- IPv6 Routing: EIGRP Support
- EIGRP MIB
- EIGRP MPLS VPN PE-CE Site of Origin
- EIGRP Support for Route Map Filtering
- EIGRP/SAF HMAC-SHA-256 Authentication
- EIGRP Wide Metrics
- EIGRP
- EIGRP IPv6 VRF-Lite
- EIGRP Stub Routing
- IP EIGRP Route Authentication
- Configuring EIGRP
- Add Path Support in EIGRP
- BFD Support for EIGRP IPv6
- EIGRP Classic to Named Mode Conversion
- EIGRP IPv6 NSF/GR
- EIGRP Loop-Free Alternate Fast Reroute
- EIGRP Support for 6PE/6VPE
- EIGRP Route Tag Enhancements
- Finding Feature Information
- Restrictions for EIGRP Loop-Free Alternate Fast Reroute
- Information About EIGRP Loop-Free Alternate Fast Reroute
EIGRP Loop-Free Alternate Fast Reroute
The EIGRP Loop-Free Alternate Fast Reroute feature allows the Enhanced Interior Gateway Routing Protocol (EIGRP) to reduce the routing transition time to less than 50 ms by precomputing repair paths or backup routes and installing these paths or routes in the Routing Information Base (RIB). Fast Reroute (FRR) is the mechanism that enables traffic that traverses a failed link to be rerouted around the failure. In EIGRP networks, precomputed backup routes or repair paths are known as feasible successors or loop-free alternates (LFAs). This module describes how to configure the EIGRP Loop-Free Alternate Fast Reroute feature and enable load-sharing and tie-breaking configurations for the feasible successors or LFAs that are identified by EIGRP.
- Finding Feature Information
- Restrictions for EIGRP Loop-Free Alternate Fast Reroute
- Information About EIGRP Loop-Free Alternate Fast Reroute
- How to Configure EIGRP Loop-Free Alternate Fast Reroute
- Configuration Examples for EIGRP Loop-Free Alternate Fast Reroute
- Additional References
- Feature Information for EIGRP Loop-Free Alternate Fast Reroute
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Restrictions for EIGRP Loop-Free Alternate Fast Reroute
Information About EIGRP Loop-Free Alternate Fast Reroute
Repair Paths Overview
When a link or a device fails, distributed routing algorithms compute new routes or repair paths. The time taken for this computation is called routing transition. Until the transition is complete and all devices are converged on a common view of the network, the connectivity between the source and destination pairs of devices is interrupted. Repair paths forward traffic during a routing transition.
When a link or a device fails, initially only the neighboring devices are aware of the failure. All other devices in the network are unaware of the nature and location of this failure until information about this failure is propagated through the routing protocol. The propagation of this information may take several hundred milliseconds. Meanwhile, packets affected by the network failure need to be steered to their destinations. A device adjacent to the failed link employs a set of repair paths for packets that would have used the failed link. These repair paths are used from the time the router detects the failure until the routing transition is complete. By the time the routing transition is complete, all devices in the network revise their forwarding data and the failed link is eliminated from the routing computation. Routing protocols precompute repair paths in anticipation of failures so that the repair paths can be activated the moment a failure is detected. In Enhanced Interior Gateway Routing Protocol (EIGRP) networks, precomputed repair paths or backup routes are known as feasible successors or loop-free alternates (LFAs).
LFA Computation
A loop-free alternate (LFA) is a precomputed next-hop route that delivers a packet to its destination without looping back. Traffic is redirected to an LFA after a network failure and the LFA makes the forwarding decision without any knowledge of the failure.
Interior Gateway Protocols (IGPs) compute LFAs in the following two ways:
Per-link (link-based) computation: In link-based LFAs, all prefixes (networks) that are reachable through the primary (protected) link share the same backup information. This means that the whole set of prefixes sharing the primary link also share the repair or the Fast Reroute (FRR) ability. The per-link approach protects only the next-hop address. It need not necessarily protect the destination node. Therefore, the per-link approach is suboptimal and not the best approach for capacity planning because all traffic from the primary link is redirected to the next hop instead of being spread over multiple paths. Redirecting all traffic to the next hop may lead to congestion on the link to the next hop
Per-prefix (prefix-based) computation: Prefix-based LFAs allow computing backup information per prefix (network) and protect the destination address. The per-prefix approach is preferred over the per-link approach because of its greater applicability and better bandwidth utilization. Per-prefix computations provide better load sharing and better protection coverage than per-link computations because per-prefix computations evaluate all possible LFAs and use tie-breakers to select the best LFA from among the available LFAs.
Note | The repair or backup information computed for a primary path by using prefix-based LFAs may be different from that computed by using link-based LFAs. |
EIGRP always computes prefix-based LFAs. EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate the successor and feasible successors. EIGRP uses the successor as the primary path and feasible successors as repair paths or LFAs.
LFA Tie-Breaking Rules
Lowest-repair-path-metric—Eliminates LFAs whose metric to the protected prefix is high. Multiple LFAs with the same lowest path metric may remain in the routing table after this tie-breaker is applied.
Shared Risk Link Group (SRLG)-disjoint—Eliminates LFAs that belong to any of the protected path SRLGs. SRLGs refer to situations where links in a network share a common fiber (or a common physical attribute). If one link fails, other links in the group may also fail. Therefore, links in a group share risks.
How to Configure EIGRP Loop-Free Alternate Fast Reroute
- Configuring LFA FRRs per Prefix
- Disabling Load Sharing Among Prefixes
- Enabling Tie-Breaking Rules for EIGRP LFAs
Configuring LFA FRRs per Prefix
Perform this task to configure loop-free alternate (LFA) Fast Reroutes (FRRs) per prefix in an Enhanced Interior Gateway Routing Protocol (EIGRP) network. You can enable LFAs for all available prefixes in the EIGRP topology or for prefixes specified by route maps.
1.
enable
2.
configure
terminal
3.
router eigrp
virtual-name
4.
address-family ipv4 autonomous-system
autonomous-system-number
5.
topology base
6.
fast-reroute per-prefix {all | route-map
route-map-name}
7.
end
8.
show ip eigrp topology frr
DETAILED STEPS
Disabling Load Sharing Among Prefixes
When the primary path is an Equal Cost Multipath (ECMP) path with multiple LFAs, prefixes (networks) are distributed equally among the LFAs because the default behavior for ECMP paths is load sharing. However, you can control the selection of LFAs by enabling tie-breaking configurations. To enable tie-breaking configurations, you should disable load sharing among prefixes. Perform this task to disable load sharing among prefixes.
1.
enable
2.
configure
terminal
3.
router eigrp
virtual-name
4.
address-family ipv4 autonomous-system
autonomous-system-number
5.
topology base
6.
fast-reroute load-sharing disable
7.
end
8.
show ip eigrp topology frr
DETAILED STEPS
Enabling Tie-Breaking Rules for EIGRP LFAs
Perform this task to enable tie-breaking rules to select a single loop-free alternate (LFA) when there are multiple LFAs for a given primary path. The Enhanced Interior Gateway Routing Protocol (EIGRP) allows you to use four attributes to configure tie-breaking rules. Each of the following keywords of the fast-reroute tie-break command allows you to configure a tie-breaking rule based on a specific attribute: interface-disjoint, linecard-disjoint, lowest-backup-path-metric, and srlg-disjoint. You can assign a priority value for each attribute. Tie-breaking rules are applied on the basis of the priority assigned to each attribute. The lower the assigned priority value the higher the priority of the tie-breaking attribute.
1.
enable
2.
configure
terminal
3.
router eigrp
virtual-name
4.
address-family ipv4 autonomous-system
autonomous-system-number
5.
topology base
6.
fast-reroute tie-break {interface-disjoint | linecard-disjoint | lowest-backup-path-metric | srlg-disjoint}
priority-number
7.
end
8.
show ip eigrp topology frr
DETAILED STEPS
Configuration Examples for EIGRP Loop-Free Alternate Fast Reroute
- Example: Configuring LFA FRRs Per Prefix
- Example: Disabling Load Sharing Among Prefixes
- Example: Enabling Tie-Breaking Rules
Example: Configuring LFA FRRs Per Prefix
The following example shows how to configure Enhanced Interior Gateway Routing Protocol (EIGRP) loop-free alternate (LFA) Fast Reroutes (FRRs) for prefixes specified by the route map named map1:
Device> enable Device# configure terminal Device(config)# router eigrp name Device(config-router)# address-family ipv4 autonomous-system 1 Device(config-router-af)# topology base Device(config-router-af-topology)# fast-reroute per-prefix route-map map1 Device(config-router-af-topology)# end
Example: Disabling Load Sharing Among Prefixes
Device> enable Device# configure terminal Device(config)# router eigrp name Device(config-router)# address-family ipv4 autonomous-system 1 Device(config-router-af)# topology base Device(config-router-af-topology)# fast-reroute load-sharing disable Device(config-router-af-topology)# end
Example: Enabling Tie-Breaking Rules
The following examples show how to enable tie-breaking configurations to allow the Enhanced Interior Gateway Routing Protocol (EIGRP) to select a loop-free alternate (LFA) when there are multiple candidate LFAs for a given primary path. The following example shows how to enable the tie-breaking rule that eliminates LFAs that share the outgoing interface with the primary path:
Device> enable Device# configure terminal Device(config)# router eigrp name Device(config-router)# address-family ipv4 autonomous-system 1 Device(config-router-af)# topology base Device(config-router-af-topology)# fast-reroute tie-break interface-disjoint 2 Device(config-router-af-topology)# end
The following example shows how to enable the tie-breaking rule that eliminates LFAs that share the linecard with the primary path:
Device> enable Device# configure terminal Device(config)# router eigrp name Device(config-router)# address-family ipv4 autonomous-system 1 Device(config-router-af)# topology base Device(config-router-af-topology)# fast-reroute tie-break linecard-disjoint 3 Device(config-router-af-topology)# end
The following example shows how to enable the tie-breaking rule that selects the LFA with the lowest metric to the the protected prefix:
Device> enable Device# configure terminal Device(config)# router eigrp name Device(config-router)# address-family ipv4 autonomous-system 1 Device(config-router-af)# topology base Device(config-router-af-topology)# fast-reroute tie-break lowest-backup-path-metric 4 Device(config-router-af-topology)# end
The following example shows how to enable the tie-breaking rule that eliminates LFAs that share any SRLGs with the primary path:
Device> enable Device# configure terminal Device(config)# router eigrp name Device(config-router)# address-family ipv4 autonomous-system 1 Device(config-router-af)# topology base Device(config-router-af-topology)# fast-reroute tie-break srlg-disjoint 1 Device(config-router-af-topology)# end
Additional References
Related Documents
Related Topic |
Document Title |
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Cisco IOS commands |
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EIGRP commands |
Technical Assistance
Description |
Link |
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Feature Information for EIGRP Loop-Free Alternate Fast Reroute
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Name |
Releases |
Feature Information |
---|---|---|
EIGRP Loop-Free Alternate Fast Reroute |
15.2(1)SY1 |
The EIGRP Loop-Free Alternate Fast Reroute feature allows the Enhanced Interior Gateway Routing Protocol (EIGRP) to reduce the routing transition time to less than 50 ms by precomputing repair paths or backup routes and installing these paths or routes in the Routing Information Base (RIB). In EIGRP networks, the precomputed backup routes are known as feasible successors or loop-free alternates (LFAs). The following commands were introduced or modified: debug eigrp frr, fast-reroute load-sharing disable (EIGRP), fast-reroute per-prefix (EIGRP), fast-reroute tie-break (EIGRP), and show ip eigrp topology. |