This document discusses route redistribution between different routing protocols and the issues that can arise, particularly with multipoint redistribution. It recommends using route tagging as the best solution to prevent routing loops from occurring when routes are redistributed in both directions between protocols. Specifically, it shows how to configure route maps to tag redistributed routes so they are not endlessly propagated between protocols. While route tagging can prevent loops, suboptimal routing may still exist, so careful metric tuning is also advised to prefer internal routes over external ones.

1. Tag Your Routes Before Redistribution
Presented by
Md. Abdullah Al Naser
Sr. Systems Specialist
MetroNet Bangladesh Limited
Founder, mn-LAB

2. Presented by – Md. Abdullah Al Naser Page # 2
Before Starting…..
● Experts say that, don’t REDISTRIBUTE IGP routes to the
upstreams while you configure BGP – That’s True.
● But many engineers still prefer this REDISTRIBUTION for their
internal networks and also for their clients’ internal networks in
complex routing scenario – Design Requirements.

3. Presented by – Md. Abdullah Al Naser Page # 3
What is Redistribution?
● It’s possible that we have multiple routing protocols in our network
and we’ll need some method to exchange routing information between
the different protocols. This method is called redistribution.
● Redistribution between routing protocols (RIP, OSPF, EIGRP, BGP).
● Static routes can be redistributed into a routing protocol.
● Directly connected routes can also be redistributed into a routing
protocol.

4. Presented by – Md. Abdullah Al Naser Page # 4
What is Redistribution?
● Redistribution is not done automatically.
● Redistribution happens outbound.
● When we redistribute from one routing protocol into another we
have to use a seed metric.
OSPF: Cost
EIGRP: K-Values (bandwidth, delay, load and reliability)
RIP: Hop count

5. Presented by – Md. Abdullah Al Naser Page # 5
Configuring Redistribution – Just an example
R1(config)#router ospf 1
R1(config-router)#network 1.1.1.0 0.0.0.255 area 0
R1(config-router)#network 192.168.12.0 0.0.0.255 area 0
R2(config)#router ospf 1
R2(config-router)#network 192.168.12.0 0.0.0.255 area 0
R2(config)#router rip
R2(config-router)#version 2
R2(config-router)#no auto-summary
R2(config-router)#network 192.168.23.0
R3(config)#router rip
R3(config-router)#version 2
R3(config-router)#network 3.3.3.0
R3(config-router)#network 192.168.23.0

6. Presented by – Md. Abdullah Al Naser Page # 6
Configuring Redistribution – Just an example
R2(config)#router rip
R2(config-router)#redistribute ospf 1 metric 5
R2(config)#router ospf 1
R2(config-router)#redistribute rip subnets
R3#show ip route rip
R 192.168.12.0/24 [120/5] via 192.168.23.2, 00:00:00, FastEthernet0/0
R 1.1.1.1 [120/5] via 192.168.23.2, 00:00:00, FastEthernet0/0
R1#show ip route ospf
O E2 3.3.3.0 [110/20] via 192.168.12.2, 00:00:21, FastEthernet0/0
O E2 192.168.23.0/24 [110/20] via 192.168.12.2, 00:00:21, FastEthernet0/0

7. Presented by – Md. Abdullah Al Naser Page # 7
Multipoint Redistribution
● So far so good.
● But the problem will arise during multipoint redistribution.
● It can mislead you to a sub optimal routing and a routing loop.

8. Presented by – Md. Abdullah Al Naser Page # 8
Multipoint Redistribution
Let’s go into
the deep
R2#show ip route rip
R 1.1.1.0 [120/6] via 192.168.12.1, 00:00:09, FastEthernet1/0

9. Presented by – Md. Abdullah Al Naser Page # 9
Multipoint Redistribution
Let’s redistribute RIP routes into OSPF
R3 & R4#
(config)#router ospf 1
(config-router)#redistribute rip subnets

10. Presented by – Md. Abdullah Al Naser Page # 10
Multipoint Redistribution
Let’s check R4’s routing table
R4#show ip route ospf
O E2 1.1.1.0 [110/20] via 192.168.34.3, 00:01:14, FastEthernet0/1

11. Presented by – Md. Abdullah Al Naser Page # 11
Sub Optimal Routing
Sub optimal routing refers to inefficient routing – not good

12. Presented by – Md. Abdullah Al Naser Page # 12
Multipoint Redistribution (cont..)
Let’s redistribute OSPF routes into RIP
R3 & R4#
(config)#router rip
(config-router)#redistribute ospf 1 metric 1

13. Let’s check R2’s routing table again
R2#show ip route rip
R 1.1.1.0 [120/1] via 192.168.24.4, 00:00:05, FastEthernet0/1
R2#traceroute 1.1.1.1
1 192.168.24.4 1388 msec 620 msec 744 msec
2 192.168.34.3 2552 msec 2308 msec 1904 msec
3 192.168.23.2 56 msec 48 msec 48 msec
4 192.168.24.4 76 msec 76 msec 72 msec
5 192.168.34.3 112 msec 124 msec 100 msec
6 192.168.23.2 96 msec 100 msec 92 msec
7 192.168.24.4 124 msec 140 msec 124 msec
8 192.168.34.3 156 msec 168 msec 176 msec
9 192.168.23.2 152 msec 136 msec 156 msec
10 192.168.24.4 176 msec 184 msec 172 msec
Presented by – Md. Abdullah Al Naser Page # 13
Routing Loop

14. Let’s check R2’s routing table again
R2#show ip route rip
R 1.1.1.0 [120/1] via 192.168.24.4, 00:00:05, FastEthernet0/1
Presented by – Md. Abdullah Al Naser Page # 14
Routing Loop

15. Be careful
You should never redistribute routes like this:
Routing Protocol X ----> Y ----> X
Presented by – Md. Abdullah Al Naser Page # 15
Important Redistribution Rules

16. ● Redistribution Filtering – not scalable
● Route Filtering – not scalable at all
● Route Tagging – the best solution
Presented by – Md. Abdullah Al Naser Page # 16
Looking for Solution

17. R3 & R4#
(config)#route-map TAGGING deny 10
(config-route-map)#match tag 1234
(config-route-map)#exit
(config)#route-map TAGGING permit 20
(config-route-map)#set tag 1234
(config)#router rip
(config-router)#redistribute ospf 1 metric 1 route-map TAGGING
(config-router)#exit
(config)#router ospf 1
(config-router)#redistribute rip subnets route-map TAGGING
Presented by – Md. Abdullah Al Naser Page # 17
Configuring Route Tagging

18. Let’s check R2’s routing table again
R2#show ip route rip
R 1.1.1.0 [120/6] via 192.168.12.1, 00:00:01, FastEthernet1/0
It looks good now…..
But sub optimal routing is still there….
Presented by – Md. Abdullah Al Naser Page # 18
Routing Loop is now Removed

19. Be careful again
Always prefer your “internal”
routes over “external” routes.
Whenever your internal route
has a higher AD than the
external route, you have to
be careful!
Presented by – Md. Abdullah Al Naser Page # 19
Another Important Redistribution Rules

20. ● Decrease the AD
of the internal route(s)
or
● Increase the AD
of the external route(s)
Presented by – Md. Abdullah Al Naser Page # 20
Possible Solution

21. Thank you very much