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Re: Doubt in ISIS
From: Savyasachi Choudhary <savyasachi.choudhary () gmail com>
Date: Thu, 21 Apr 2011 14:46:15 +0530

Thank you for the response. Its very detailed, and I am yet to understand
it completely.

Following is the problem I am facing on Ericsson Routers.

                                (static route)
Router1---------------------------------------------------------------------Router2
                    ip route 202.1.1.0/24 null0 cost 9

In this 2 router topology, I have imported a static route 202.1.1.0/24
with cost 9.

And I am giving the following redistribute command on Router1, and later
observe show ip route command on Router2

PROTOCOL COMMAND (in Router1)
             COST (Router2)                            OBSERVATION

ISIS             #redistribute static
                       10
 default
                #redistribute static metric 7 mertic-type external
               17
 configured in redist  + 10
                #redistribute static metric 7 mertic-type internal
             17
 configured in redist  + 10

What is observed is, it completely ignores the cost (9) that is configured
with the static route. In case of OSPF, they consider cost 9 also.
So, I have a doubt, whether ISIS RFC describes to ignore the route cost? Or
it is just implementation dependent.


Regards,
Savyasachi
7676077879



On Thu, Apr 21, 2011 at 2:14 PM, Vitkovsky, Adam <avitkovsky () emea att com>wrote:

Isis doesn't have type-1 and type-2 external routes
It has something similar though
Metric of a prefix can be marked as internal metric or external metric

These are my isis notes section regarding internal and external:

the metric type for ext(redistributed) routes can be set as int/ext
-by default internal metric is asigned during redistribution
 if nothing is specified

ext metric has the I/E bit set  -bit 7
=> higher metric value +64
--------------------------------------
but cisto doesn't set bit 7 but bit 8
-when narrow metric is used bit 8 of the metric field is set by cisco
=> external metric than appears to be increased by 128
--------------------------------------
_____________________________________________________________________
Narrow metric:
8th bit is S-bit -support for qos metrics (only 0 is supported)
7th bit is the internal/external bit
And the remaining 8 bits are for the metric

1b 1b        6bits      => max 63 values
0 i/e default metric value
1 i/e delay metric value
1 i/e expense metric value
1 i/e error metric value
-------neighbor ID--------

Internal is default on cisco
If you set external the bit 7 is set -thus the metric appears to be +64
higher compared to the same route redistributed as internal


______________________________________________
TLV specified by ISO 10589 contain metric info:
ES neighbor     type 2
IS neigh        tyep 3
prefix neigh    type 5

______________________________________________
TLV specified by RFC 1195 contain metric info:
ip internal reachability        type 128
ip external reachability        type 130



__________________________________________________________
ISIS metric extensions

extended IS reachability tlv            type 22
extended ip reachability tlv            type 135
and
trafic engineering rotuer ID tlv type 134


-the borrowed the metric fields for delay, expense, error
 -as they are not used

-but the first S-bit and the I/E-bit remained
So the same apply to the extended metric as well



These are my notes form the labs:
_____________________________________________________________
R1
route-map static-routes permit 10
 match ip address prefix-list static-routes
!
router isis 1
 redistribute static ip route-map static-routes metric-type external
level-1

-everything explicitely set in the redistribute cmd
_____________________________________________________________
R2
route-map static-routes permit 10
 match interface Null0
 set metric-type external
 set level level-1
!
router isis 1
 redistribute static ip route-map static-routes

-everything set in the route-map used during the redistribution
_____________________________________________________________

R7#sh isis dat ver

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime
 ATT/P/OL
R1.00-00              0x00000007   0x23CA        642               1/0/0
 Area Address: 49.0001
 NLPID:        0xCC
 Hostname: R1
 IP Address:   1.1.1.1
 Metric: 10         IP 13.0.0.0 255.255.255.0
 Metric: 10         IP 17.0.0.0 255.255.255.0
 Metric: 0          IP 1.1.1.1 255.255.255.255
 Metric: 10         IS R7.00
 Metric: 0          IP-External 200.10.0.0 255.255.255.0
 Metric: 0          IP-External 200.20.0.0 255.255.255.0
 Metric: 0          IP-External 200.30.0.0 255.255.255.0
R2.00-00              0x00000015   0x5DA2        1049              1/0/0
 Area Address: 49.0001
 NLPID:        0xCC
 Hostname: R2
 IP Address:   2.2.2.2
 Metric: 10         IP 24.0.0.0 255.255.255.0
 Metric: 10         IP 27.0.0.0 255.255.255.0
 Metric: 0          IP 2.2.2.2 255.255.255.255
 Metric: 10         IS R7.00
 Metric: 64         IP-External 200.10.0.0 255.255.255.0
 Metric: 64         IP-External 200.20.0.0 255.255.255.0
 Metric: 64         IP-External 200.30.0.0 255.255.255.0


-so definition of external metric in redistribution cmd
 did not touch the metric -not sure how come it's still marked as ext
 -as the bit 7 or 8 should have been set

-and definition of external metric in route-map
 did set the bit 7 of the metric -thus we see increase in metric of 64

-but none of the cmds have set the bit 8 as stated in the book :)




adam

-----Original Message-----
From: Savyasachi Choudhary [mailto:savyasachi.choudhary () gmail com]
Sent: Thursday, April 21, 2011 8:56 AM
To: nanog () nanog org
Subject: Doubt in ISIS

I have a doubt in ISIS.
While redistributing routes from other protocols, how the metric is
decided?
OSPF has deccribed this in RFC 2328 Section 16.4 :

              '4) Let X be the cost specified by the preferred routing
table

          entry for the ASBR/forwarding address, and Y the cost
          specified in the LSA.  X is in terms of the link state
          metric, and Y is a type 1 or 2 external metric.

      (5) Look up the routing table entry for the destination N.  If
          no entry exists for N, install the AS external path to N,
          with next hop equal to the list of next hops to the
          forwarding address, and advertising router equal to ASBR.
          If the external metric type is 1, then the path-type is set
          to type 1 external and the cost is equal to X+Y.  If the
          external metric type is 2, the path-type is set to type 2
          external, the link state component of the route's cost is X,

and the type 2 cost is Y.'

What is the behavior in ISIS?
Regards,
Savyasachi
7676077879




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