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New Member

CEF creating host route for /24 prefix! Is that normal?

Hi,

I have a strange CEF behaviour and I need to know if it's normal or not:

This is a subnet of a PE router's loopback interface in the MPLS network. As you see, I have configured the loopback with /24 length intentionally and have received it via ISIS on another PE router.

PE2#sh ip route isis

      10.0.0.0/24 is subnetted, 1 subnets

i L2     10.10.10.0 [115/30] via 172.17.22.1, 00:03:51, GigabitEthernet1/0

Now, when I check the CEF, I find that CEF has created a host route for this particular route:

PE2#show ip cef 10.10.10.10 internal

10.10.10.10/32, epoch 0, refcount 5, per-destination sharing

  sources: RR

  subblocks:

   1 RR source [active source]

    Dependent covered prefix type rr cover 10.10.10.0/24

    non-eos chain 18

  ifnums:

   GigabitEthernet1/0(3): 172.17.22.1

  path 6673E928, path list 66741244, share 1/1, type recursive, for IPv4, flags doesnt-source-via, cef-internal

  recursive via 10.10.10.0/24<nh:10.10.10.10>[IPv4:Default], fib 66744B30, 1 terminal fib, v4:Default:10.10.10.0/24

    path 6673E8B8, path list 667411F4, share 1/1, type attached nexthop, for IPv4

      MPLS short path extensions: MOI flags = 0x0 label 18

    nexthop 172.17.22.1 GigabitEthernet1/0 label 18, adjacency IP adj out of GigabitEthernet1/0, addr 172.17.22.1 657398A0

  output chain: label 18 TAG adj out of GigabitEthernet1/0, addr 172.17.22.1 65739720

The question, Is this a default behaviour for CEF? and if not, why CEF has done this?

Thank you.

  • MPLS
2 REPLIES
Cisco Employee

CEF creating host route for /24 prefix! Is that normal?

Hi,

I believe you have a typo in your show ip cef command: you asked for 10.10.10.10 (which is a particular IP within 10.10.10.0/24). Try with

show ip cef 10.10.10.0/24

or

show ip cef 10.10.10.0 255.255.255.0

Best regards,

Peter

New Member

CEF creating host route for /24 prefix! Is that normal?

Hi Peter,

After some investigations and trials, I found that  this is the default CEF behaviour when the /24 prefix in the routing  table contains BGP router-id for MP-BGP peer that advertise VPNv4 route.

So the following conditions must be met for the CEF to, automatically, insert /32 host entry in the CEF table:

1- MP-BGP peering is established over using a router-id that falls into the /24 subnet.

2- VPNv4 routes are received.

Thanks for your help    

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