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Beginner

ospf question

Hi All

Is there anyway with OSPF to know what's the backup route for a specific destination like eigrp does with the "sh ip eigrp topo [subnet]" ?

Thanks

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Hall of Fame Cisco Employee

Hello,

In general, I do not believe that in a normal OSPF implementation, you would be able to see a backup path simply because the SPF does not compute any backup paths by itself. EIGRP uses an approximative condition, or a heuristic, to select a neighbor guaranteedly providing a loop-free path towards a destination; any neighbor passing this condition is a prospective next hop on a (backup) path towards that destination. OSPF does not use this; instead, it uses a deterministic algorithm that systematically extends the shortest path found so far, not producing any prospective backup paths in the process.

There are extensions the OSPF that focus on preparing backup paths for local failures beforehand, so that when a particular failure occurs, the backup path is already prepared. These are, however, relatively complex and are not universally supported yet. Look for their description here:

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_ospf/configuration/15-s/iro-lfa-frr.html

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_ospf/configuration/15-s/iro-ipfrr-lfa.html

Best regards,

Peter

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Highlighted
Beginner

show ip route [subnet]

Highlighted

Sorry I should have specified it; I also want to know even if the route has a different cost than the first put in routing table.

I won't see this information in the routing table.  The only time I'll see this information in the routing table is when both route has the same cost.

Highlighted
Hall of Fame Cisco Employee

Hello,

In general, I do not believe that in a normal OSPF implementation, you would be able to see a backup path simply because the SPF does not compute any backup paths by itself. EIGRP uses an approximative condition, or a heuristic, to select a neighbor guaranteedly providing a loop-free path towards a destination; any neighbor passing this condition is a prospective next hop on a (backup) path towards that destination. OSPF does not use this; instead, it uses a deterministic algorithm that systematically extends the shortest path found so far, not producing any prospective backup paths in the process.

There are extensions the OSPF that focus on preparing backup paths for local failures beforehand, so that when a particular failure occurs, the backup path is already prepared. These are, however, relatively complex and are not universally supported yet. Look for their description here:

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_ospf/configuration/15-s/iro-lfa-frr.html

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_ospf/configuration/15-s/iro-ipfrr-lfa.html

Best regards,

Peter

View solution in original post

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