In this session we will explain how to setup route tags with OSPF to avoid routing loops we might face when re-distributing the external routes.

1. Route Tags with OSPF
Dhruv Sharma

2. Introduction
• We use Route Tags to avoid route loops when we re-distribute network from one routing
algorithm into another. Like OSPF into RIP or EIGRP into OSPF.
• As routes are redistributed into OSPF, we can tag them with a numeric value of our choice, and
then deny values with that same tag from being redistributed back into the RIP/EIGRP domain.

3. Network Topology

4. • Pre-Tag configuration

5. R2 & R3 ASBR

6. R1- RIP

7. R4 - OSPF

8. Network Topology

9. • Route Redistribute

10. Route Redistribute
• Impact on R3 Pre-Post change

11. • Route Loop

12. R1 Route Setup
• Route Loop

13. • Fix Route Loop

14. Fix Route Loop using Route Maps
• When R2 will distribute the RIP route 2.0.0.0/8 into OSPF, it should use a TAG.
• When R3 re-distributes the routes back into RIP and when it will see the TAG it will skip
redistributing 2.0.0.0/8.

15. Route Tags
• Sequence number 10 says that when it matches tag number 1 that
it should be denied.
• Sequence number 20 says that we need to set tag number 1.
There’s no “match” statement so EVERYTHING will match.
• Note: We set R2 to set TAG when it redistribute routes into OSPF
and RIP- both ways.
R2 config
!
route-map TAG deny 10
match tag 1
!
route-map TAG permit 20
set tag 1
!
router ospf 13
log-adjacency-changes
redistribute rip route-map TAG
network 13.0.0.0 0.255.255.255 area 0
router rip
redistribute ospf 13 metric 5 route-map TAG
network 15.0.0.0

16. Verify
• We can verify if Router R3 can view the Tag in it’s routing updates or not.
• Next time if you configure R3 to redistribute the OSPF routes into RIP due to TAG it cannot allow
it to happen.

17. Quick Tips
• Route tagging when redistributing into BGP is not supported.
• RIP version 1 doesn’t support route tagging.
• With previous configuration all routes will be tagged, but if you wish to tag only specific network,
use an ACL within a route map.