Quick Convergence

IS-IS Fast Convergence Overview

IS-IS fast convergence enhances routing recalculation speed through extensions including I-SPF, PRC, intelligent timers, and LSP rapid flooding. Combined with BFD integration, these features enable sub-second failure detection and backup path activation via IS-IS Auto FRR.

I-SPF

I-SPF recalculates routes only for affected nodes during topology changes instead of processing all nodes. When new device R6 connects to R5, routers flood updated LSPs containing new neighbor relationships. Upon receiving these, R1 performs I-SPF calculations exclusively for R5 and R6, preserving the existing shortest path tree structure for unchanged nodes.

LSP Fast Flooding

This mechanism accelerates LSP distribution across the network. When newer LSPs are received, devices immediately flood a configured number of LSPs before route computation. Configuration example:

<Router-isis-config>
  flash-flood 25 max-timer-interval 100 level-2
</Router-isis-config>

The lsp-count parameter limits flooded LSPs per event, while max-timer-interval controls flooding delays.

Routing Control

IS-IS Routing Optimization

IS-IS provides granular route management through metrics adjustment, equal-cost routing, route leaking, default route advertisemment, external route injection, and filter policies to optimize forwarding paths and table sizes.

Equal-Cost Routing

Multiple equal-cost paths enable either:

• Load balancing across interfaces
• Priority-based path selection using weighted preferences

Priority values are set via:

<Router-isis-config>
  nexthop 192.168.1.5 weight 10
  nexthop 192.168.1.6 weight 20
</Router-isis-config&gt>

Lower wieghts indicate higher priority. During overload scenarios, paths are selected by: 1) Priority 2) System ID 3) Egress interface index.

Configuration Example

For dual-path load balancing between R1-R3 and R1-R2-R3:

<R1-isis-config>
  maximum load-balancing 2
</R1-isis-config>

For priority-based path selection:

<R1-isis-config>
  nexthop 10.0.13.3 weight 5
  nexthop 10.0.12.2 weight 10
</R1-isis-config>

Default Routing

Default routes are managed through ATT bit manipulation and explicit advertisement.

ATT Bit Control

Level-1-2 routers set ATT bits in LSPs when multiple areas are reachable via Level-2. Level-1 routers then generate default routes toward the ATT-set device. Configuration:

<Level12-Router-isis>
  attached-bit advertise always
</Level12-Router-isis>

<Level1-Router-isis>
  attached-bit avoid-learning
</Level1-Router-isis>

Default Route Advertisement

Border routers advertise 0.0.0/0 into IS-IS domains:

<BorderRouter-isis>
  default-route-advertise always cost 30
</BorderRouter-isis>

Additional Features

Multi-Instance and Multi-Process

Multiple IS-IS processes operate independently within single devices, enabling VPN segmentation and isolated routing domains.

LSP Fragmentation

Extended LSP fragmentation overcomes 256-fragment limits using virtual systems.

Fragmentation Mechanism

Virtual systems with additional System IDs generate supplementary LSP fragments. The IS Alias ID TLV (Type 24) maintains originator-virtual system relationships.

Configuration

<Router-isis-config>
  lsp-fragments-extend mode-1
  virtual-system 0000.0000.000A
</Router-isis-config>

Virtual System IDs must be domain-wide unique with maximum 50 per process.

Graceful Restart

IS-IS GR maintains forwarding during control-plane restarts using Restart TLVs and timers.

Restarting Process

For in-service upgrades (FIB preserved):

1. Restarter sends IIH packets with RR bit set
2. Helpers maintain adjacency and send full LSP sets
3. Synchronization completes when T2 timers expire

Starting Process

For device reboots (FIB rebuilt):

1. Restarter sends IIH with SA bit set
2. Helpers re-establish adjacencies
3. LSDB synchronization triggers SPF recalculation

Configuration

<Router-isis-config>
  graceful-restart
  graceful-restart no-impact-holdtime
  graceful-restart interval 400
</Router-isis-config>

Verification command: display isis graceful-restart status