The document explains the use of route tags in OSPF to prevent routing loops during redistribution between different routing protocols, such as OSPF and RIP. It details the configuration of route maps to manage tags effectively and the verification process to ensure routes are handled correctly without being re-distributed. Additionally, it notes limitations regarding route tagging with BGP and RIP version 1.

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.