Route tagging questions - "blank" tag, and tags change?

[PAUL TRIVINO]

I have a couple questions about route tagging.  I don't currently use it but it may help with some issues.  As I understand it:

• BGP tags routes "automagically" with the AS number
• EIGRP does not tag routes unless explicitly told to
• OSPF does not tag routes unless explicitly told to

(Actually I do use tagging in a few places but as of now, only to tag static routes, so that I do or do not redistribute them).

Questions:

Does a tag of "0" indicate an "untagged" route? I.e., using "match tag 0" in a route-map would find ONLY routes not previously tagged? (When I do "sh ip route tag 0" on my EIGRP-only router, this seems to be the case.)

If one redistributes a route, say between EIGRP and BGP or vice versa, and you don't *explicitly" tag or retag the route, does the tag it has ever change?

Thanks.

Paul

[milan.kulik]

Hi,

AFAIK, BGP does not use simple route tags (you can use communities, e.g., instead).

But when you are redistributing a prefix from BGP to some IGB, it's tagged by a value equal to the router BGP AS number by default.

I believe tag=0 is the default value used for each route in the RIB.

It might be interesting to configure a route-map used for a route redistribution and setting the tag value to 0!

See here http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_pi/command/iri-cr-book/iri-cr-s1.html#wp4048265148

and here

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_pi/command/iri-cr-book/iri-cr-s1.html#wp7981727100

for more details.

Best regards,

Milan