Nov 182009
 

Did you ever tag your EIGRP routes to prevent routingloops in an enterprise network? And did you mind about the TAG-value? YOU HAVE TO!

If you tag your external EIGRP routes (that ones you already redistributed from another routing protocol or from static) with a routemap you can choose your TAG-value from 0-4294967295.

But if you try to tag internal EIGRP routes, you can only use the range 0-255. Currently I wasn’t able to find any documentation about this – but thats the thing I had to learn while trying to use greater values.

Let’s do an example:

2 Routers connected via Eth 1/0, Transfernet 192.168.12.0/29, Lo 0 on both routers, EIGRP 1

router eigrp 1
network 10.0.0.0
network 192.168.0.0 0.0.255.255
distribute-list route-map settag out
no auto-summary
!
route-map settag permit 10
set tag 199

The topology-table on Router2 for the Loopback-IP of Router1 shows this:

R2#sh ip eig top 10.1.1.1/32
IP-EIGRP (AS 1): Topology entry for 10.1.1.1/32
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 409600
Routing Descriptor Blocks:
192.168.12.1 (Ethernet1/0), from 192.168.12.1, Send flag is 0x0
Composite metric is (409600/128256), Route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 6000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
Internal tag is 199
R2#

Now Let’s try it with tag-value 256

route-map settag permit 10
set tag 256

Topology-table of Router2:

R2#sh ip eig top 10.1.1.1/32
IP-EIGRP (AS 1): Topology entry for 10.1.1.1/32
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 409600
Routing Descriptor Blocks:
192.168.12.1 (Ethernet1/0), from 192.168.12.1, Send flag is 0x0
Composite metric is (409600/128256), Route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 6000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
R2#

As we can see, internal TAG-values greater than 255 are ignored.
If you tag an external Route, you can use the whole range suggested from the set tag command (0-4294967295). This limitation applies only to internal routes.

All tests were made with 12.4(24)T2

 Posted by at 18:25