Redistribution

General Notes

  • Routers doing mutual redistribution will not readvertise routes between protocols (Ex: EIGRP > OSPF > EIGRP)
  • Routes will not be redistributed if they don’t exist in the RIB
  • Not possible to create loops with a single redistribution point
  • EIGRP and OSPF will not accept routes advertised from their own router ID for loop prevention
  • Tags can be used to prevent loops caused by advertising routes from one domain back into the same domain
  • Modifying AD can prevent loops caused by higher AD protocols being redistributed into lower AD protocols and then preferred through that protocol
  • Normally applied with route-map to advertise specific routes, set tags, etc

Redistribution per Protocol

  • EIGRP
    • Redistribution into EIGRP can be done anywhere in the topology
    • Redistributed routes are marked external, AD of 170
    • When redistributing into EIGRP, must assign metric either directly or with the default-metric command
  • OSPF
    • Redistribution can happen anywhere aside from stubby, totally stubby networks - redistributing router becomes ASBR and has type 4 LSA advertised to any areas outside of its own local area to ensure reachability to border router
    • External routes will be LSA 5 in regular area, LSA 7 in NSSA and translated to type 5 at ABR
    • By default routes will be external type 2, e1 always preferred over e2 in OSPF algorithm
    • Must use the subnets flag or only classful routes will be redistributed
  • BGP
    • Redistribution from BGP can be dangerous if using eBGP as IGPs will not be able to handle a full BGP table
    • Must manually match externals if redistributing OSPF, otherwise only internal routes redistributed

Useful show commands

  • ip route profile - Shows changes in routing topology, useful to see route flapping

Standard Redistribution Config

RouteMaps

ip prefix-list PL_REDIST seq 5 permit 10.10.10.0/24 ge 28
!
route-map RM_REDIST permit 10  
match ip address prefix-list PL_REDIST 
set tag 100
!
redistribute connected route-map RM_REDIST

EIGRP

redistribute connected
redistribute static
redistribute ospf 1 metric 10000 100 255 1 1500
default-metric 10000 100 255 1 1500

OSPF

redistribute connected metric 10 subnets
redistribute static metric 200 metric-type 1 subnets
redistribute eigrp 1 metric 100 subnets
distance ospf external 171 ! for AD manipulation if required

BGP

redistribute connected
redistribute static
redistribute ospf 1 match external 1 external 2s