Continuing on this topic – I have been reading through Cisco Press’ Fault Tolerant IP & MPLS Networks by Hussain and some interesting facts in the book. He cites a University of Michigan one-year reliability study of IP core routers conducted in a regional IP service provider network where the main causes of outages are listed as
- 23 percent for router failure (software/hardware faults, denial-of-service attack),
- 32 percent for link failures (fiber cuts, network congestion),
- 36 percent for router maintenance (software and hardware upgrade, configuration errors),
- The remaining 9 percent for other miscellaneous reasons.
He states that for carrier-class routers the following characteristics should apply.
- No single hardware fault should result in a loss or degradation of user traffic or a loss of control-plane and management functions.
- System downtime should be less than 5.256 minutes per year.
- Line cards, switching fabric, and control processor cards should be redundant with capability to monitor standby cards.
- The control-plane software/hardware module should not be a single point of failure, and the service (forwarding plane) should not be disrupted due to failure of the control plane.
- The router should be capable of service recovery from link/node failures.
By the way we suffered a carrier issue during last week when a 34MB SDH tributary card failed causing a number of hours downtime so it feels appropriate to cover this topic now.
NSF – Nonstop Forwarding works with the Stateful Switchover (SSO) feature in Cisco IOS software. NSF works with SSO to minimize the amount of time a network is unavailable to its users following a switchover. The main objective of Cisco NSF is to continue forwarding IP packets following a Route Processor (RP) switchover. It maintains and updates Layer 3 routing and forwarding information in the backup route processor. This ensures that the forwarding of IP packets and routing protocol information are continuous during the switchover and route convergence process. It eliminates router downtime, and increases network availability during scheduled maintenance of a route processor, or a route processor failure.
When a networking device restarts, all routing peers of that device detect that the device went down and then came back up. This transition results in what is called a routing flap, which could spread across multiple routing domains. Routing flaps caused by routing restarts create routing instabilities, which are detrimental to the overall network performance. Cisco NSF helps to suppress routing flaps in SSO-enabled devices, thus reducing network instability. Cisco NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored following a switchover. With Cisco NSF, peer networking devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards or dual forwarding processors (FPs) while the standby RP assumes control from the failed active RP during a switchover. The ability of line cards and FPs to remain up through a switchover and to be kept current with the Forwarding Information Base (FIB) on the active RP is key to Cisco NSF operation. During switchover, system control and routing protocol execution is transferred from the active processor to the standby processor. The time required by the device to switch over from the active to the standby processor ranges from just a few seconds to approximately 30 seconds, depending on the platform
Pre-requisites include that NSF must be configured on a networking device that has been configured for SSO. On platforms supporting the Route Switch Processor (RSP), and where the CEF switching mode is configurable, configure distributed CEF (dCEF) switching mode using the ip cef distributed command.
There are several restrictions for the various routing protocols such as they must be running an NSF software image, not supported on OSPF virtual links, HSRP does not work with NSF, etc. See the reference links below for more information.
From the CCIE SP Lab perspective it is only supported on the 7200 router on the lab equipment list.
Configuration Example: [For OSPF]
Router# show cef state <- Verifies that router is NSF capable
Router# configure terminal
Router(config)# router ospf 400
Router# show running-config <- Verifies NSF for OSPF
router ospf 120
network 192.168.20.0 0.0.0.255 area 0
network 192.168.30.0 0.0.0.255 area 1
network 192.168.40.0 0.0.0.255 area 2
Router> show ip ospf <- Verifies NSF for OSPF, look for the line “Non-Stop Forwarding enabled”
Note: Both Fast re-route and link/node protection will be covered under MPLS as they are part of the MPLS-TE Section.