IGP Flexible Algorithm in IP Networks
Feature Name |
Release Information |
Feature Description |
IGP IP Flexible Algorithm for IS-IS Protocol |
Release 7.6.1 |
With IS-IS protocol extensions supporting Interior Gateway Protocol (IGP) Flexible Algorithm (Flex-Algorithm) on the IP data plane, you can now use the Algorithm to calculate IGP paths in an IP network without running Segment Routing. The IGP Flex-Algorithm allows for user-defined algorithms where the IGP computes paths based on a user-defined combination of metric type and constraints. Earlier, you could calculate IGP only using the Shortest Path First (SPF), which meant that you didn't have any choice except to use the default IGPpath calculated based on a native IGP metric. The following command is introduced: The following commands are modified: |
Flexible Algorithm allows operators to customize IGP shortest path computation according to their own needs. The Flexible Algorithm allows Link State IGPs (OSPF, ISIS) to compute paths using various constraints. Each Flexible Algorithm represents the triplet (Calculation-Type, Metric-Type, Constraints) which all routers in the area consistently elect using the defined selection algorithm.
In IP networks, the router uses IGP Flexible Algorithm by computing the paths to the IPv4 address [RFC0791] and IPv6 address [RFCS8200].
Each interface may be associated with one or more IP addresses, and each IP address may be associated with one Flexible Algorithm.
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Packets sent to an address that is associated with a Flexible Algorithm follow the constraint-based path as calculated by the Flexible Algorithm.
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Packet sent to an address, that is not associated with a Flexible Algorithm, follow the IGP least-cost path to the egress node.
IGP IP Flexible Algorithm allows forwarding of the voice traffic and data traffic over different paths in the IP network. It can, for example, provide a low latency path for the voice traffic.
Note |
This feature is supported on Cisco Network Convergence System 5700 Series routers and routers with the Cisco NC57 line cards operating in native mode. |
This document describes the IS-IS protocol extensions to support IGP Flexible Algorithm in IP networks.
Prerequisites for IP Flexible Algorithm
Configure the data-plane ip command under IGP flex-algo sub-mode to enable participation of the router with the IP Flexible Algorithm.
Flexible Algorithm Definition
The set consisting of (a) calculation-type, (b) metric-type and (c) a set of constraints is referred to as a Flexible-Algorithm Definition.
Flexible-Algorithm is a numeric identifier in the range 128-255 that is associated via provisioning with the Flexible-Algorithm Definition.
To guarantee the loop free forwarding for paths computed for a particular Flex-Algorithm, all routers that are configured to participate in a particular Flex-Algorithm, and in the same Flex-Algorithm definition advertisement scope must agree on the definition of the Flex-Algorithm as in ietf-lsr-flex-algo.
Flexible Algorithm Definition Advertisement
To guarantee the loop free forwarding for paths computed for a particular Flexible Algorithm, all routers in the network must share the same definition of the Flexible Algorithm. This is achieved by dedicated router(s) advertising the definition of each Flexible Algorithm. Such advertisement is associated with the priority to make sure that all routers agree on a single and consistent definition for each Flexible Algorithm.
Definition of Flexible Algorithm includes:
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Metric type
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Affinity constraints
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Exclude SRLG constraints
To enable the router to advertise the definition for the particular Flexible Algorithm, advertise-definition command is used. At least one router in the area, preferably two for redundancy, must advertise the Flexible Algorithm definition. Without the valid definition being advertised, the Flexible Algorithm are not be functional.
IP Flexible Algorithm Prefix Advertisement
The IPv4 and IPv6 Algorithm Prefix Reachability TLVs defined in draft-ietf-lsr-ip-flexalgo are used to advertise prefix reachability associated with a Flexible Algorithm.
IP Flexible Algorithm Participation
Each application using the Flexible Algorithm must use its own participation signaling. IP Flexible Algorithm uses ISIS IP Algorithm Sub-TLV as specified in draft-ietf-lsr-ip-flexalgo.
Computing IP Flexible Algorithm Paths
Each Flexible Algorithm maintains a separate set of paths—One set per each data-plane.
The IP Flexible Algorithm computation uses only the IP Flex-Algorithm prefixes that are advertised in IPv4 and IPv6 Algorithm Prefix Reachability TLV.
Note |
The performance impact is proportional to the number of IP Flexible Algorithms in the participating router. Routers using the algorithm may use additional CPU cycles to:
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IP Flexible Algorithm Forwarding
IP Flexible Algorithm uses the base IPv4 and IPv6 packets for forwarding.
IP Flexible Algorithm prefixes are advertised in IGPs. The forwarding plane installs the IP Flex-Algorithm prefixes advertised by the receiving routers participating in the associated topology and algorithm.
The IP Flex-Algorithm prefixes can be protected by local LFA. When the prefix is associated with an algorithm, the LFA paths to such a prefix are calculated using the Flexible Algorithm in the associated topology. Thus, ensuring that the algorithm follows the same constraints as the calculation of the primary paths.
The IP Flex-Algorithm prefixes can be protected by TI-LFA. For more information on protecting the IP Flex-Algorithm prefixes, see Protecting Flexible Algorithm IP Prefixes.