As the name suggests, this feature allows routes with a specific tag to be installed in the RIB faster than other prefixes.  By default, the highest priority prefixes are ones with /32 masks. As we will see here, we can tag routes to give them a higher priority than /32 prefixes.

The command we will be using to see this in action is:

R3#debug isis local-updates

IS-IS local updates debugging is on

R3 is in an L1 area with 2 routers, R1 and R2.

R1 is advertising 10.1.1.1/32, 10.1.11.1/32 and redistributing 200.0.0.0/24.

R2 is advertising 10.1.1.2/32 and 10.1.20.0/24.

The other routes you see in the debugs are connected routes between routers.

Let's clear IS-IS on R3 and see what routes have the highest priority:

*Mar  1 17:35:50.883: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 10.1.1.1

*Mar  1 17:35:50.891: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 10.1.11.1

*Mar  1 17:35:50.895: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 10.1.1.2

*Mar  1 17:35:50.899: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 192.168.113.0

*Mar  1 17:35:50.903: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 192.168.12.0

*Mar  1 17:35:50.907: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 200.0.0.0

*Mar  1 17:35:50.915: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 10.1.20.0

Notice all the /32 prefixes were processed first. Then rest were processed in no particular order. Suppose we want the 200.0.0.0 route processed first. We could do the following (Wide-metrics must be enabled):

On R1 tag the route during redistribution:

R1(config)#route-map con2isis 10

R1(config-route-map)#set tag 1000

R1(config-route-map)#router isis

R1(config-router)#redistribute connected route-map con2isis level-1

On R3, we configure the router to give priority to tag 1000:

R3(config)#router isis

R3(config-router)#ip route priority high tag 1000

Clear ISIS on R3:

R3#cle isis *

*Mar  1 17:40:27.475: ISIS-Upd: Building L1 LSP

*Mar  1 17:40:27.955: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->1000) change, 200.0.0.0

*Mar  1 17:40:27.959: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 10.1.1.2

*Mar  1 17:40:27.963: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 10.1.1.1

*Mar  1 17:40:27.967: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 10.1.11.1

*Mar  1 17:40:27.971: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 192.168.12.0

*Mar  1 17:40:27.971: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 10.1.20.0

*Mar  1 17:40:27.971: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 192.168.113.0

The routes with tag 1000 are processed first every time. Let's tag 10.1.20.0 on R2, it is loopback 20.

R2(config)#int lo 20

R2(config-if)#isis tag 1000

R3#cle isis *

*Mar  1 17:41:45.259: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->1000) change, 10.1.20.0

*Mar  1 17:41:45.263: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->1000) change, 200.0.0.0

*Mar  1 17:41:45.267: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 10.1.1.2

*Mar  1 17:41:45.271: ISIS-Loc: L2 non-summarized metric(4294967295->10)or route-type(->L1) or tag (0->0) change, 10.1.1.1

*Mar  1 17:41:45.275: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 10.1.11.1

*Mar  1 17:41:45.283: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 192.168.12.0

*Mar  1 17:41:45.287: ISIS-Loc: L2 non-summarized metric(4294967295->20)or route-type(->L1) or tag (0->0) change, 192.168.113.0

So to summarize:

• Enable wide metrics for this to work, otherwise tag is not advertised.
• You can set tag in a route-map during redistribution or at the interface level with “isis tag”
• Use the "ip route priority high" command on the router which needs to give priority to the prefixes.

Bryan Bartik
CCIE #23707 (R&S), CCNP
Sr. Support Engineer - IPexpert, Inc.
URL: http://www.IPexpert.com