Two-point redistribution between OSPF and EIGRP without loops

Tasks

1. On R4, add a static route to the partner network and redistribute it into EIGRP.
2. On R2 and R3, define with prefix lists which prefixes may cross in each direction. Do not redistribute the transit /30 links.
3. On R2 and R3, tag routes as they enter each domain and refuse to send a route back into the domain it came from.
4. Redistribute both ways on R2 and R3 with explicit seed metrics.
5. Stop R2 and R3 from preferring an OSPF copy of a route that started in EIGRP.
6. Verify tags, paths and the absence of feedback.

Solution

1. 1Partner network into EIGRPR4

   ip route 192.0.2.0 255.255.255.0 10.4.0.254
   !
   router eigrp 100
    redistribute static metric 1000000 1 255 1 1500

   The seed is 1 Gbps and a delay of 1 (tens of microseconds). R2 and R3 see 192.0.2.0/24 as D EX, AD 170.

2. 2Prefix lists for each directionR2 and R3

   ip prefix-list EIGRP-NETS seq 5 permit 10.4.0.0/16 le 24
   ip prefix-list EIGRP-NETS seq 10 permit 192.0.2.0/24
   !
   ip prefix-list OSPF-NETS seq 5 permit 10.1.0.0/16 le 24

   10.4.0.0/16 le 24 matches any route inside 10.4.0.0/16 from /16 to /24, so the 10.4.0.0/24 LAN passes. The 10.0.x.x/30 transit links match neither list and stay in their own domain.

3. 3Route-maps that tag and block the way backR2 and R3

   route-map EIGRP-TO-OSPF deny 10
    match tag 110
   route-map EIGRP-TO-OSPF permit 20
    match ip address prefix-list EIGRP-NETS
    set tag 90
   !
   route-map OSPF-TO-EIGRP deny 10
    match tag 90
   route-map OSPF-TO-EIGRP permit 20
    match ip address prefix-list OSPF-NETS
    set tag 110

   The deny entries come first. A route that entered OSPF from EIGRP (tag 90) can never be sent back into EIGRP, and the reverse for tag 110.

4. 4Redistribution with seed metricsR2 and R3

   router ospf 1
    redistribute eigrp 100 subnets route-map EIGRP-TO-OSPF
   !
   router eigrp 100
    redistribute ospf 1 metric 100000 10 255 1 1500 route-map OSPF-TO-EIGRP

   Into OSPF the default is E2 with metric 20. Into EIGRP nothing is advertised without a metric; this seed is 100 Mbps and 100 microseconds.

5. 5Do not install EIGRP routes that come back through OSPFR2 and R3

   route-map NO-EIGRP-ORIGIN deny 10
    match tag 90
   route-map NO-EIGRP-ORIGIN permit 20
   !
   router ospf 1
    distribute-list route-map NO-EIGRP-ORIGIN in

   OSPF (110) beats EIGRP external (170), so without this, whichever of R2 and R3 redistributes 192.0.2.0/24 second reaches it through R1 and the other redistributor. The filter only changes what each router puts in its own routing table; the LSAs still flood to R1. No mirror filter is needed in EIGRP, because OSPF routes (110) already beat their EIGRP external copies (170).

Verify

R1# show ip route ospf | begin Gateway
Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 7 subnets, 3 masks
O E2     10.4.0.0/24 [110/20] via 10.0.13.2, 00:04:10, GigabitEthernet0/0/1
                     [110/20] via 10.0.12.2, 00:04:10, GigabitEthernet0/0/0
O E2  192.0.2.0/24 [110/20] via 10.0.13.2, 00:04:10, GigabitEthernet0/0/1
                   [110/20] via 10.0.12.2, 00:04:10, GigabitEthernet0/0/0

R1# show ip route 10.4.0.0
Routing entry for 10.4.0.0/24
  Known via "ospf 1", distance 110, metric 20
  Tag 90, type extern 2, forward metric 1
  Last update from 10.0.13.2 on GigabitEthernet0/0/1, 00:04:10 ago
  Routing Descriptor Blocks:
    10.0.13.2, from 10.255.0.3, 00:04:10 ago, via GigabitEthernet0/0/1
      Route metric is 20, traffic share count is 1
  * 10.0.12.2, from 10.255.0.2, 00:04:10 ago, via GigabitEthernet0/0/0
      Route metric is 20, traffic share count is 1

R1# show ip ospf database external 10.4.0.0 | include Advertising|Tag
Advertising Router: 10.255.0.2
      External Route Tag: 90
Advertising Router: 10.255.0.3
      External Route Tag: 90

R4# show ip route 10.1.0.0
Routing entry for 10.1.0.0/24
  Known via "eigrp 100", distance 170, metric 28416
  Tag 110, type external
  Redistributing via eigrp 100
  Last update from 10.0.34.1 on GigabitEthernet0/0/1, 00:04:10 ago
  Routing Descriptor Blocks:
    10.0.34.1, from 10.0.34.1, 00:04:10 ago, via GigabitEthernet0/0/1
      Route metric is 28416, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1
  * 10.0.24.1, from 10.0.24.1, 00:04:10 ago, via GigabitEthernet0/0/0
      Route metric is 28416, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

R3# show ip route eigrp | include 192.0.2.0
D EX  192.0.2.0/24 [170/3072] via 10.0.34.2, 00:03:02, GigabitEthernet0/0/1

Mistakes people make

• Configuring the tags and route-maps on only one of the two redistributors. Feedback then happens through the other one.
• Writing the prefix list as permit 10.4.0.0/16 without le 24, so the /24 LANs do not match and nothing is redistributed.
• Leaving out the EIGRP seed metric. The OSPF routes are taken but never advertised.
• Putting the permit entry before the tag deny in a route-map. First match wins, so the deny never runs.
• Expecting an OSPF distribute-list in to stop LSAs. It only filters what that router installs in its own table.
• Leaving out the empty permit 20 in NO-EIGRP-ORIGIN. A route-map ends with an implicit deny, so the distribute-list would keep every OSPF route out of the routing table.

On other platforms

set policy-options policy-statement TO-OSPF term no-feedback from tag 110
set policy-options policy-statement TO-OSPF term no-feedback then reject
set policy-options policy-statement TO-OSPF term partner from protocol static
set policy-options policy-statement TO-OSPF term partner from route-filter 192.0.2.0/24 exact
set policy-options policy-statement TO-OSPF term partner then tag 90
set policy-options policy-statement TO-OSPF term partner then accept
set protocols ospf export TO-OSPF