Redistribution into EIGRP
ccnp-routing
Lab on mutual redistribution between OSPF, EIGRP, and RIP, the suboptimal routing it can cause, and route tags that prevent routing domain loops.
hostname R1 ! interface GigabitEthernet0/1 ip address 10.10.10.1 255.255.255.252 duplex auto speed auto media-type rj45 ! interface GigabitEthernet0/2 ip address 10.10.20.1 255.255.255.252 duplex auto speed auto media-type rj45 ! router ospf 1 network 10.10.10.1 0.0.0.0 area 0 network 10.10.20.1 0.0.0.0 area 0RD1 Configuration
hostname RD1 ! interface GigabitEthernet0/0 ip address 10.10.11.1 255.255.255.252 duplex auto speed auto media-type rj45 ! interface GigabitEthernet0/1 ip address 10.10.10.2 255.255.255.252 duplex auto speed auto media-type rj45 ! router eigrp RD1 ! address-family ipv4 unicast autonomous-system 1 ! topology base redistribute ospf 1 metric 1000000 20 255 1 1500 exit-af-topology network 10.10.11.1 0.0.0.0 exit-address-family ! router ospf 1 redistribute eigrp 1 subnets network 10.10.10.2 0.0.0.0 area 0RD2 Configuration
hostname RD2 ! interface GigabitEthernet0/0 ip address 10.10.21.1 255.255.255.0 duplex auto speed auto media-type rj45 ! interface GigabitEthernet0/1 ip address 10.10.20.2 255.255.255.252 duplex auto speed auto media-type rj45 ! router eigrp RD2 ! address-family ipv4 unicast autonomous-system 1 ! topology base redistribute ospf 1 metric 1000000 20 255 1 1500 exit-af-topology network 10.10.21.1 0.0.0.0 exit-address-family ! router ospf 1 redistribute eigrp 1 subnets network 10.10.20.2 0.0.0.0 area 0RD3 Configuration
hostname RD3 ! interface GigabitEthernet0/0 ip address 172.20.0.1 255.255.0.0 duplex auto speed auto media-type rj45 ! interface GigabitEthernet0/1 ip address 10.10.11.2 255.255.255.252 duplex auto speed auto media-type rj45 ! interface GigabitEthernet0/2 ip address 10.10.21.2 255.255.255.252 duplex auto speed auto media-type rj45 ! router eigrp RD3 ! address-family ipv4 unicast autonomous-system 1 ! topology base redistribute rip metric 1000000 2000 255 1 1500 exit-af-topology network 10.10.11.2 0.0.0.0 network 10.10.21.2 0.0.0.0 exit-address-family ! service-family ipv4 autonomous-system 1 ! topology base exit-sf-topology exit-service-family ! router rip version 2 redistribute eigrp 1 metric 2 network 172.20.0.0 no auto-summary !R2 Configuration
hostname R2 ! interface GigabitEthernet0/0 ip address 172.20.0.255 255.255.0.0 duplex auto speed auto media-type rj45 ! router rip version 2 network 172.20.0.0 no auto-summary !Let us verify the end-to-end reachability
R1#traceroute 172.20.0.255 1 10.10.10.2 5 msec 6 msec 4 msec 2 10.10.11.2 8 msec 7 msec 5 msec 3 172.20.0.255 9 msec 13 msec 9 msecWe see that the connectivity between R1 and R2 passes from RD1. Everything seems to be okay and we have end-to-end reachability! But there is a problem. We have inefficient routing. Let us check how RD2 reaches 172.20.0.0
RD2#traceroute 172.20.0.255 1 10.10.20.1 7 msec 6 msec 22 msec 2 10.10.10.2 10 msec 9 msec 9 msec 3 10.10.11.2 10 msec 9 msec 8 msec 4 172.20.0.255 13 msec 9 msec 8 msec
We see that if RD2 wants to reach R2 (instead of using RD3) it goes to R1, RD1, then RD3, which is sub-optimal!
Here is the reason.
- When you redistribute RIP into EIGRP on RD3, both RD1 and RD2 see the 172.16.0.0 having AD=170 if we did not have OSPF. But here we are having OSPF and EIGRP mutual redistribution. The router which first started the redistribution process (RD1) is in a privileged position. RD2 is going to get RIP from OSPF because it offers a better AD=110 than AD=170.
RD1#show ip route 172.20.0.0 Routing entry for 172.20.0.0/16 Known via "eigrp 1", distance 170, metric 10250240, type external Redistributing via ospf 1, eigrp 1 Advertised by ospf 1 subnets Last update from 10.10.11.2 on GigabitEthernet0/0, 00:12:30 ago Routing Descriptor Blocks: * 10.10.11.2, from 10.10.11.2, 00:12:30 ago, via GigabitEthernet0/0 Route metric is 10250240, traffic share count is 1 Total delay is 20010 microseconds, minimum bandwidth is 1000000 Kbit Reliability 255/255, minimum MTU 1500 bytes Loading 1/255, Hops 1 RD2#show ip route 172.20.0.0 Routing entry for 172.20.0.0/16 Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2 Redistributing via eigrp 1 Advertised by eigrp 1 metric 1000000 20 255 1 1500 Last update from 10.10.20.1 on GigabitEthernet0/1, 00:12:19 ago Routing Descriptor Blocks: * 10.10.20.1, from 10.10.11.1, 00:12:19 ago, via GigabitEthernet0/1 Route metric is 20, traffic share count is 1
Here are the solutions
- Use per-route Administrative Distance (AD)
- Use route map to filter the subnet
- Both RD1 and RD2 apply a route map to their redistribution from OSPF into EIGRP, filtering routes with prefix 172.20.0.0
- Use route tags to prevent domain loops
- A route tag is a unitless 32-bit integer. Most routing protocols can assign one to any given route. It is like a sticky note on a specific route
RD1(config-router)#router ospf 1 RD1(config-router)#redistribute eigrp 1 subnets route-map TAG11 RD1(config)#route-map TAG11 permit 10 RD1(config-route-map)#set tag 11
RD1(config)#router eigrp RD1 RD1(config-router)#a ipv4 a 1 RD1(config-router-af)#topology base RD1(config-router-af-topology)#redistribute ospf 1 metric 1000000 20 255 1 1500 route-map STOPTAG12 RD1(config)#route-map STOPTAG12 deny 10 RD1(config-route-map)#match tag 12 RD1(config)#route-map STOPTAG12 permit 20
RD2(config)#router eigrp RD2 RD2(config-router)#a ipv4 a 1 RD2(config-router-af)#topology base RD2(config-router-af-topology)#redistribute ospf 1 metric 1000000 20 255 1 1500 route-map STOPTAG11 RD2(config)#route-map STOPTAG11 deny 10 RD2(config-route-map)#match tag 11 RD2(config)#route-map STOPTAG11 permit 20 RD2(config)#router ospf 1 RD2(config-router)#redistribute eigrp 1 subnets route-map TAG12 RD2(config)#route-map TAG12 permit 10 RD2(config-route-map)#set tag 12