Setting the metric on the IS-IS default route generated by the attached bit

[jon_panes24]

Hi All,

I have 3 routers in a shared LAN segment. R1 is a level1 only router while R2 and R3 are level-1-2 routers but their links in the LAN segment are level1 only. R2 and R3 have level-2 only links going to other routers. Ive attached a sample topolgy for reference

Since R2 and R3 are level1-2 routers they will set the attached bit for R1 to have a default route. This causes R1 to have 2 equal cost default routes.

Is there a way to set the metric on the default route that is generated by the attached bit?

Thanks!

[Julio E. Moisa]

Hi,

I think is not possible to configure a metric for the automatic default route but have you tried configure a metric on the interface of R2 or R3? the default metric is 10 so you could use higher to manipulate the traffic:

interface g0/0

isis metric 25

If im not wrong the maximum metric with normal style is 63 and with wide is 16700000 aprox. 

[Georg Pauwen]

Hello,

on a side note, and maybe I miss what you are asking, are you referring to the default route that is generated with the 'default-information originate' command ?

If so, a route map attached to the command with a metric and the set-level command will change the metric of the default route and add the default route to the level 1 database:

router isis
default-information originate route-map ISIS_DEFAULT_METRIC
!
route-map ISIS_DEFAULT_METRIC permit 10
set metric 21
set level level-1

R2#show isis database level-1 detail

IS-IS Level-1 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
R1.00-00 0x0000000A 0x78FF 663 0/0/0
Area Address: 49.0001
NLPID: 0xCC
Hostname: R1
IP Address: 192.168.1.2
Metric: 10 IP 192.168.1.0 255.255.255.0
Metric: 10 IP 10.1.1.0 255.255.255.0
Metric: 10 IS R2.00
R2.00-00 * 0x0000000C 0xCC7E 1156 0/0/0
Area Address: 49.0001
NLPID: 0xCC
Hostname: R2
IP Address: 172.21.1.1
Metric: 10 IP 192.168.1.0 255.255.255.0
Metric: 10 IP 172.21.1.0 255.255.255.0
Metric: 10 IP 172.17.1.0 255.255.255.0
Metric: 10 IS R1.00
Metric: 10 IS R3.00
Metric: 21 IP 0.0.0.0 0.0.0.0
R3.00-00 0x00000009 0xA06C 698 0/0/0
Area Address: 49.0001
NLPID: 0xCC
Hostname: R3
IP Address: 172.21.1.2
Metric: 10 IP 172.21.1.0 255.255.255.0
Metric: 10 IP 172.22.1.0 255.255.255.0
Metric: 10 IS R2.00

[Julio E. Moisa]

Hi Georg

Thank you, but I think the original post is related to when a IS-IS router is connected to the backbone or other area and it will attach the ATT bit, it will create a default route automatically and it is installed on the other Level 1 routers, this default route will be pointing toward the nearest Level 1/Level 2 routers.

[jon_panes24]

Thanks Julio and Georg for the quick responses.

Yes i was reffering to the default route generated by the attached bit.

Ill test whether changing the inteface metric will also change the defaul route metric.

[Georg Pauwen · Accepted solution]

Hello,

as an alternative, you could also get rid of the ATT-bit generated default routes on R1 altogether, and then use the route maps in combination with the default-information originate. For that, you first need to configure R1 with the hidden command 'ignore-attached-bit'. This will get rid of both default routes. Subsequently, you configure the route maps with different metrics. Both default routes will be installed in the database, but only the one with the lower metric in the routing table:

R1

router isis 1
ignore-attached-bit

R2

router isis 1
default-information originate route-map ISIS_DEFAULT_METRIC
!
route-map ISIS_DEFAULT_METRIC permit 10
set metric 21
set level level-1

R3

router isis 1
default-information originate route-map ISIS_DEFAULT_METRIC
!
route-map ISIS_DEFAULT_METRIC permit 10
set metric 31
set level level-1

[jon_panes24]

I think its the best way to go. Thanks!

Why are there even hidden commands in Cisco. :-)

[PaulP]

I have just finished going through a few books and videos on ISIS so I have a fresh packet capture which I hope will help with this.

The ATT bit as you can see below only sets the bit to 1 but it is down to the SPF to calculate the cost to the L1/L2 routers (or Intermediate systems). The solutions provided already are valid (route-map or interface cost). Cisco shows an example where cost is modified (probably because it's easier) but a route-map will give you more granularity.

PCAP available here (Level 1): http://packetlife.net/captures/protocol/isis/

Futher reading:

https://www.cisco.com/c/en/us/support/docs/ip/ip-routing/200472-Configure-the-Attach-bit-set.html

I hope it helps,

Paul