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Advanced Network Design

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The document discusses techniques for achieving fast convergence in networks including graceful restart, fast down detection using Bidirectional Forwarding Detection (BFD), and exponential backoff. It explains how these techniques work in routing protocols like OSPF, IS-IS, and EIGRP to limit instability during convergence events and prevent issues like network meltdowns. Key aspects covered are separating the control and forwarding planes to ensure continuous forwarding, signaling between routers to resynchronize routing information, and throttling routing updates and SPF calculations to introduce stability.

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Advanced Network Design Eugene Odnoralets Routing Protocols Team 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.
Introduction 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.2
High Availability and Fast Convergence Analyzing potential problems you could face trying to deploy fast convergence. Several techniques that have been developed to allow high availability and fast convergence, including: Β§β€― Graceful restart Β§β€― Fast down detection Β§β€― Exponential backoff Β§β€― Speeding up route selection
Considerations in Fast Convergence Β§β€― Scale and speed are contradictory goals. Β§β€― The faster a network converges the less stable it is likely to be. Fast reactions to changes in the network topology tend to create positive feedback loops, which result in a network that simply will not converge. The pieces of a network you need to be concerned about when considering subsecond (fast) convergence: Β§β€― The physical layer how fast can a down link be detected ? Β§β€― Routing protocol convergence how fast can a routing protocol react to the topology change ? Β§β€― Forwarding how fast can the forwarding engine on each router in the network adjust to the new paths that the routing protocol calculates?
Network Meltdown Definition A state in which a network grinds to a halt due to excessive traffic. A network meltdown generally starts as a broadcast storm that gets out of control but even legitimate network messages can cause a meltdown if the network hasn't been designed to accommodate that level of traffic.
Network Meltdowns Link between Routers D and G flaps, it cycles between the "down" and "up" states slow enough Β§β€― for a routing adjacency to be formed Β§β€― for the new link to be advertised as part of the topology too quickly Β§β€― for the link to be used Adjacency between D and G forms and tears down as quickly as the routing protocol allows B C A D G F E
Slow down How to work around this sort of a problem in the routing protocol ? The answer is simple: Slow down ! Methods of slowing down: Β§β€― Not reporting all interface transitions from the physical layer up to the routing protocol. This is called debouncing the interface. Β§β€― Slow down neighbor timers. Β§β€― Slow down the distribution of information about topology changes. Β§β€― Slow down the time that the routing protocol reacts to information about topology changes.
To provide stability within a routing system Methods are typically used in routing protocol design and implementation to provide stability within a routing system Β§β€― IS-IS Β§β€― a timer regulates how often a router can originate new routing information lsp-gen-interval { level-1 | level-2 } lsp-max-wait [ lsp-initial-wait lsp-second-wait ] lsp-max-wait maximum interval between two consecutive occurrences of an LSP being generated lsp-initial-wait initial LSP generation delay lsp-second-wait hold time between the first and second LSP generation Β§β€― how often a router can run the shortest path first (SPF) algorithm that calculates the best paths through the network spf-interval [level-1 | level-2] spf-max-wait [spf-initial-wait spf-second-wait] spf-max-wait maximum interval between two consecutive SPF calculations spf-initial-wait initial SPF calculation delay after a topology change spf-second-wait hold time between the first and second SPF calculation
To provide stability within a routing system (cont) Β§β€― OSPF Β§β€― similar timers regulate the rate at which topology information can be transmitted and the frequency at which the shortest path first algorithm can be run. Β§β€― EIGRP Β§β€― the simple rule β€œNo route may be advertised until it is installed in the local routing table” dampens the the speed at which routing information is propagated through the network. Β§β€― routing information is also paced when being transmitted through the network based on the bandwidth between two routers. EIGRP uses 50% of the bandwidth reported by the software.
Do not report everything Reporting the changes more slowly when they occur quickly or not report some events at all makes routing converge much faster providing the expected stability Β§β€― Router should not immediately report all the events of which it is aware: Β§β€― link failure Β§β€― neighbor failures Β§β€― Let’s sort out which events are in some sense Β§β€― important Β§β€― not Β§β€― Example: Β§β€― if a router loses contact with an adjacent router because the adjacent router restarted for some reason do not report the resulting change in topology until it’s clear the neighbor is not coming back
The classic questions Β§β€― How long do you wait before deciding the problem is real ? Β§β€― What happens to traffic you would normally forward to that neighbor while you are waiting ? Β§β€― How do you reconnect in a way that allows the network to continue operating correctly ? Two technologies incorporated in routing protocols can answer these questions: Β§β€― Graceful Restart (GR) Β§β€― Non-Stop Forwarding (NSF)
Control plane / forwarding plane What happens to traffic received by a router while it is restarting ? well, normally Β§β€― this traffic is dropped Β§β€― any applications that are impacted must retransmit lost data Prevent this by taking advantage of the separation between the control plane and the forwarding plane: if the control plane fails or restarts for any reason, the data plane can continue forwarding traffic based on the last known good information.
Separation of the control & forwarding plane 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.13 locally generated packets packets for processing in a distributed router architechture
Non-Stop Forwarding NSF implemented through Stateful Switchover (SSO) in Cisco products. NSF allows continuous forwarding to take place regardless of the state of the control plane. When the control plane resets it sends a signal to the data plane that it should clear its tables and reset. With NSF enabled this signal from the control plane acts as a signal to mark the current data as stale and to begin aging out the information.
Non-Stop Forwarding (cont) After we have gotten this far Route Processor (RP) should be able to Β§β€― bring the control plane back up Β§β€― resynchronize the routing protocol databases Β§β€― rebuild the routing table without disturbing the packets that are still being switched by the data plane on the router. This is accomplished through Graceful Restart.
Graceful Restart 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.16
Graceful Restart for any routing protocol 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.17 sent hello indicate GR capable build adjacency mark as GR capable A B sent helloreset hold timer control-plane resethold timer is counting down sent hello indicate GR capable reset hold timer signal database resyncset up for database resync resync databaseresync database continue normal operationcontinue normal operation
Graceful Restart for any routing protocol (cont) Β§β€― Router A & B exchange some form of signaling noting that they are capable of understanding GR signaling and are responding to it correctly. Β§β€― This signaling does not imply that the router is capable of restarting gracefully or forwarding traffic through a local failure Only that it can support a neighboring router performing Graceful Restart Β§β€― However a router where the control and data plane are not cleanly separated, cannot fully support GR it can support the signaling that is necessary for a neighboring router to restart gracefully.
How EIGRP neighbor restart normally occurs 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.19 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane resethold timer is counting down place new neighbor in pending state send hello send empty update with initialization bit set set up for database resync send topology information new neighbor send topology table continue normal operationcontinue normal operation
How Graceful Restart resolves the same 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.20 sent hello indicate GR capable build adjacency mark as GR capable A B sent helloreset hold timer control-plane resethold timer is counting down sent hello with restart bit set reset hold timer place A in local neighbor table send hello empty update with init & restart bit set setup for database resync resync database resync database continue normal operation continue normal operation
OSPF Graceful Restart Two styles of OSPF Graceful Restart are available: Β§β€― Graceful Restart using link local signaling Β§β€― Graceful Restart using opaque link-state advertisements (LSAs)
Normal OSPF Restart 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.22 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane resethold timer is counting down send hello with an empty neighbor list reset adjacency place new neighbor in neighbor list send hello with router-id of new neighbor send hello with router-id of new neighbor place new neighbor in neighbor list exchange databases exchange databases continue normal operation continue normal operation negotiate db exchange negotiate db exchange
OSPF Graceful Restart using Link Local Signalling This method of signaling GR, described in the IETF Internet-Draft, β€œOSPF Restart Signaling,” (draft-nguyen-ospf-restart-04.txt) relies on two mechanisms: Β§β€― Link Local Signaling (LLS) a mechanism described in the IETF Internet-Draft, β€œOSPF Link-local Signaling” (draft-nguyen-ospf-lls-02.txt). This draft extends the OSPF hello packet format to include TLVs, which can then be used to include additional signaling of various types, such as graceful restart capability and a graceful restart. Β§β€― Out of Band Resynchronization a mechanism described in the IETF Internet-Draft, β€œOSPF Out-of-Band LSDB Resynchronization” (draft-nguyen-ospf-oob-resync-04.txt). This draft describes a mechanism through which two OSPF routers can resynchronize their link-state databases at any point.
OSPF Graceful Restart using Link Local Signalling 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.24 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane resethold timer is counting down send hello with an empty neighbor list & the RS bit set reset hold timer place new neighbor in neighbor list send hello with router-id of restarting neighbor send hello with router-id of new neighbor place new neighbor in neighbor list exchange databases using out of band sync exchange databases using out of band sync continue normal operation continue normal operation negotiate db exchange negotiate db exchange
OSPF Graceful Restart using Opaque LSA 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.25 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane reset send Grace LSA exchange databases using Grace LSA exchange databases using Grace LSA continue normal operation continue normal operation reset hold timer GR timer couting down
Fast Down Detection 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.26
Fast Down Detection Before you can route around a failed link or device, however, you need to detect its failure. Detecting failure is a major concern in the highly available network. You can detect a neighbor or link failure in two ways: Β§β€― Polling through fast hellos or other packets, transmitted at Layer 2 or Layer 3 Β§β€― Event-driven notification through monitoring some link property, such as the link carrier
Detecting a Link or Adjacency Failure Using Polling One common method to detect a link or adjacency failure is polling, or periodically sending hello packets to the adjacent device and expecting a periodic hello packet in return. The two determining factors in the speed at which polling can discover a failed link or device are as follows: Β§β€― The rate at which hello packets are transmitted Β§β€― The number of hello packets missed before declaring a link or adjacency as failed
How Fast Does Polling Detect a Down Neighbor ? A Bhellos transmitted A B C D last hellos transmitted 10 second hello interval 30 second hold interval E F
Fast hellos Using faster times than the defaults in most protocols: Β§β€― OSPF can transmit a hello every 330 milliseconds and set the dead interval to 1 second ip ospf dead-interval minimal hello-multiplier multiplier Β§β€― IS-IS can transmit a hello every 330 millisecond and set the dead interval to 1 second isis hello-interval minimal [level-1 | level-2] isis hello-multiplier multiplier [level-1 | level-2] the hello multiplier is set to 3 by default. Β§β€― EIGRP can transmit a hello every second and set the dead interval to 3 sec ip hello-interval eigrp [autonomous system] [seconds] ip hold-time eigrp [autonomous system] [seconds]
Bidirectional Forwarding Detection - BFD What's BFD ? Β§β€― Lightweight hello protocol designed to run over multiple transport protocols Β§β€― Designed for sub-second Layer 3 failure detection Β§β€― Any interested client Β§β€― EIGRP Β§β€― IS-IS Β§β€― OSPF Β§β€― etc registers with BFD and is notified as soon as BFD detects a neighbor loss Β§β€― All registered clients benefit from uniform failure detection Β§β€― Runs on physical, virtual and bundle interfaces Β§β€― Uses UDP port 3784 / 3785 (for echo)
BFD in a distributed router architechture Route Processor OSPF IS-IS Telnet SNMP BFD Master Linecard BFD Agent FIB Downloader Linecard BFD Agent FIB Downloader Linecard BFD Agent FIB Downloader
Event-driven notification through monitoring link Rather than periodically polling rely on event-driven notification of link failures. Rely on lower-layer devices to monitor the link status and notify the routing protocol when the link fails. Β§β€― SONET/SDH Β§β€― DWDM probably the best known of the fast convergence technologies available; it not only allows the fast detection of down links and devices, but it also provides for link protection, which allows traffic to quickly be switched to a backup fiber link if the primary path fails. APS protected link unprotected link
Exponential Backoff 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.34
Exponential Backoff in Link-State Protocols step 2 2nd link flap step 1 1st link flap initial timer set to 1 sec send notification add increment of 1 sec and set timer here send notification double time and set timer here step 3 3d link flap send notification set timer to max of 5 sec A B C flapping link step 4 set timer to initial 2x maximum (10 seconds)
Exponential Backoff in Link-State Protocols (cont) Exponential backoff mechanizm can be applied to two different timers in link-state protocols: Β§β€― The Link-state generation timer, the case just examined Β§β€― The SPF timer, which determines how often a router runs the SPF algorithm in response to changes in the network
OSPF Exponential Backoff for LSA Generation OSPF exponential backoff for LSA generation is called LSA throttling Two configuration commands are related to this capability: Β§β€― timers throttle lsa all [start-interval] [hold-interval] [max-interval] start-interval is the initial time hold-interval is the increment max-interval is the maximum time Β§β€― timers lsa arrival [milliseconds] the rate at which a router accepts LSAs with the same LSA-ID
OSPF Exponential Backoff for Running SPF OSPF exponential backoff for SPF is implemented as OSPF SPF throttling Β§β€― timers throttle spf spf-start spf-hold spf-max-wait Β§β€― spf-start is the initial SPF schedule delay in milliseconds Β§β€― spf-hold is the minimum hold time between two consecutive SPF calculations Β§β€― spf-max-wait is the maximum wait time between two consecutive SPF calculations
IS-IS Exponential Backoff for Running SPF IS-IS also implements exponential backoff as throttling Three commands are used to configure: Β§β€― LSP generation lsp-gen-interval [level-1 | level-2] lsp-max-wait [lsp-initial-wait lsp-second-wait] Β§β€― SPF run spf-interval [level-1 | level-2] spf-max-wait [spf-initial-wait spf-second-wait] Β§β€― PRC throttling prc-interval prc-max-wait [prc-initial-wait prc-second-wait]
Speeding up route selection 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.40
Calculating the Route Faster Another area where the convergence speed of a network could be decreased is in route calculation. How long does it take to calculate the best path to a destination in the network after you have detected and reported an event ? Consider tuning: Β§β€― feasible successors in EIGRP Β§β€― link-state partial SPF Β§β€― link-state incremental SPF
EIGRP Feasible Successors EIGRP calculates not only the best path to each reachable destination but also feasible successors, which are known as loop-free routes to the same destination. The route to 172.17.1.0/24 Β§β€― through 172.17.3.1 has reported distance of 2167296 Β§β€― through 172.18.8.4 feasible distance of 2172416 router#show ip eigrp topo 172.17.1.0 IP-EIGRP (AS 100): Topology entry for 172.17.1.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416 Routing Descriptor Blocks: 172.17.2.1 (Serial0/0), from 172.18.8.4, Send flag is 0x0 Composite metric is (2172416/18944), Route is Internal .... 172.17.1.0 (Serial0/3), from 172.17.3.1, Send flag is 0x0 Composite metric is (2684416/2167296), Route is Internal Because the reported distance through 172.17.3.1 is less than the feasible distance through 172.18.8.4, the route through 172.17.3.1 must be loop free. It is a feasible successor.
How EIGRP determines that a nonfeasible successor is loop free It always takes time to query neighbors and to receive replies which slows down network convergence. Apply this knowledge to network design by considering not only the best path to each destination from a given area in the network but also where the feasible successors are and how to tweak the metrics so that you have a feasible successor where possible.
How EIGRP determines that a nonfeasible successor is loop free (cont) One such possible situation with a pair of equal cost links: Β§β€― A to B link Β§β€― A to C link router-b#show ip eigrp topo 172.17.1.0 IP-EIGRP (AS 100): Topology entry for 172.17.1.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416 Routing Descriptor Blocks: 10.1.1.1 (Serial0/0), from 10.1.1.1, Send flag is 0x0 Composite metric is (2172416/18944), Route is Internal .... 10.3.3.1 (Serial0/3), from 10.1.3.1, Send flag is 0x0 Composite metric is (2684416/2172416), Route is Internal The feasible distance through Router A is equal to the reported distance through Router C, so the route through Router C is not considered a feasible successor. If the Router A to B link or the Router A to C link fails, at least one query is required to re-converge. 172.17.1.0/24 B C A 10.1.1.1 10.1.2.1 10.1.3.1
Modifying the Delay to Create an EIGRP-Feasible Successor Modifying the metrics on the Router A to C link by decreasing the delay slightly produces the results router-b#show ip eigrp topo 172.17.1.0 IP-EIGRP (AS 100): Topology entry for 172.17.1.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416 Routing Descriptor Blocks: 10.1.1.1 (Serial0/0), from 10.1.1.1, Send flag is 0x0 Composite metric is (2172416/18944), Route is Internal .... 10.1.3.1 (Serial0/3), from 10.1.3.1, Send flag is 0x0 Composite metric is (2684416/2167296), Route is Internal The reported distance through Router C is now lower than the feasible distance through Router A, so the path through Router C is considered a feasible successor. 172.17.1.0/24 B C A 10.1.1.1 10.1.2.1 10.1.3.1
Link-State Partial SPF Three types of objects along directed graph built using SPF: Β§β€― Nodes Β§β€― Edges Β§β€― Leaves IS-IS treats all IP subnets as leaves off the SPF tree Β§β€― 172.17.1.0/24 leaf Β§β€― 172.17.2.0/24 leaf OSPF treats an external (redistributed) as leaves Β§β€― 172.17.1.0/24 leaf Β§β€― 172.17.2.0/24 treated as a node in OSPF (network statement) 172.17.1.0/24 B C A D 172.17.2.0/24 redistributed brought into OSPF through a network statement
Node and a Leaf in the SPF Removing and adding leaf nodes without recalculating the entire SPF tree is called Partial SPF Β§β€― a feature of implementation of OSPF and IS-IS Β§β€― the distinction between a node and a leaf in the SPF matters !!! Β§β€― changes in leaves in the SPF tree do not cause a complete recalculation of the SPF tree Β§β€― if 172.17.1.0/24 fails it is simply removed from the SPF tree Β§β€― parts of the tree that contain the nodes A, B, C, and D are not impacted by this change 172.17.1.0/24 B C A D 172.17.2.0/24 redistributed brought into OSPF through a network statement
Link-State Incremental SPF Incremental SPF takes the concept of a partial SPF one step further. If a specific piece of the SPF tree changes, rather than recalculating the entire tree recompute just a section of the tree Β§β€― link to router B fails Β§β€― no alternate path exists to router B Β§β€― it is unnecessary to recalculate the entire SPF tree Β§β€― Instead, SPF can safely remove the branch behind router B Β§β€― adjust the routing table accordingly without further calculations 172.17.1.0/24 B C A D E
Link-State Incremental SPF (cont) In summary: Β§β€― iSPF is more efficient than the full SPF algorithm thereby allowing OSPF/IS-IS to converge faster Β§β€― iSPF also provides a significant advantage when the changes in the network topology are further away from the root of the SPT - the larger the network the more significant the impact Β§β€― iSPF provides greater improvements in convergence time for networks with a high number of nodes and links a segment of 400-1000 nodes should see improvements
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Π–Π΄Π΅ΠΌ Π²Π°ΡˆΠΈΡ… сообщСний с Ρ…Π΅ΡˆΡ‚Π΅Π³ΠΎΠΌ #CiscoConnectRu CiscoRu Cisco CiscoRussia CiscoRu 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.
Advanced Network Design

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Advanced Network Design

  • 1. Advanced Network Design Eugene Odnoralets Routing Protocols Team 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.
  • 2. Introduction 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.2
  • 3. High Availability and Fast Convergence Analyzing potential problems you could face trying to deploy fast convergence. Several techniques that have been developed to allow high availability and fast convergence, including: Β§β€― Graceful restart Β§β€― Fast down detection Β§β€― Exponential backoff Β§β€― Speeding up route selection
  • 4. Considerations in Fast Convergence Β§β€― Scale and speed are contradictory goals. Β§β€― The faster a network converges the less stable it is likely to be. Fast reactions to changes in the network topology tend to create positive feedback loops, which result in a network that simply will not converge. The pieces of a network you need to be concerned about when considering subsecond (fast) convergence: Β§β€― The physical layer how fast can a down link be detected ? Β§β€― Routing protocol convergence how fast can a routing protocol react to the topology change ? Β§β€― Forwarding how fast can the forwarding engine on each router in the network adjust to the new paths that the routing protocol calculates?
  • 5. Network Meltdown Definition A state in which a network grinds to a halt due to excessive traffic. A network meltdown generally starts as a broadcast storm that gets out of control but even legitimate network messages can cause a meltdown if the network hasn't been designed to accommodate that level of traffic.
  • 6. Network Meltdowns Link between Routers D and G flaps, it cycles between the "down" and "up" states slow enough Β§β€― for a routing adjacency to be formed Β§β€― for the new link to be advertised as part of the topology too quickly Β§β€― for the link to be used Adjacency between D and G forms and tears down as quickly as the routing protocol allows B C A D G F E
  • 7. Slow down How to work around this sort of a problem in the routing protocol ? The answer is simple: Slow down ! Methods of slowing down: Β§β€― Not reporting all interface transitions from the physical layer up to the routing protocol. This is called debouncing the interface. Β§β€― Slow down neighbor timers. Β§β€― Slow down the distribution of information about topology changes. Β§β€― Slow down the time that the routing protocol reacts to information about topology changes.
  • 8. To provide stability within a routing system Methods are typically used in routing protocol design and implementation to provide stability within a routing system Β§β€― IS-IS Β§β€― a timer regulates how often a router can originate new routing information lsp-gen-interval { level-1 | level-2 } lsp-max-wait [ lsp-initial-wait lsp-second-wait ] lsp-max-wait maximum interval between two consecutive occurrences of an LSP being generated lsp-initial-wait initial LSP generation delay lsp-second-wait hold time between the first and second LSP generation Β§β€― how often a router can run the shortest path first (SPF) algorithm that calculates the best paths through the network spf-interval [level-1 | level-2] spf-max-wait [spf-initial-wait spf-second-wait] spf-max-wait maximum interval between two consecutive SPF calculations spf-initial-wait initial SPF calculation delay after a topology change spf-second-wait hold time between the first and second SPF calculation
  • 9. To provide stability within a routing system (cont) Β§β€― OSPF Β§β€― similar timers regulate the rate at which topology information can be transmitted and the frequency at which the shortest path first algorithm can be run. Β§β€― EIGRP Β§β€― the simple rule β€œNo route may be advertised until it is installed in the local routing table” dampens the the speed at which routing information is propagated through the network. Β§β€― routing information is also paced when being transmitted through the network based on the bandwidth between two routers. EIGRP uses 50% of the bandwidth reported by the software.
  • 10. Do not report everything Reporting the changes more slowly when they occur quickly or not report some events at all makes routing converge much faster providing the expected stability Β§β€― Router should not immediately report all the events of which it is aware: Β§β€― link failure Β§β€― neighbor failures Β§β€― Let’s sort out which events are in some sense Β§β€― important Β§β€― not Β§β€― Example: Β§β€― if a router loses contact with an adjacent router because the adjacent router restarted for some reason do not report the resulting change in topology until it’s clear the neighbor is not coming back
  • 11. The classic questions Β§β€― How long do you wait before deciding the problem is real ? Β§β€― What happens to traffic you would normally forward to that neighbor while you are waiting ? Β§β€― How do you reconnect in a way that allows the network to continue operating correctly ? Two technologies incorporated in routing protocols can answer these questions: Β§β€― Graceful Restart (GR) Β§β€― Non-Stop Forwarding (NSF)
  • 12. Control plane / forwarding plane What happens to traffic received by a router while it is restarting ? well, normally Β§β€― this traffic is dropped Β§β€― any applications that are impacted must retransmit lost data Prevent this by taking advantage of the separation between the control plane and the forwarding plane: if the control plane fails or restarts for any reason, the data plane can continue forwarding traffic based on the last known good information.
  • 13. Separation of the control & forwarding plane 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.13 locally generated packets packets for processing in a distributed router architechture
  • 14. Non-Stop Forwarding NSF implemented through Stateful Switchover (SSO) in Cisco products. NSF allows continuous forwarding to take place regardless of the state of the control plane. When the control plane resets it sends a signal to the data plane that it should clear its tables and reset. With NSF enabled this signal from the control plane acts as a signal to mark the current data as stale and to begin aging out the information.
  • 15. Non-Stop Forwarding (cont) After we have gotten this far Route Processor (RP) should be able to Β§β€― bring the control plane back up Β§β€― resynchronize the routing protocol databases Β§β€― rebuild the routing table without disturbing the packets that are still being switched by the data plane on the router. This is accomplished through Graceful Restart.
  • 16. Graceful Restart 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.16
  • 17. Graceful Restart for any routing protocol 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.17 sent hello indicate GR capable build adjacency mark as GR capable A B sent helloreset hold timer control-plane resethold timer is counting down sent hello indicate GR capable reset hold timer signal database resyncset up for database resync resync databaseresync database continue normal operationcontinue normal operation
  • 18. Graceful Restart for any routing protocol (cont) Β§β€― Router A & B exchange some form of signaling noting that they are capable of understanding GR signaling and are responding to it correctly. Β§β€― This signaling does not imply that the router is capable of restarting gracefully or forwarding traffic through a local failure Only that it can support a neighboring router performing Graceful Restart Β§β€― However a router where the control and data plane are not cleanly separated, cannot fully support GR it can support the signaling that is necessary for a neighboring router to restart gracefully.
  • 19. How EIGRP neighbor restart normally occurs 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.19 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane resethold timer is counting down place new neighbor in pending state send hello send empty update with initialization bit set set up for database resync send topology information new neighbor send topology table continue normal operationcontinue normal operation
  • 20. How Graceful Restart resolves the same 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.20 sent hello indicate GR capable build adjacency mark as GR capable A B sent helloreset hold timer control-plane resethold timer is counting down sent hello with restart bit set reset hold timer place A in local neighbor table send hello empty update with init & restart bit set setup for database resync resync database resync database continue normal operation continue normal operation
  • 21. OSPF Graceful Restart Two styles of OSPF Graceful Restart are available: Β§β€― Graceful Restart using link local signaling Β§β€― Graceful Restart using opaque link-state advertisements (LSAs)
  • 22. Normal OSPF Restart 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.22 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane resethold timer is counting down send hello with an empty neighbor list reset adjacency place new neighbor in neighbor list send hello with router-id of new neighbor send hello with router-id of new neighbor place new neighbor in neighbor list exchange databases exchange databases continue normal operation continue normal operation negotiate db exchange negotiate db exchange
  • 23. OSPF Graceful Restart using Link Local Signalling This method of signaling GR, described in the IETF Internet-Draft, β€œOSPF Restart Signaling,” (draft-nguyen-ospf-restart-04.txt) relies on two mechanisms: Β§β€― Link Local Signaling (LLS) a mechanism described in the IETF Internet-Draft, β€œOSPF Link-local Signaling” (draft-nguyen-ospf-lls-02.txt). This draft extends the OSPF hello packet format to include TLVs, which can then be used to include additional signaling of various types, such as graceful restart capability and a graceful restart. Β§β€― Out of Band Resynchronization a mechanism described in the IETF Internet-Draft, β€œOSPF Out-of-Band LSDB Resynchronization” (draft-nguyen-ospf-oob-resync-04.txt). This draft describes a mechanism through which two OSPF routers can resynchronize their link-state databases at any point.
  • 24. OSPF Graceful Restart using Link Local Signalling 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.24 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane resethold timer is counting down send hello with an empty neighbor list & the RS bit set reset hold timer place new neighbor in neighbor list send hello with router-id of restarting neighbor send hello with router-id of new neighbor place new neighbor in neighbor list exchange databases using out of band sync exchange databases using out of band sync continue normal operation continue normal operation negotiate db exchange negotiate db exchange
  • 25. OSPF Graceful Restart using Opaque LSA 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.25 normally operating neighbor relationship normally operating neighbor relationship A B sent helloreset hold timer control-plane reset send Grace LSA exchange databases using Grace LSA exchange databases using Grace LSA continue normal operation continue normal operation reset hold timer GR timer couting down
  • 26. Fast Down Detection 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.26
  • 27. Fast Down Detection Before you can route around a failed link or device, however, you need to detect its failure. Detecting failure is a major concern in the highly available network. You can detect a neighbor or link failure in two ways: Β§β€― Polling through fast hellos or other packets, transmitted at Layer 2 or Layer 3 Β§β€― Event-driven notification through monitoring some link property, such as the link carrier
  • 28. Detecting a Link or Adjacency Failure Using Polling One common method to detect a link or adjacency failure is polling, or periodically sending hello packets to the adjacent device and expecting a periodic hello packet in return. The two determining factors in the speed at which polling can discover a failed link or device are as follows: Β§β€― The rate at which hello packets are transmitted Β§β€― The number of hello packets missed before declaring a link or adjacency as failed
  • 29. How Fast Does Polling Detect a Down Neighbor ? A Bhellos transmitted A B C D last hellos transmitted 10 second hello interval 30 second hold interval E F
  • 30. Fast hellos Using faster times than the defaults in most protocols: Β§β€― OSPF can transmit a hello every 330 milliseconds and set the dead interval to 1 second ip ospf dead-interval minimal hello-multiplier multiplier Β§β€― IS-IS can transmit a hello every 330 millisecond and set the dead interval to 1 second isis hello-interval minimal [level-1 | level-2] isis hello-multiplier multiplier [level-1 | level-2] the hello multiplier is set to 3 by default. Β§β€― EIGRP can transmit a hello every second and set the dead interval to 3 sec ip hello-interval eigrp [autonomous system] [seconds] ip hold-time eigrp [autonomous system] [seconds]
  • 31. Bidirectional Forwarding Detection - BFD What's BFD ? Β§β€― Lightweight hello protocol designed to run over multiple transport protocols Β§β€― Designed for sub-second Layer 3 failure detection Β§β€― Any interested client Β§β€― EIGRP Β§β€― IS-IS Β§β€― OSPF Β§β€― etc registers with BFD and is notified as soon as BFD detects a neighbor loss Β§β€― All registered clients benefit from uniform failure detection Β§β€― Runs on physical, virtual and bundle interfaces Β§β€― Uses UDP port 3784 / 3785 (for echo)
  • 32. BFD in a distributed router architechture Route Processor OSPF IS-IS Telnet SNMP BFD Master Linecard BFD Agent FIB Downloader Linecard BFD Agent FIB Downloader Linecard BFD Agent FIB Downloader
  • 33. Event-driven notification through monitoring link Rather than periodically polling rely on event-driven notification of link failures. Rely on lower-layer devices to monitor the link status and notify the routing protocol when the link fails. Β§β€― SONET/SDH Β§β€― DWDM probably the best known of the fast convergence technologies available; it not only allows the fast detection of down links and devices, but it also provides for link protection, which allows traffic to quickly be switched to a backup fiber link if the primary path fails. APS protected link unprotected link
  • 34. Exponential Backoff 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.34
  • 35. Exponential Backoff in Link-State Protocols step 2 2nd link flap step 1 1st link flap initial timer set to 1 sec send notification add increment of 1 sec and set timer here send notification double time and set timer here step 3 3d link flap send notification set timer to max of 5 sec A B C flapping link step 4 set timer to initial 2x maximum (10 seconds)
  • 36. Exponential Backoff in Link-State Protocols (cont) Exponential backoff mechanizm can be applied to two different timers in link-state protocols: Β§β€― The Link-state generation timer, the case just examined Β§β€― The SPF timer, which determines how often a router runs the SPF algorithm in response to changes in the network
  • 37. OSPF Exponential Backoff for LSA Generation OSPF exponential backoff for LSA generation is called LSA throttling Two configuration commands are related to this capability: Β§β€― timers throttle lsa all [start-interval] [hold-interval] [max-interval] start-interval is the initial time hold-interval is the increment max-interval is the maximum time Β§β€― timers lsa arrival [milliseconds] the rate at which a router accepts LSAs with the same LSA-ID
  • 38. OSPF Exponential Backoff for Running SPF OSPF exponential backoff for SPF is implemented as OSPF SPF throttling Β§β€― timers throttle spf spf-start spf-hold spf-max-wait Β§β€― spf-start is the initial SPF schedule delay in milliseconds Β§β€― spf-hold is the minimum hold time between two consecutive SPF calculations Β§β€― spf-max-wait is the maximum wait time between two consecutive SPF calculations
  • 39. IS-IS Exponential Backoff for Running SPF IS-IS also implements exponential backoff as throttling Three commands are used to configure: Β§β€― LSP generation lsp-gen-interval [level-1 | level-2] lsp-max-wait [lsp-initial-wait lsp-second-wait] Β§β€― SPF run spf-interval [level-1 | level-2] spf-max-wait [spf-initial-wait spf-second-wait] Β§β€― PRC throttling prc-interval prc-max-wait [prc-initial-wait prc-second-wait]
  • 40. Speeding up route selection 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.40
  • 41. Calculating the Route Faster Another area where the convergence speed of a network could be decreased is in route calculation. How long does it take to calculate the best path to a destination in the network after you have detected and reported an event ? Consider tuning: Β§β€― feasible successors in EIGRP Β§β€― link-state partial SPF Β§β€― link-state incremental SPF
  • 42. EIGRP Feasible Successors EIGRP calculates not only the best path to each reachable destination but also feasible successors, which are known as loop-free routes to the same destination. The route to 172.17.1.0/24 Β§β€― through 172.17.3.1 has reported distance of 2167296 Β§β€― through 172.18.8.4 feasible distance of 2172416 router#show ip eigrp topo 172.17.1.0 IP-EIGRP (AS 100): Topology entry for 172.17.1.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416 Routing Descriptor Blocks: 172.17.2.1 (Serial0/0), from 172.18.8.4, Send flag is 0x0 Composite metric is (2172416/18944), Route is Internal .... 172.17.1.0 (Serial0/3), from 172.17.3.1, Send flag is 0x0 Composite metric is (2684416/2167296), Route is Internal Because the reported distance through 172.17.3.1 is less than the feasible distance through 172.18.8.4, the route through 172.17.3.1 must be loop free. It is a feasible successor.
  • 43. How EIGRP determines that a nonfeasible successor is loop free It always takes time to query neighbors and to receive replies which slows down network convergence. Apply this knowledge to network design by considering not only the best path to each destination from a given area in the network but also where the feasible successors are and how to tweak the metrics so that you have a feasible successor where possible.
  • 44. How EIGRP determines that a nonfeasible successor is loop free (cont) One such possible situation with a pair of equal cost links: Β§β€― A to B link Β§β€― A to C link router-b#show ip eigrp topo 172.17.1.0 IP-EIGRP (AS 100): Topology entry for 172.17.1.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416 Routing Descriptor Blocks: 10.1.1.1 (Serial0/0), from 10.1.1.1, Send flag is 0x0 Composite metric is (2172416/18944), Route is Internal .... 10.3.3.1 (Serial0/3), from 10.1.3.1, Send flag is 0x0 Composite metric is (2684416/2172416), Route is Internal The feasible distance through Router A is equal to the reported distance through Router C, so the route through Router C is not considered a feasible successor. If the Router A to B link or the Router A to C link fails, at least one query is required to re-converge. 172.17.1.0/24 B C A 10.1.1.1 10.1.2.1 10.1.3.1
  • 45. Modifying the Delay to Create an EIGRP-Feasible Successor Modifying the metrics on the Router A to C link by decreasing the delay slightly produces the results router-b#show ip eigrp topo 172.17.1.0 IP-EIGRP (AS 100): Topology entry for 172.17.1.0/24 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416 Routing Descriptor Blocks: 10.1.1.1 (Serial0/0), from 10.1.1.1, Send flag is 0x0 Composite metric is (2172416/18944), Route is Internal .... 10.1.3.1 (Serial0/3), from 10.1.3.1, Send flag is 0x0 Composite metric is (2684416/2167296), Route is Internal The reported distance through Router C is now lower than the feasible distance through Router A, so the path through Router C is considered a feasible successor. 172.17.1.0/24 B C A 10.1.1.1 10.1.2.1 10.1.3.1
  • 46. Link-State Partial SPF Three types of objects along directed graph built using SPF: Β§β€― Nodes Β§β€― Edges Β§β€― Leaves IS-IS treats all IP subnets as leaves off the SPF tree Β§β€― 172.17.1.0/24 leaf Β§β€― 172.17.2.0/24 leaf OSPF treats an external (redistributed) as leaves Β§β€― 172.17.1.0/24 leaf Β§β€― 172.17.2.0/24 treated as a node in OSPF (network statement) 172.17.1.0/24 B C A D 172.17.2.0/24 redistributed brought into OSPF through a network statement
  • 47. Node and a Leaf in the SPF Removing and adding leaf nodes without recalculating the entire SPF tree is called Partial SPF Β§β€― a feature of implementation of OSPF and IS-IS Β§β€― the distinction between a node and a leaf in the SPF matters !!! Β§β€― changes in leaves in the SPF tree do not cause a complete recalculation of the SPF tree Β§β€― if 172.17.1.0/24 fails it is simply removed from the SPF tree Β§β€― parts of the tree that contain the nodes A, B, C, and D are not impacted by this change 172.17.1.0/24 B C A D 172.17.2.0/24 redistributed brought into OSPF through a network statement
  • 48. Link-State Incremental SPF Incremental SPF takes the concept of a partial SPF one step further. If a specific piece of the SPF tree changes, rather than recalculating the entire tree recompute just a section of the tree Β§β€― link to router B fails Β§β€― no alternate path exists to router B Β§β€― it is unnecessary to recalculate the entire SPF tree Β§β€― Instead, SPF can safely remove the branch behind router B Β§β€― adjust the routing table accordingly without further calculations 172.17.1.0/24 B C A D E
  • 49. Link-State Incremental SPF (cont) In summary: Β§β€― iSPF is more efficient than the full SPF algorithm thereby allowing OSPF/IS-IS to converge faster Β§β€― iSPF also provides a significant advantage when the changes in the network topology are further away from the root of the SPT - the larger the network the more significant the impact Β§β€― iSPF provides greater improvements in convergence time for networks with a high number of nodes and links a segment of 400-1000 nodes should see improvements
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  • 51. Π–Π΄Π΅ΠΌ Π²Π°ΡˆΠΈΡ… сообщСний с Ρ…Π΅ΡˆΡ‚Π΅Π³ΠΎΠΌ #CiscoConnectRu CiscoRu Cisco CiscoRussia CiscoRu 23.11.15 Β© 2015 Cisco and/or its affiliates. All rights reserved.