Administrative Distance, Access Lists, Route Manipulation, and IP Multicast - CCIE Routing and Switching Exam Certification Guide - page 488

Route Tagging
Routes can be configured with an administrative tag that identifies their origination. Routes
originated from one external source can be set with a tag of 1, and routes from another external
source can be set with a tag of 2. This is useful in identifying routes to be filtered when
redistributing into other networks. The tag can be matched during redistribution. OSPF, RIPv2,
and EIGRP carry tags on external routes. The tag can set policy on routes redistributed into
BGP. Also, static routes can be configured with the tag keyword. Route tagging is another
method for the internetwork administrator to mark or tag certain routes, usually redistributed
routes, and then to use the tag as the deciding factor on route policy decisions. As shown in
Figure 10-3, OSPF external routes from RIP are set with a tag of 5, and routes from EIGRP are
set with a tag of 3. At the redistribution, a route map is built to filter the routes with a tag set to 5.

Route Tags

Figure 10-3

RIP

Tag=5

OSPF external routes with
tag=5 are redistributed into BGP.

Tag= 5

OSPF

BGP

Tag=3

Tag=3 X

EIGRP

The following commands set a tag value of 5 to routes redistributed from EIGRP into OSPF:

router ospf 100
redistribute eigrp 50 tag 5
The following command sets a tag value of 10 to a static route:

ip route 1.1.1.1 0.0.0.0 2.2.2.2 200 tag 10
The following commands redistribute OSPF routes that have a tag value of 10 and set the BGP
local preference to 200:

router bgp 1000
redistribute ospf 100 route-map ospftag2bgp
!
route-map ospftag2bgp permit 10
match tag 10
set local-preference 200
route-map ospftag2bgp permit 20