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Configuration Guide - IP Unicast Routing

S7700 and S9700 V200R011C10

This document describes IP Unicast Routing configurations supported by the switch, including the principle and configuration procedures of IP Routing Overview, Static Route, RIP, RIPng, OSPF, OSPFv3, IS-IS(IPv4), IS-IS(IPv6), BGP, Routing Policy ,and PBR, and provides configuration examples.
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Association Between OSPF and Other Protocols

Association Between OSPF and Other Protocols

BFD for OSPF

Bidirectional Forwarding Detection (BFD) detects communication faults between forwarding engines. BFD monitors the connectivity of a data protocol on a path between two systems. This path can be a tunnel, a physical link, or a logical link.

BFD for OSPF enables BFD sessions to be associated with OSPF. If a BFD session detects a link fault, it notifies OSPF of the fault, allowing OSPF to quickly respond to the change in network topology.

Devices re-calculate routes in the event of a link fault or topology change. Network performance is improved if the route convergence process of routing protocols is completed faster.

When BFD is associated with OSPF, BFD can speed up OSPF route convergence if a fault occurs between neighbors.

Table 5-14  Comparison before and after BFD for OSPF is enabled

BFD Association

Link Fault Detection Conditions

Convergence Speed

No association

OSPF Dead timer expires. The default timeout period is 40 seconds.

Measured in seconds

Associated

BFD session goes Down.

Measured in milliseconds

Figure 5-53 shows the process of BFD for OSPF.

Figure 5-53  BFD for OSPF

The process of BFD for OSPF is:

  1. OSPF neighbor relationships are established between switches (SwitchA, SwitchB, and SwitchC in the preceding figure).

  2. A full neighbor relationship triggers BFD, which then establishes a BFD session.

  3. The outbound interface of the route from SwitchA to SwitchB is GE2/0/0. If the link fails, BFD detects the fault and then notifies SwitchA of the fault.

  4. SwitchA processes the Down neighbor relationship event, and then re-calculates routes. Following re-calculation, the outbound interface of the route becomes GE1/0/0, with the route to SwitchB traversing SwitchC.

OSPF-BGP Synchronization

A device restart, or even a device itself, may cause the loss of network traffic during BGP convergence. This happens because OSPF converges faster than BGP.

The solution to this problem is synchronization between OSPF and BGP.

If there is a backup link, BGP traffic is lost during traffic switchback because BGP route convergence is slower than OSPF route convergence.

In Figure 5-54, SwitchA, SwitchB, SwitchC, and SwitchD are running OSPF and have established IBGP connections. SwitchC functions as the backup of SwitchB. When the network is stable, BGP and OSPF routes converge completely on the device.

Traffic from SwitchA to 10.3.1.0/30 normally passes through SwitchB. If SwitchB becomes faulty, traffic is switched to SwitchC. After SwitchB recovers, traffic is switched back to SwitchB.

During the switchback process, packets are lost because IGP route convergence is faster than BGP route convergence. This means that the convergence of OSPF routes is already complete while BGP routes are still converging. As a result, SwitchB does not learn the route to 10.3.1.0/30. Therefore, upon receipt of packets from SwitchA to 10.3.1.0/30, SwitchB discards these packets.

Figure 5-54  OSPF-BGP synchronization

A device that has OSPF-BGP synchronization enabled remains a stub router within the set synchronization period. The link metric in the LSA advertised by the device is the maximum value 65535. Therefore, the device instructs other OSPF devices not to use it for data forwarding.

In Figure 5-54, OSPF-BGP synchronization is enabled on SwitchB. SwitchA continues to use the backup link SwitchC until BGP route convergence on SwitchB is complete.

OSPF-LDP Synchronization

On networks that use primary and backup links, OSPF-LDP synchronization ensures that, when a faulty primary link recovers, traffic interruptions are minimized.

As shown in Figure 5-55, the primary link travels along the path PE1→P1→P2→P3→PE2, and the backup link travels along the path PE1→P1→P4→P3→PE2.

When the primary link is faulty, traffic is switched to the backup link. After the primary link recovers, traffic is switched back to the primary link. During this process, traffic is interrupted for an extended period of time.

Figure 5-55  OSPF-LDP synchronization

Synchronizing LDP and IGP on P1 and P2 can shorten the duration of traffic interruption caused by switching traffic from the backup link to the primary link. OSPF-LDP synchronization shortens this duration from seconds to milliseconds.

OSPF-LDP synchronization delays route switchback by suppressing the establishment of IGP neighbor relationships until LDP convergence is complete. Before an LSP is established on the primary link, the backup link continues to forward traffic. The backup link is then deleted following LSP establishment.

Synchronization of LDP and OSPF involves three timers:

  • Hold-down

  • Hold-max-cost

  • Delay

After the primary link recovers, a router responds as follows:

  1. Starts the hold-down timer. The IGP interface does not establish IGP neighbors but waits for the establishment of an LDP session. The hold-down timer specifies the period that the IGP interface waits.

  2. Starts the hold-max-cost timer after the hold-down timer expires. The hold-max-cost timer specifies the interval for advertising the maximum link metric of the interface in the Link State Advertisement (LSA) to the primary link.

  3. Starts the delay timer to allow time for establishing an LSP after an LDP session is re-established for the faulty link.

  4. After the delay timer expires, LDP notifies IGP that synchronization is complete regardless of the status of IGP.

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Updated: 2019-04-01

Document ID: EDOC1000178324

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