Prevención de loop de ruteo con redistribución OSPF

7/28/2019 Prevencin de loop de ruteo con redistribucin OSPF

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Prevencin de loop de ruteo con redistribucin OSPF - EIGRP

Issue:

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

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, Loopback010.0.0.0/24 is subnetted, 1 subnets

C 10.1.1.0 is directly connected, FastEthernet1/0D 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, FastEthernet1/0150.1.0.0/24 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 asexternal EIGRP network through redistributionin other words R1 and R10 supposed to see this network as external EIGRP, lets see the routing table ofR1 and R10:

R1#show ip route 150.1.1.0

Routing entry for 150.1.1.0/24Known via "ospf 1", distance 110, metric 20Tag 190, type extern 2, forward metric 1Redistributing via eigrp 10Advertised by eigrp 10 metric 1 1 1 1 1Last update from 192.168.1.10 on FastEthernet1/0, 00:03:23 agoRouting 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 1Route tag 190

R10#show ip route 150.1.1.0Routing entry for 150.1.1.0/24Known via "eigrp 10", distance 170, metric 2560002816

Tag 190, type externalRedistributing via ospf 1, eigrp 10Advertised by ospf 1 subnets tag 190
https://supportforums.cisco.com/servlet/JiveServlet/showImage/102-8322-1-2595/redist.jpg


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Last update from 10.1.1.1 on FastEthernet1/1, 00:03:35 agoRouting 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 1Total delay is 110 microseconds, minimum bandwidth is 1 KbitReliability 1/255, minimum MTU 1 bytesLoading 1/255, Hops 1

10.1.1.1, from 10.1.1.1, 00:03:35 ago, via FastEthernet1/1Route metric is 2560002816, traffic share count is 1Total delay is 110 microseconds, minimum bandwidth is 1 KbitReliability 1/255, minimum MTU 1 bytesLoading 1/255, Hops 1Route tag 190

As it shown in the routing table of R1 and R10 the network 150.1.1.0 is looping nowR1 see it through OSPF with distance 110 and R10 see it from R1 and R3 and this will result in route loop

R10#ping 150.1.1.1Type 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 150.1.1.1Type 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 isknow as mutual redistribution between OSPF and EIGRP in both routers this issue will happen withExternal EIGRP route only here, because each of R1 and R10 will see this route with less distance fromeach 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 howcomplex 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 network150.1.1.0 in R1 and R10 the is becausewhen the redistributin from EIGRP into OSPF configured its taged the traffic using he bellow commands:R1:router ospf 1redistribute eigrp 10 subnets tag 1090

R10:router ospf 1redistribute 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 ofthe higher distance of external EIGRP AD

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

R10#show ip route 150.1.1.0Routing entry for 150.1.1.0/24


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Known via "eigrp 10", distance 170, metric 2560002816Tag 190, type externalRedistributing via ospf 1, eigrp 10Advertised by ospf 1 subnets tag 190Last update from 10.1.1.1 on FastEthernet1/1, 00:17:49 agoRouting Descriptor Blocks:* 10.1.1.3, from 10.1.1.3, 00:17:49 ago, via FastEthernet1/1


10.1.1.1, from 10.1.1.1, 00:17:49 ago, via FastEthernet1/1Route metric is 2560002816, traffic share count is 1Total delay is 110 microseconds, minimum bandwidth is 1 KbitReliability 1/255, minimum MTU 1 bytesLoading 1/255, Hops 1Route 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 backwhat that mean, in our example R1 need to block any learned route with tag 190 to be redistributed backinto 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 beredistributed back into EIGRP domain by R10same with R10 redistributionlets 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 190match tag 190!route-map EIGRP permit 20 -- permit anything else

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

R10:

route-map EIGRP deny 10match tag 1090!

route-map EIGRP permit 20router eigrp 10redistribute ospf 1 metric 1 1 1 1 1 route-map EIGRP

lets pig now :

R1#ping 150.1.1.1Type 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 150.1.1.1Type 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


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All good !!!!!

lets see the routing tables of R1 and R10


Known via "eigrp 10", distance 170, metric 2560002816, type externalRedistributing via ospf 1, eigrp 10Advertised by ospf 1 subnets tag 190Last update from 10.1.1.3 on FastEthernet1/1, 00:03:46 agoRouting Descriptor Blocks:* 10.1.1.3, from 10.1.1.3, 00:03:46 ago, via FastEthernet1/1


R1#show ip route 150.1.1.0Routing entry for 150.1.1.0/24Known via "ospf 1", distance 110, metric 20

Tag 190, type extern 2, forward metric 1Redistributing via eigrp 10Last update from 192.168.1.10 on FastEthernet1/0, 00:30:37 agoRouting 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 1Route tag 190

Although we have just solved the looping issue but we still see this network in R1 through OSPFas long as there is no route loop this is fine UNLESS you are looking for optimal route path in the casethere 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 underOSPF 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 wellBut keep in mind that you need to consider redundancy and backup links when you do a real networkdesign because with the filtering above you might block a redundant link ( just be ware and careful whenyou 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 toconfigure this network for optimal path selection.

router ospf 1distribute-list route-map EIGRP in


Known via "eigrp 10", distance 170, metric 2560002816, type externalRedistributing via ospf 1, eigrp 10

Advertised by ospf 1 subnets tag 1090Last update from 10.1.1.3 on FastEthernet1/1, 00:00:36 agoRouting Descriptor Blocks:* 10.1.1.3, from 10.1.1.3, 00:00:36 ago, via FastEthernet1/1


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

same config need to be done in R10

Prevención de loop de ruteo con redistribución OSPF

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