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Configuration Guide - IP Unicast Routing

CloudEngine 8800, 7800, 6800, and 5800 V200R005C10

This document describes the configurations of IP Unicast Routing, including IP Routing, Static Route, RIP, RIPng, OSPF, OSPFv3, IPv4 IS-IS, IPv6 IS-IS, BGP, Routing Policy, and PBR.
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Association Between LDP and Static Routes

Association Between LDP and Static Routes

Background

If a backup path exists between LSRs, packet loss may occur due to status inconsistency between static routes and LDP sessions. To resolve this issue, you can configure association between LDP and static routes.

Typical Networking

Figure 2-3 shows the typical networking of association between LDP and static routes. LSRA and LSRD interwork through static routes. Primary and secondary static routes are deployed on LSRA, with the next-hop devices being LSRB and LSRC, respectively. Primary and secondary LDP LSPs are established based on the static routes. The primary LSP uses Link A, and the secondary LSP uses Link B. In normal cases, Link A is preferred. Association between LDP and static routes in switchover and switchback scenarios is described as follows.

Figure 2-3 Networking diagram for LSP switching scenario where association between LDP and static routes is configured

Switchover scenario

In a switchover scenario, traffic of the primary static route is not switched to the secondary link when the LDP session on the primary link fails (not because of a link fault). As a result, traffic on the LSP over the primary link is interrupted.

After an LDP session is established, LSP traffic travels along the primary link Link A (LSRA → LSRB → LSRD). If the LDP session between LSRA and LSRB is interrupted, traffic of the primary LSP is immediately switched to the secondary link Link B (LSRA → LSRC → LSRD). However, because the link between LSRA and LSRB is normal, traffic of the primary static route is not switched to the secondary link. The asynchronous state between LDP and the primary static route causes an LSP traffic interruption.

If association between LDP and static routes is enabled, traffic is automatically switched to the secondary link when the LDP session goes Down, ensuring uninterrupted traffic forwarding.

Switchback scenario

In a switchback scenario, when the primary link recovers from a fault, the traffic of the primary static route is switched back to Link A earlier than the traffic of the primary LSP because the convergence of static routes is faster than that of LDP LSPs. As a result, the secondary LSP on Link B cannot be used, and the LSP on Link A has not been set up yet. As a result, LSP traffic is interrupted.

If the link between LSRA and LSRB is faulty, traffic is switched immediately to the secondary link Link B (LSRA → LSRC → LSRD). After the link between LSRA and LSRB recovers, traffic of the primary static route is immediately switched back to the primary link Link A (LSRA → LSRB → LSRD). However, the secondary LSP cannot be used, and the LSP on Link A has not recovered yet. As a result, traffic is interrupted.

If association between LDP and static routes is enabled, the static route on Link A becomes active only when the LDP session on Link A goes Up. In this manner, the states of the primary static route and LSP are asynchronous during the switchback, which prevents traffic loss.

Application Scenario

Association between LDP and static routes applies to scenarios where a static route backup path exists between LSRs.

Benefits

Association between LDP and static routes ensures status consistency between LDP and static routes, prevents traffic loss, and improves network reliability.

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Updated: 2019-04-20

Document ID: EDOC1100074760

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