Doubt in ISIS

Savyasachi Choudhary savyasachi.choudhary at gmail.com
Thu Apr 21 04:16:15 CDT 2011


> 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 at 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 at gmail.com]
>> Sent: Thursday, April 21, 2011 8:56 AM
>> To: nanog at 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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