Route redistribution and routing loops.

[JohnTylerPearce]

I have a few questions about route redistributions and routing loops. Please correct me if I'm wrong.

Example 1: Let's say that we have two redistribution points in the network and two different routing protocols. The routing protocols

will be RIP and OSPF. So let's say we have mutual redistribution on both redistribution points. If you left the network like this, I would

assume that you would create a routing loop, since both redistribution points (RPs) will have multiple routes for the same networks,

and install the OSPF route in the routing table since it has the lower AD. So if RP1 wants to send a packet to the RIP network,

it will look in its routing table, and obviously choose the OSPF External route since it should be the only one installed in the routing

table, and forward it to RP2. RP2 would do the same, and forward it back to RP1, hence creating a routing loop. Now I'm assuming

the best fix for this would be, to either lower the AD on the RIP routes on both RPs or raise the OSPF AD on both RPs. I would also

assume that we would want to create a route-map to make sure that when we for instance, redistribute the OSPF routes into RIP

that it doesn't include the OSPF External RIP routes, and vice versa for OSPF into RIP? I'm assuming that would be the cast.

Example 2: Let's say we have the same setup and our two RPs are running EIGRP and OSPF. We have mutual redistribution as

before. Now when EIGRp redistributes its routes into OSPF, they will come in as 110 by default. With EIGRP the external routes

will come in as 170 by default. So both RPs should have all the internal EIGRP routes with an AD of 90, and all OSPF routes with

an AD of 110; they will not have the external EIGRP routes, since OSPF will win out by default, when you have multiple routes for

the same network (EIGRP/OSPF), since OSPF has the lowest AD. So you should not get route feedback when EIGRP gets

redistributed into OSPF, since the external OSPF routes will not be in the RPs routing table(hence not redistribusted). But you

may need to create route-maps to not allow the External OSPF routes to go back into EIGRP.

Sorry for the long post btw....

[nkarpysh · Accepted solution]

Hello,

You are basically correct and answering your questions your self. When you redistribute between to IGP you  need to think of the routes coming back with better AD. There are few ways to filter that:

1) Applying distance to the routes coming from different IGP. E.G. you redistribute OSPF to RIP - you can apply AD (distance) 121 to these routes on router rip process so those would not be preferred to internal routes. Same within other IGP

2) Distribute list - you can filter out networks of particular IGP (e.g. EIGRP AS running it's own subnet) to come from different IGP. So you just apply distribute list and block incoming subnets which should be originated within this IGP

3) Route-map doing same as above with ability to add some specific filters. E.G. you can set tag to outgoing redistributed routes (E.G set tag 90 or 170 to routes redistributed from EIGRP to OSPF). Then you with similar route-map filter routes with this tag from coming back

4) Possibly smth else

Normally you need to check your topology for IGP loops. If you don't have any loops between IGPs - then there should not be much redistribution problems. AS OSPF will not send back routes to the EIGRP domain if it received those on same router. That only possible if there is another router redistributing between two.

So first you look for loops between IGPs, then consider where you need to apply those filters.

Possibly if you have several routers working both in different IGP, E.G R1, R2 and R3 are all working in both OSPF and EIGRP AS 100. You may want to redistribute only on one of them. That will solve any loops afaik as long as you don't do mutual redistribution on others.

That's really a question for discussion so will wait for other answers/questions.

Hope this helps,

Nik

[JohnTylerPearce]

Thanks, I was more wanting someone to verify what I was thinking.

[nkarpysh]

I see,

If I just look closely to your initial post than my comments are below:

JohnTylerPearce написал(а):
Example 1: Let's say that we have two redistribution points in the network and two different routing protocols. The routing protocols
will be RIP and OSPF. So let's say we have mutual redistribution on both redistribution points. If you left the network like this, I would
assume that you would create a routing loop, since both redistribution points (RPs) will have multiple routes for the same networks,
and install the OSPF route in the routing table since it has the lower AD. So if RP1 wants to send a packet to the RIP network,
it will look in its routing table, and obviously choose the OSPF External route since it should be the only one installed in the routing
table, and forward it to RP2. RP2 would do the same, and forward it back to RP1, hence creating a routing loop. Now I'm assuming
the best fix for this would be, to either lower the AD on the RIP routes on both RPs or raise the OSPF AD on both RPs. I would also
assume that we would want to create a route-map to make sure that when we for instance, redistribute the OSPF routes into RIP
that it doesn't include the OSPF External RIP routes, and vice versa for OSPF into RIP? I'm assuming that would be the cast.

That is correct and the way to solve this changing AD will work. Just keep in mind for loop to happen you will need another point of redistribution between these or other protocols. E.G  RIP ---R1 --- OSPF--- R2---RIP - Doing redistribute on both R1 and R2 you can have RIP routes leaking back with OSPF AD and changing AD on redistribution will help. Possibly other techincs I mentioned before. Other possible redistribution challenge will occur here : RIP --- R1 --- OSPF --- R2--- EIGRP---R3--- OSPF . This will make RIP routes to come to EIGRP on R2 and return back to OSPF and then again to RIP on R1. Again similar problem

Example 2: Let's say we have the same setup and our two RPs are running EIGRP and OSPF. We have mutual redistribution as
before. Now when EIGRp redistributes its routes into OSPF, they will come in as 110 by default. With EIGRP the external routes
will come in as 170 by default. So both RPs should have all the internal EIGRP routes with an AD of 90, and all OSPF routes with
an AD of 110; they will not have the external EIGRP routes, since OSPF will win out by default, when you have multiple routes for
the same network (EIGRP/OSPF), since OSPF has the lowest AD. So you should not get route feedback when EIGRP gets
redistributed into OSPF, since the external OSPF routes will not be in the RPs routing table(hence not redistribusted). But you
may need to create route-maps to not allow the External OSPF routes to go back into EIGRP.

Challenge here is that if you originally had routes with AD 170 in EIGRP (e.g. from some other domain). Those routes going through OSPF and coming back to EIGRP will have now AD 110 and will kick of the old routes with AD 170. So some measures should be taken here as well.

Nik