TroubleShooting Route RedistribuTion with Multiple RedistribuTion Points

               If we were designing a network ourselves, we might look at that Redistribution Point and think that’s the potential single Point of failure instead we might want to have 2 or more Routers that are Redistributing between the different Autonomous Systems, different Routing Domains we can certainly do that but in some cases, that might introduce a bit of Troubleshooting issue for example, let say that Router2 on Picture.

                    Wants to send traffic to the 10.1.1.0/24 network and Router R2’s IP Routing Table say’s that the next Hop is Router RD1

                 Which is a Router that’s doing Redistribution, RD1 sends the Packets down to Router R1 and interestingly Router R1 sends the packet on to Router RD2 and Router RD2 sends this packet back into our original Autonomous System

      That could be sub-optimal Routing we leaving our Routing Domain going through another Routing Domain, another Autonomous System and then coming back into our own Routing Domain and you might run into a situation like this

Ø  Depending on how many Domains you have?

Ø  How many boundary Routers you have?

Ø  What specific Routing Protocols you using?

Ø  How things are configured?

In this topic, we want to talk mostly in theory about how we would Troubleshoot an issue like this.

                         What we could do is, set really really high Metric values on a Routes that being Redistributed into a Routing Domain that way if i am in Autonomous System number 2 and i am trying to send packets to a destination that also lives in Autonomous System number 2, i am probably not going to be leaving my Autonomous System because the Metric would be too high to go through another Autonomous System, it would be a much lower Metric to stay with my own Autonomous System something else we might do to combat situation like this, is to statically set the Administrative Distance for a Routing Protocol

     Remember that , RIP has a default Administrative Distance of 120, OSPF has a default Administrative Distance of 110 and EIGRP has default Administrative Distance of 90 except for (external Routes that get injected into EIGRP those have an Administrative Distance of 170) infect since, EIGRP does distinguish between externally learned Routes and Routes leaned within the Autonomous System and it gives a higher Administrative Distance to those externally learned Routes EIGRP by itself does a great job for preventing a situation like this from happing but depending on what Routing Protocol you using and how things configured, you might in some cases need to statically configured the Administrative Distance for Routing Protocol so let’s Hope out to interface and take a look at how easy is to set the Administrative Distance for Routing Protocol.

I am sitting on Router RD1 and its configured for Route Redistribution and if we do a

Ø  RD1#show ip route

                     We have learned Routes via OSPF, we have learned Routes via EIGRP and notice the Administrative Distances of these different Routing Protocols however, in some Troubleshooting scenarios we might want to make OSPF more believable than EIGRP, here how we could do that, we can give the “Distance” command, that command works not just with OSPF, it also works with EIGRP and it works with RIP, here how we can do it

Ø  RD1(config)#router ospf 1

Ø  RD1(config-router)#distance 80

Now if we look at IP Routing Table again

Ø  RD1#show ip route

                        Look at this my OSPF Routes now have a more believable Administrative Distance then EIGRP, it’s now AD of 80 and you can see that we don’t have any EIGRP learned Routes in the IP Routing Table now because i have learned all of them via OSPF.

                         But something important to realize is that this Administrative Distance of 80 is only locally significant, it only applies to Router RD1 Route configured it, another words this Administrative Distance is not being advertised out to other Routers this is not going to influence the Routing decision that other Routers make, this only influences RD1

                        Another fairly clever way to overcome this Routing Loop issue is to “Tag a Route” as its being Redistributed from one Routing Domain into another and when we talk about Tags, please realize we can use this for lots of different reason not just for Redistribution but what is a Tag?

                      It’s a value that we can assign to a Route and Tag isn’t a really measuring anything it’s not a specific unit of measure for bandwidth or delay or anything like that it’s just a Label that we put on a Route let’s checkout an example, what we can do with Tags.

                         What we could is say that, we want to assign a Tag of 10 to Routes being Redistributed into Autonomous System 1 on picture.

            And i have just drown for Router RD1 but we would do the same thing for router RD2 but in addition to setting the Tag to 10 for Routes going into Autonomous System 1. What if we did this also, what if we said that we going to deny any Routes from being Redistributed that had a Tag of 10 in the example of picture.

                  We got a Route going from Autonomous System 2 into Autonomous System 1 but as that Route gets Redistributed it’s given a Tag of 10 and RD1 in the example, is assigning that Tag if that’s same Route were to then try to comeback into Autonomous System 2 via RD2 that’s not going to be allowed because RD2 is gonna have a Route-Map that says we are not going to allow the Redistribution of any Route that has a Tag of 10 going into Autonomous System 2.

Let’s go out an interface now and take a look at how we can configure these Tags

                       We here again on Router RD1 and what we want to do is to say if we were Redistributing our Route from EIGRP into OSPF we want to give it a Label we gonna give it a Tag of 10 and we can do that with Route-Map, let’s do that

Ø  RD1(config)#route-map TAG10

Ø  RD1(config-route-map)#set tag 10

ü  TAG10: Route-Map Name

ü  10: Tag Number

Let’s create a second Route Map

Ø  RD1(config)#route-map DENYTAG10 deny 10

ü  DENYTAG10: Route-Map Name

ü  deny: Deny Statement

ü  10: Sequence Number

What I am wanting to deny, i am wanting to deny Routes that have a Tag of 10

Ø  RD1(config-route-map)#match tag 10

                      We gonna deny that being Redistributed from OSPF back into EIGRP thus breaking that potential Routing loop however, i want to allow other traffic to be Redistributed, i need to allow everything else so, we need to give a second Route-Map statement for the deny tag 10

Ø  RD1(config)#route-map DENYTAG10 permit 20

ü  DENYTAG10: Previous Route-Map Name

ü  Permit: Permit everything

ü  20: higher Sequence Number because (10 Sequence number is denying)

                    Now need to match anything because the default is going to match everything now the we got those Route-Map created, let’s apply them to our Routing Process.

Ø  RD1(config)#route ospf 1

Ø  RD1(config-route-map)#redistribute eigrp 1 subnet route-map TAG10

                          I am assigning Tag of 10 to Routes being Redistributed into OSPF from EIGRP, now let’s apply the other Route-Map to the EIGRP Routing Process.

Ø  RD1(config)#router eigrp 1

Ø  RD1(config-route-map)#redistribute ospf 1 route-map DENYTAG10

I have already set the Default Metric

                   I am saying if i am Redistributed into EIGRP from OSPF, i am going to deny any Routes that have a Tag of 10 and in this example, i did this two one of my Redistribution point RD1, to complete the Configuration i would need to do the same thing on Router RD2 which is my other Redistribution Point.

   That’s the look at a few different ways that we can Troubleshoot a Routing Loop that might occurred when where doing Mutual Route Redistribution between a couple of different Routing Domains and we have more than 1 Redistribution Point.

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