The inspiration for this post comes directly from IPexpert's wonderfully insane Volume 1 Section 12 Lab. THIS LAB IS INSANE. Not too bad really, but see for yourself :)
Here is the topology:
All I am going to do in this post is explain how I go about "prepping" for redistribution. These are just a random set of miscellaneous notes, in no particular order. I will number them just for identification purposes :)
Also, remember you can do ALL of this in a notepad file before you configure the routers. I try to do it like that, that way my head is truly "around" the whole scenario. Of course, there is some tweaking that will always need to be done once you do get down to business.
1) ASSIGN TAGSFirst thing I like to do is think about tagging a routing domain. Remember we always drop the tag coming in so if you like to use the AD for a tag, be careful if you have multiple domains with the same protocol.
For example, if you set a tag of 110 on R4 and R7 for OSPF routes going into EIGRP. If you deny this tag from EIGRP into OSPF, you prevent R7 and R4 from learning about each other's OSPF networks. In this lab I used 110 for the R2/R4 process. And I used 115 for the R5/R6/R7 process.
Here are the tags I assigned, these were applied at the END of every redistribution route-map.
RIP = 110
OSPF AREA 0 = 110
BGP = 256
OSPF AREA 567 = 567
EIGRP 7800 = 7800
EIGRP 12348 = 12348
On R4, I might have something like this:
route-map ospf2eigrp permit 50 set tag 1102) WHEN TO DENY TAG ON RE-ENTRYAnother thing to remember is when to deny the tag. I only deny the tag when it's entering that protocol from which it came. Unless, of course, I am specifically asked to block routes from entering a certain domain.
Example Rip routes are tagged with 120 going into OSPF (or community 120 in BGP). The only place I deny 120 from entering a routing domain is on R2 from OSPF to RIP, or BGP to RIP (matching community value, not tag).
So building on the above route-map I would have this now:
route-map ospf2eigrp deny 10 match tag 12348route-map ospf2eigrp permit 50 set tag 110Everything else is allowed.
3) ALLOW TAGS TO PASS THROUGHAnother thing to remember is to allow the tags to "pass through" each domain. In the above route-map, R4 removes any tags that R2 may have placed on RIP routes from R1. This prevents us from identifying and blocking the re-entry RIP routes in BGP on R2!
So on R4 we could do this:
route-map ospf2eigrp deny 10 match tag 12348 route-map ospf2eigrp permit 20 match tag 120 route-map ospf2eigrp permit 30 match tag 567 route-map ospf2eigrp permit 40 match tag 7800 route-map ospf2eigrp permit 45 match tag 115 route-map ospf2eigrp permit 50 set tag 110Here I have "pre-identified" or "pre-classified" all the tags I want pass-through. This way we can identify the originating protocol of every route in the EIGRP 12348 domain.
4) WHEN TO CONVERT TAGSAnother note to remember is that BGP does not use tags, but it does community values to identify routes. So on R2, R5, and R6 we will need to "convert" the tag to community value. And when we redistribute the other way, we need to make a community-list to match and drop that route.
For example on R2 we set the community of OSPF routes into BGP as follows:
route-map ospf2bgp permit 50 set community 110On R5 and R6 this community is converted to a tag as follows:
ip community-list 1 permit 110route-map bgp2ospf permit 20 match community 1 set tag 110Now as long as we allow this tag to pass through on R7 and R8, R4 can identify these OSPF routes so it will not redistribute them back into OSPF area 0.
5) OPTIMAL ROUTINGLastly, we are not concerned with optimal routing here. If you are tasked with making paths optimal, you will have to work with metrics or administrative distances to do so. But that's easy right? :)
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