isn't able to send one of the notifications that itnormally sends during software maintenance. However, both learned and advertisedroutes remain for the duration of the negotiatedholdtimer.