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Nexus 7K ISIS Router ID set an IP address under a VRF instance. Is this normal?

d.elefsiniotis
Level 1
Level 1

Dear all,

hopefully this might be an easy question for whoever deals with Nexus switches. I have noticed the following about the isis protocol and how nexus 7K seems to set the router-id for this protocol and i would like clarifications from you if possible. 

Within the LSPs that ISIS creates, the router ID seems to be an ip address which is configured under a VRF interface and not the looback of the box. Of course I know that ISIS protocol and router ID concept are alike (router ID is set mainly for IP protocols, while ISIS utilizes as "router-id" the NET address. Hence, you cannot set actually the router ID in ISIS like you do explicitly with CLI for OSPF and EIGRP. 

You may see below the commands taken out of a nexus 7K, that I'm operating. 

nexus7K1# show isis database detail

 

…output omitted….

 

nexus7K1.00-00     * 0x00042CCD   0x0515    720        0/0/0/3

    Instance      :  0x00004A4D

    Area Address  :  49.0001

    NLPID         :  0xCC

    Router ID     :  192.168.1.251

    IP Address    :  192.168.1.251

    Hostname      :  nexus7K1          Length : 9

 

…output omitted….

 

nexus7K1# show running-config interface vlan 107

 

!Command: show running-config interface Vlan107

!Time: Mon Mar 23 16:28:54 2015

 

version 5.2(5)

 

interface Vlan107

  no shutdown

  description xxxxx

  vrf member MGMT

  no ip redirects

  ip address 192.168.1.251/24

  hsrp version 2

  hsrp 107

    priority 255

    ip 192.168.1.240

 

As a result in other network routers, we may see that networks towards Nexus are advertised with the router id 192.1681.251

 

E.g for IP address 194.30.193.3 @ Nexus7k1 (vlan xx)

 

RP/0/RSP0/CPU0:CRS-00#show route 194.30.193.3

 

Routing entry for 194.30.193.0/24

  Known via "isis 3329", distance 115, metric 40, type level-2

  Installed Mar  5 06:00:26.487 for 2w4d

  Routing Descriptor Blocks

    62.38.93.97, from 192.168.1.251, via TenGigE0/1/0/0

      Route metric is 40

  No advertising proto

 

Do you know why this is happening? Or what is the reason for nexus picking over this IP address (under a VRF?) for ISIS protocol router-id? Is it some kind of default mechanism picking over specific e.g max or lower IP address from those configured?

I couldn't locate any useful information in Nexus 7K configuration guide regarding ISIS router-id concept. 

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/unicast/configuration/guide/l3_cli_nxos/l3_isis.html#wp1063980

I have searched the running configuration to find out the whole banch of IP addresses configured. The above address lies in the middle so nor max or lowest rule is followed:

ip address 10.112.60.2/24

ip address 194.30.193.3/24

Finally, do you know any way to configure explicitly this via CLI in order to be the loopback? 

 

Thanks in advance

Dimitris 

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