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Redistributing from OSPF & EIGRP into BGP

richard.foster
Level 1
Level 1

If I apply the following configuration 10.1.0.0/17 is correctly filtered (not shown in the BGP table):

ip prefix-list FILTER deny 10.1.0.0/17
ip prefix-list FILTER permit 0.0.0.0/0 le 32

router eigrp 100
distribute-list prefix FILTER out eigrp 100

router bgp 100
redistribute eigrp 100

If I follow the same approach with OSPF it doesn't work (I see no matches against the prefix-list and 10.1.0.0/17 is in the BGP table).  I tried the following two ways:

router ospf 1
distribute-list prefix FILTER out bgp 100
distribute-list prefix FILTER out ospf 1

Whereas if I apply a distribute list in BGP the OSPF filtering does work:

ip prefix-list FILTER deny 10.1.0.0/17
ip prefix-list FILTER permit 0.0.0.0/0 le 32

router bgp 100
redistribute ospf 1 metric 50 match external 2
distribute-list prefix FILTER out ospf 1


These discrepancies frustrate me!  Can anyone please explain why this is?  Thanks.

1 Accepted Solution

Accepted Solutions

Hello Richard,

if you are redistributing into BGP the right place to implement route filtering is the BGP process the line red OSPF allows for invoking a route-map

What you see with EIGRP is the effect of

distribute-list prefix FILTER out

this has a direct effect on EIGRP domain: the filtered prefix is not sent out by this node in EIGRP updates

this can be seen as a side effect of this command

For OSPF only meaningful cases are:

distribute-list in

or

distribute-list out xx

that is it applies for routes redistributed from source-protocol into OSPF not the opposite

hint:

the filtered prefix is still sent out in OSPF updates

for this reason the commands are ineffective for redistribution of OSPF into BGP

see

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800943c5.shtml

Hope to help

Giuseppe

View solution in original post

4 Replies 4

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Richard,

OSPF is link state EIGRP is distance vector

so the use of distribute-list with OSPF provides different effects

An OSPF router is constrained in having an OSPF database in sync with the OSPF peers in same area

distribute-list in can be used to avoid to install an OSPFroute in the IP routing table but does not change the DB content (no filter in DB)

distribute-list out can work in conjunction of routes being redistributed into OSPF

so if you have

router ospf 10

red bgp 100

distribute-list out prefix FILTER out

that is the command distribute is not effective alone and applies to routes  coming from BGP in deciding what OSPF LSA 5 external routes to inject in OSPF domain.

when performing redistribution you add a point where route filtering is possible and BGP again has DV properties

Hope to help

Giuseppe

Hi Giuseppe,

In this scenario I'm trying to filter routes as they leave the IGP.  With EIGRP an outbound distribute-list affects the routes redistributed, or received into BGP.  When I try a similar configuration with OSPF it doesn't work - it does work when I use a distribute-list in BGP.

When redistributing routes out of EIGRP or OSPF into BGP would the behavior not be similar as it's just prefixes and metrics that are being passed on?

Thanks,

Richard

Hello Richard,

if you are redistributing into BGP the right place to implement route filtering is the BGP process the line red OSPF allows for invoking a route-map

What you see with EIGRP is the effect of

distribute-list prefix FILTER out

this has a direct effect on EIGRP domain: the filtered prefix is not sent out by this node in EIGRP updates

this can be seen as a side effect of this command

For OSPF only meaningful cases are:

distribute-list in

or

distribute-list out xx

that is it applies for routes redistributed from source-protocol into OSPF not the opposite

hint:

the filtered prefix is still sent out in OSPF updates

for this reason the commands are ineffective for redistribution of OSPF into BGP

see

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800943c5.shtml

Hope to help

Giuseppe

Thanks Giuseppe - I'm studying towards my R&S lab which is why I'm trying to think of lots of ways of doing things...!

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