Route Tagging and Redistribution Loops

[Mitrixsen]

Hello, everyone!

If my understanding here is correct, Route Tagging is used to prevent redistribution loops where routes are being redistributed from, for example, Protocol A -> Protocol B -> back to Protocol A.

Source: NetworkLessons.com

However, I've encountered some confusion here about how these redistribution loops even happen. Take a look at this scenario

There really does appear to be a loop because R1 also considers the path through

G0/1

as valid for the

192.168.1.0/24

destination.

However, in the next scenario

Nor EIGRP or OSPF are encountering any of these redistribution loops that are supposed to happen if no tagging is configured. R1 has just a single route which leads to the correct destination.

So how exactly do these loops even work? Why is it that with OSPF and EIGRP, the routes are not getting redistributed back into them, while they do with RIP?

Thank you in advance for your help.

David

[Giuseppe Larosa · Accepted solution]

Hello David @Mitrixsen ,

>> Why is it that with OSPF and EIGRP, the routes are not getting redistributed back into them, while they do with RIP?

Both EIGRP and OSPF are able to distinguish internal routes from external routes and EIGRP uses different AD 170 for D EX prefixes. OSPF uses a single AD but it uses  an ordered list of preferences

O > O IA > O Ex  , O E1 are preferred over O E2 prefixes

EIGRP internal routes AD is 90 < OSPF AD 110.

However, mutual redistribution in two routers of two dynamci protocols is a dynamic process too , that does not happen once but it occurs over time.

A safe configuration must be able to deal with whatever event happens.

In your example what happens if the interface directly connected to

192.168.1.0/24

in EIGRP domain is shut down ?

EIGRP uses its own way and it goes active for the

prefix

during the time this happens OSPF can

re-inject the prefix

into EIGRP domain for the redistribution.

The purpose of the route tags when used correctly is to manage all scenarios and to avoid unwanted

re-injections of prefixes

in the IGP that should be origin of the route.

So if the OSPF route is tagged with a route tag value that is denied in redistributing into EIGRP this

re-injection

does not occur.

The route tag is an integer  32bit value for OSPF, EIGRP and IS-IS it might be 16 bit value for RIPv2.

Hope to help

Giuseppe

[M02@rt37]

Hello @Mitrixsen,

EIGRP implement a mechanism known as

split horizon

to avoid redistribution loops by default. The split horizon rule dictates that routes learned from one routing protocol should not be readvertised back into the same routing protocol. Due to the split horizon rule, EIGRP will not readvertise routes that they have learned via redistribution back into the same OSPF or EIGRP domain. As a result, redistribution loops are naturally avoided.

In OSPF, there is no built-in mechanism like

split horizon

to prevent redistribution loops. OSPF redistributes external routes into the OSPF domain by default, which could lead to potential redistribution loops if there are multiple redistribution points. In the second scenario you mentioned that there are no redistribution loops even though R1 receives routes from both EIGRP and OSPF. The reason for this behavior is due to the default OSPF behavior for Type 2 external routes. When redistributing routes into OSPF, they are treated as Type 2 external routes by default. Type 2 external routes have a fixed cost (metric) within OSPF, and OSPF will not readvertise them back into the same OSPF domain. This behavior prevents redistribution loops for Type 2 external routes.

By tagging the redistributed routes and filtering them during redistribution, you can prevent certain routes from being readvertised back into OSPF, providing additional control over the redistribution process.

RIP, on the other hand, does not have built-in mechanisms to prevent redistribution loops. When RIP receives routes from another routing protocol (such as OSPF or EIGRP), it will readvertise them back into the RIP domain. This is why redistribution loops are more likely to occur with RIP than with EIGRP or OSPF.

[paul driver]

Hello
You can use

route map

tagging to negate

route-reeedback

from either IGPs

Example:
route-map OSPFintoEIGRP deny 10 -( eigrp routes- now ospf routes with a tag of 90 will be denied from going back into eigrp
match tag 90

route-map OSPFintoEIGRP permit 99 - ( All other OSPF routes are tagged with 110 into eigrp)
set tag 110


route-map EIGRPintoOSPF deny 10 - (ospf routes- now eigrp routes with a tag of 110 will be denied from going back into ospf
match tag 110

route-map EIGRPintoOSPF permit 99 - ( All other eigrp routes are tagged with 90 into ospf)
set tag 90