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Route Redistribution in ISIS

Route Redistribution in ISIS

Route Redistribution in ISIS: In general, Route Redistribution is a technique using which the boundary routers connecting different routing domains can exchange and advertise routing information. In this article, we will learn how to redistribute routes from other IGPs or BGP into ISIS and vice versa. This blog explains the method and configuration for Route Redistribution in ISIS.

Below syntax is used for redistributing any other routing protocol routes into ISIS. The command has to be configured in router configuration mode for the route redistribution in ISIS.

 

Route Redistribution in ISIS - Syntax

redistribute protocol [process-id] [level  level-value] [metric metric-value] [metric-type type-value]  [route-map name]

Description:

  • protocol: For example the protocol you want to redistribute into ISIS (i.e. EIGRP, OSPF,  RIP, or BGP etc.)
  • process-id: This field is specifically used while redistributing the protocols EIGRP and BGP into ISIS. The value of this field is AS number in the case of EIGRP and BGP and process-id in the case of OSPF. No value is required for RIP.
  • level: This is an optional field. It specifies how external routes are redistributed. The level types exist. The routers can be Level 1 (level-1), Level 1/Level 2 (level-1-2), or Level 2 (level-2) routes. The default type is level-2.
  • metric: This is an optional field that specifies the ISIS metric or the default Seed Metric used for the redistributed route. ISIS uses a default metric of 0. Unlike RIP and EIGRP, a default metric of 0 is not treated as infinity or unreachable, so the route is redistributed. The metric is incremented as the route is propagated into the ISIS domain. The IS-IS default metric value is cost.
  • metric-type: This field is optional and specifies the ISIS metric-type as external or internal. The default metric type is internal.
  • route-map: This is again an optional field and used when we have to do any route filtering while redistribution.

 

Redistribute EIGRP into ISIS

Route Redistribution in ISIS

In the above lab scenario, we are redistributing EIGRP routes into ISIS. 


R2(config)#router ISIS
R2(config-router)#redistribute eigrp 10 route-map FILTER

 

Imp Point: Kindly also note that you have to configure the route-map 'FILTER'. An empty route-map or wrong route-map configured will not allow redistribution of routes.

Using the above topology, let us try a simple redistribution of EIGRP into ISIS. While checking the ISIS database for R1's prefix 10.10.0.1/32, we can see the route is installed with a default seed metric of 0.
 
R2(config)#router ISIS
R2(config-router)#redistribute eigrp 10 level-2


R2#sh isis database level-2 R2.00-00 detail
IS-IS Level-2 LSP R2.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000008   0x8BFD        725               0/0/0
  Area Address: 49.00aa
  NLPID:        0xCC
  Hostname: R2
  IP Address:   10.10.23.2
  Metric: 10        IS R3.02
  Metric: 0         IP-External 10.10.0.1 255.255.255.255
  Metric: 0          IP-External 10.10.0.2 255.255.255.255
  Metric: 0          IP-External 10.10.12.0 255.255.255.0
  Metric: 10        IP 10.10.23.0 255.255.255.0



Imp Point:  The default link cost of any link in the ISIS domain is 10 (irrespective of link bandwidth) when ISIS is configured with metric-style narrow (0-63).

If we check the route of prefix 10.10.0.1/32 on R3, the link cost metric will be added to default seed metric(0). This metric is cumulative and keeps on adding as the route propagates more links in the network.

R3#show ip route 10.10.0.1
  Routing entry for 10.10.0.1/32
  Known via "isis", distance 115, metric 10, type level-2
  Redistributing via isis
  Last update from 10.10.23.2 on FastEthernet1/0, 00:05:53 ago
  Routing Descriptor Blocks:
  * 10.10.23.2, from 10.10.23.2, 00:05:53 ago, via FastEthernet1/0
     Route metric is 10, traffic share count is 1



Redistributing ISIS into EIGRP

When ISIS routes are redistributed in other IGP protocols such as EIGRP or OSPF, we have the option to include Level 1, Level 2, or both Level 1 and Level 2 routes.  If no level is specified during redistribution, all routes are redistributed.
 
R2(config)#router eigrp 10
R2(config-router)#redistribute isis ?
  WORD       ISO routing area tag
  level-1    IS-IS level-1 routes only
  level-1-2  IS-IS level-1 and level-2 routes
  level-2    IS-IS level-2 routes only
  metric     Metric for redistributed routes
  route-map  Route map reference
  <cr>
 

R2(config-router)#redistribute isis metric 10000 10 255 1 1500


As specified above, we didn't choose any level type while redistributing ISIS routes in EIGRP, so all routes are distributed by default.
 
R1#show ip route eigrp
D        10.10.0.2/32 [90/156160] via 10.10.12.2, 00:22:33, FastEthernet1/0
D EX     10.10.0.3/32 [170/261120] via 10.10.12.2, 00:01:56, FastEthernet1/0
D        10.10.23.0/24 [90/30720] via 10.10.12.2, 00:22:33, FastEthernet1/0 FastEthernet1/0
 Route metric is 2, traffic share count is 1


If you want to restrict the number of redistributed routes, you can use the below router configuration command in ISIS.

R2(config)#router isis
R2(config-router)#redistribute maximum-prefix maximum [threshold] [warning-only | withdraw]


By default, we will a logging warning when a threshold of 75%  of  the defined maximum value is reached.  Also, after reaching the defined maximum number, no further routes are redistributed. The optional withdraw field will also cause IS-IS to rebuild link-state Protocol Data Units (PDUs) (Link-State Packets [LSPs]) without the external (redistributed) IP prefixes. If the warning-only field is configured, no limitation is placed on redistribution; the maximum value number simply becomes a second point where another warning messaged is logged.

Imp Point: By default, iBGP routes do not get redistributed into ISIS (or OSPF). We need to add bgp redistribute-internal command under the BGP router configuration to enable the BGP internal routes get redistributed in ISIS (or OSPF).


Route Tagging in ISIS

Route Tagging in ISIS  is a feature that enables you to tag route prefixes and use those tags in a route map to control ISIS Route Redistribution or route leaking. The results are network scalability and faster convergence for device updates. In ISIS, devices are allowed to redistribute external prefixes or the prefixes which are learned from any other routing protocol such as RIP, EIGRP etc. static configuration, or connected interfaces. The redistributed routes are allowed in either a Level 1 device or a Level 2 device. Level 2 routes injected as Level 1 routes is called Route Leaking in ISIS.

Kindly note that to use the route tagging feature in IS-IS, you must change to the 'metric-style wide command' from 'metric-style narrow' command which is configured by default under the ISIS router configuration. The tag value is set into sub-TLV 1 for type, length, values (TLV) Type 135.

R1(config)#router isis
R1(config-router)#metric-style wide

R1#conf t
R1(config)#int gi0/0/1
R1(config-if)#ip address 10.10.12.1 255.255.255.0
R1(config-if)#isis tag 100


ISIS route tagging Commands

R1#show isis database verbose

R# show ip route x.x.x.x


After the tag is set to an ISIS prefix, we can filter the route or set any routing policy to this tagged route using a route-map during redistribution.
 
R2(config)#route-map TAG [permit | deny ] [sequence-number]
R2(config-route-map)#match tag tag-value
set next-hop x.x.x.x
end

 

I hope you have found this article informative and useful and now have a fair understanding of Route Redistribution in ISIS, Redistributing ISIS routes into other protocols and how ISIS Route Tagging works. For any of the related queries or feedback, kindly write to us at networkurge@gmail.com

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