Preventing route looping by using route tagging

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Dec 21, 2009 4:14 AM
Dec 21st, 2009

Issue:

External EIGRP route not reachable by R1 and R10 in the topology bellow after redistribution between OSPF and EIGRP configured in R1 and R10

redist.jpg

Solution:

Before we start with the solution lets see the routing tables of each of the routers above

R3:

R3#sho ip route
     3.0.0.0/24 is subnetted, 1 subnets
C       3.3.3.0 is directly connected, Loopback0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, FastEthernet1/0
D EX 192.168.1.0/24 [170/2560002816] via 10.1.1.10, 00:00:43, FastEthernet1/0
                    [170/2560002816] via 10.1.1.1, 00:00:42, FastEthernet1/0
     150.1.0.0/24 is subnetted, 1 subnets
C       150.1.1.0 is directly connected, Loopback1

as we can see the network 150.1.1.0 is a directly connected network in R3 and this network advertised as external EIGRP network through redistribution

in other words R1 and R10 supposed to see this network as external EIGRP, lets see the routing table of R1 and R10:

R1#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
Known via "ospf 1", distance 110, metric 20
  Tag 190, type extern 2, forward metric 1
  Redistributing via eigrp 10
  Advertised by eigrp 10 metric 1 1 1 1 1
  Last update from 192.168.1.10 on FastEthernet1/0, 00:03:23 ago
  Routing Descriptor Blocks:
  * 192.168.1.10, from 192.168.1.10, 00:03:23 ago, via FastEthernet1/0
      Route metric is 20, traffic share count is 1
      Route tag 190

R10#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
  Known via "eigrp 10", distance 170, metric 2560002816
  Tag 190, type external
  Redistributing via ospf 1, eigrp 10
  Advertised by ospf 1 subnets tag 190
  Last update from 10.1.1.1 on FastEthernet1/1, 00:03:35 ago
  Routing Descriptor Blocks:
  * 10.1.1.3, from 10.1.1.3, 00:03:35 ago, via FastEthernet1/1
      Route metric is 2560002816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1 bytes
      Loading 1/255, Hops 1
    10.1.1.1, from 10.1.1.1, 00:03:35 ago, via FastEthernet1/1
      Route metric is 2560002816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1 bytes
      Loading 1/255, Hops 1
      Route tag 190

As it shown in the routing table of R1 and R10 the network 150.1.1.0 is looping now

R1 see it through OSPF with distance 110 and R10 see it from R1 and R3 and this will result in route loop

R10#ping 150.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.1.1.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

R1#ping 150.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.1.1.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

the problem here is that EIGRP External Route has Administrative Distance of 170 and we did what is know as mutual redistribution between OSPF and EIGRP in both routers this issue will happen with External EIGRP route only here, because each of R1 and R10 will see this route with less distance from each other and a route looping will form in this case

there are several ways to solve issues like this and always it depends on the network design and how complex it is. In this document we will see how we can solve this issue by using route filtering with Route tagging

as we can see from the abvoe routing tales of  R1 and R10 there is a tag associated with the network 150.1.1.0 in R1 and R10 the is because

when the redistributin from EIGRP into OSPF configured its taged the traffic using he bellow commands:

R1:

router ospf 1

redistribute eigrp 10 subnets tag 1090

R10:

router ospf 1

redistribute eigrp 10 subnets tag 190

now if we look the show ip route result again we can see both routers have the tagging which means that the routes is learned via OSPF after being redistributed and the reason as mentioned above because of the higher distance of external EIGRP AD

R1#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
  Known via "ospf 1", distance 110, metric 20
  Tag 190, type extern 2, forward metric 1
  Redistributing via eigrp 10
  Advertised by eigrp 10 metric 1 1 1 1 1
  Last update from 192.168.1.10 on FastEthernet1/0, 00:16:49 ago
  Routing Descriptor Blocks:
  * 192.168.1.10, from 192.168.1.10, 00:16:49 ago, via FastEthernet1/0
      Route metric is 20, traffic share count is 1
      Route tag 190    --- ----190 fom R10

R10#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
  Known via "eigrp 10", distance 170, metric 2560002816
  Tag 190, type external
  Redistributing via ospf 1, eigrp 10
  Advertised by ospf 1 subnets tag 190
  Last update from 10.1.1.1 on FastEthernet1/1, 00:17:49 ago
  Routing Descriptor Blocks:
  * 10.1.1.3, from 10.1.1.3, 00:17:49 ago, via FastEthernet1/1
      Route metric is 2560002816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1 bytes
      Loading 1/255, Hops 1
    10.1.1.1, from 10.1.1.1, 00:17:49 ago, via FastEthernet1/1
      Route metric is 2560002816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1 bytes
      Loading 1/255, Hops 1
      Route tag 190 --- this means that R1 advertising this ntwork back to R10 after its leaned it from R10 thorugh OSPF (tag 190)

Now the first step is that we know the route tags

R1  advertising EIGRP route with tag 1090

R10 advertising EGRP route with tag 190

next we need to prevent the loop by block the route from being learned back

what that mean, in our example R1 need to block any learned route with tag 190 to be redistributed back into EIGRP

R10 need to block any route with tag 1090 from being redistributed back into EIGRP

in this case we will make sure that any EIGRP route redistributed into OSPF Domain by R1 will not be redistributed back into EIGRP domain by R10

same with R10 redistribution

lets see how we can recover our network above from the route looping with this concept:

R1:

route-map EIGRP deny 10    -------- this means deny any route has tag 190
match tag 190
!
route-map EIGRP permit 20   -- permit anything else

router eigrp 10
redistribute ospf 1 metric 1 1 1 1 1 route-map EIGRP

R10:

route-map EIGRP deny 10
match tag 1090
!
route-map EIGRP permit 20

router eigrp 10
redistribute ospf 1 metric 1 1 1 1 1 route-map EIGRP

lets pig now :

R1#ping 150.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 92/137/188 ms

R10#ping 150.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/64/156 ms

All good !!!!!

lets see the routing tables of R1 and R10

R10#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
  Known via "eigrp 10", distance 170, metric 2560002816, type external
  Redistributing via ospf 1, eigrp 10
  Advertised by ospf 1 subnets tag 190
  Last update from 10.1.1.3 on FastEthernet1/1, 00:03:46 ago
  Routing Descriptor Blocks:
  * 10.1.1.3, from 10.1.1.3, 00:03:46 ago, via FastEthernet1/1
      Route metric is 2560002816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1 bytes
      Loading 1/255, Hops 1

R1#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
Known via "ospf 1", distance 110, metric 20
  Tag 190, type extern 2, forward metric 1
  Redistributing via eigrp 10
  Last update from 192.168.1.10 on FastEthernet1/0, 00:30:37 ago
  Routing Descriptor Blocks:
  * 192.168.1.10, from 192.168.1.10, 00:30:37 ago, via FastEthernet1/0
      Route metric is 20, traffic share count is 1
      Route tag 190

Although we have just solved the looping issue but we still see this network in R1 through OSPF

as long as there is no route loop this is fine UNLESS you are looking for optimal route path in the case there are some extra steps need to configured

we can use the same route-maps we created above that match the tagged traffic and then we apply under OSPF process using distribute list with a route map

lets do it in R1 to see the effect and then we can apply it in R10 as well

But keep in mind that you need to consider redundancy and backup links when you do a real network design because with the filtering above you might block a redundant link ( just be ware and careful when you do it in production network with multiple paths)

As this document not for best path selection and optimal route there will be no details about how to configure this network for optimal path selection.

router ospf 1
distribute-list route-map EIGRP in

R1#show ip route 150.1.1.0
Routing entry for 150.1.1.0/24
  Known via "eigrp 10", distance 170, metric 2560002816, type external
  Redistributing via ospf 1, eigrp 10
  Advertised by ospf 1 subnets tag 1090
  Last update from 10.1.1.3 on FastEthernet1/1, 00:00:36 ago
  Routing Descriptor Blocks:
  * 10.1.1.3, from 10.1.1.3, 00:00:36 ago, via FastEthernet1/1
      Route metric is 2560002816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 1 Kbit
      Reliability 1/255, minimum MTU 1 bytes
      Loading 1/255, Hops 1

here we go the route now through R3 and through EIGRP in R1

same config need to be done in R10

Thank you

Marwan Alshawi

Average Rating: 5 (2 ratings)

Comments

davidfarjecalderon Fri, 05/04/2012 - 14:18

great article.  I was looking for a practical example of the use of route tagging.  This article nailed it.

thanks.

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Posted December 21, 2009 at 4:14 AM
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