Route tag value question

Answered Question
Nov 14th, 2008

Hi All,

Is a tag value unique to the local routing process or it is unique for the entire network? In another word, if I use a tag value to tag a route, can I use the same tag value on another router for a different route tagging? It seems like OSPF LSA packet doesnt have a tag field.

Regards,

Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Iqbal,

think of it as similar to BGP community a route tag is a way to provide a color to a group of routes.

The routes travel with their route tags in all the OSPF domain (they are LSA type 5 they cannot be filtered they can be blocked at any type of stub area boundary).

Another ASBR has to mark routes that is injecting into OSPF.

the typical route-map with route tags has the logic

route-map asbr-tags deny 10

match tag xxxx

route-map asbr-tags permit 20

set tag xxxx

in this way you don't send back the routes you have learned before from out of OSPF.

if you use the same route-tag the risk is that you can filter some routes that actually haven't come from this ASBR boundary but they had travelled in the domain.

This can cause ip reachability problems.

For this reason is better to use different route tag values in different redistribution points.

However, if you have two ASBR nodes that redistribute from the same EIGRP domain into OSPF in this case you use the same route-tag to avoid to send back info learned on ASBR1 via ASBR2 to EIGRP domain.

So the suggestion is to use different route tags for different external routing domains.

Hope to help

Giuseppe

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Giuseppe Larosa Sat, 11/15/2008 - 01:18

Hello Iqbal,

protocols that are declared able to support route tagging like OSPF, EIGRP, RIPv2 are able to carry the route tag within the routing domain.

This is important because it provides a way to control multiple mutual redistribution points (ASBR nodes in OSPF terminology) by simply denying some specific route tags values.

Actually the support of the route tag is present on LSA type 5 where there is a 32bit field.

So they are useful in complex redistribution scenarios as I have written above.

About the route tags is usually a good policy to associate a value to a specific origin. If you reuse the same value you will have both set of routes treated at the same manner by other ASBR routers.

Hope to help

Giuseppe

i-chowdhury Sat, 11/15/2008 - 02:18

Hi Guiseppe,

Thank you for details. Yes, you are right ospf type 5 LSA carries the tag. The way I understood it, tags carries the same value within the routing domain, if I reuse the same tag value with different match and set criteria on another router but within the same domain, it will produce unexpected routing behavior. Please correct me if I am wrong.

Correct Answer
Giuseppe Larosa Sat, 11/15/2008 - 02:31

Hello Iqbal,

think of it as similar to BGP community a route tag is a way to provide a color to a group of routes.

The routes travel with their route tags in all the OSPF domain (they are LSA type 5 they cannot be filtered they can be blocked at any type of stub area boundary).

Another ASBR has to mark routes that is injecting into OSPF.

the typical route-map with route tags has the logic

route-map asbr-tags deny 10

match tag xxxx

route-map asbr-tags permit 20

set tag xxxx

in this way you don't send back the routes you have learned before from out of OSPF.

if you use the same route-tag the risk is that you can filter some routes that actually haven't come from this ASBR boundary but they had travelled in the domain.

This can cause ip reachability problems.

For this reason is better to use different route tag values in different redistribution points.

However, if you have two ASBR nodes that redistribute from the same EIGRP domain into OSPF in this case you use the same route-tag to avoid to send back info learned on ASBR1 via ASBR2 to EIGRP domain.

So the suggestion is to use different route tags for different external routing domains.

Hope to help

Giuseppe

i-chowdhury Sat, 11/15/2008 - 02:47

Hi Giuseppe,

Thats totally answers my question, thank you very much.

Regards,

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