EIGRP and Null0

Answered Question
Aug 27th, 2010

Hi Everyone,

I have router A->B->C running eigrp 1 on all of them.  Router C is running Tunnel gre multipoint and all of its spokes connecting to C get summarization of certain subnets.

Router A is advertising 192.218.96.0/22 and 192.110.88.0/24

RouterC tunnel configuration

interface Tunnel0
bandwidth 1000
ip address 10.253.252.1 255.255.252.0
no ip redirects
ip mtu 1280
ip nhrp authentication Xxxxxx
ip nhrp map multicast dynamic
ip nhrp network-id 2
ip nhrp holdtime 300
ip nhrp server-only
ip nhrp max-send 500 every 10
ip nhrp shortcut
ip nhrp redirect
ip route-cache policy
ip route-cache flow
ip tcp adjust-mss 1240
no ip split-horizon eigrp 1
ip summary-address eigrp 1 192.218.96.0 255.255.252.0 5 
ip summary-address eigrp 1 192.110.88.0 255.255.252.0 5 
ip summary-address eigrp 1 0.0.0.0 0.0.0.0 5
ip policy route-map BB_Internet
delay 1001
tunnel source GigabitEthernet0/3
tunnel mode gre multipoint
tunnel key 2
tunnel protection ipsec profile PROFILE1
RouterC#sh ip route 192.218.96.0  
Routing entry for 192.218.96.0/22
  Known via "eigrp 1", distance 5, metric 2223872, type internal
  Redistributing via eigrp 1
  Routing Descriptor Blocks:
  * directly connected, via Null0
      Route metric is 2223872, traffic share count is 1
      Total delay is 20210 microseconds, minimum bandwidth is 1500 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 2/255, Hops 3

RouterC#sh ip route 192.110.88.0
Routing entry for 192.110.88.0/24
  Known via "eigrp 1", distance 90, metric 31488, type internal
  Redistributing via eigrp 1
  Last update from RouterB on GigabitEthernet0/1, 00:10:34 ago
  Routing Descriptor Blocks:
  * RouterB, from RouterB, 00:10:34 ago, via GigabitEthernet0/1
      Route metric is 31488, traffic share count is 1
      Total delay is 230 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 3/255, Hops 4

Question? Why 192.218.96.0/22 had a directly connected to Null0 and the other doesn't? Can someone help me understand the reason?

Thank you....

Cheers!!!

I have this problem too.
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Correct Answer by Jeremy Buck about 6 years 3 months ago

mguzman4158,

I just set this up on a few routers and the reason why you see the /24 is because IOS uses a longest-prefix match (/24 is longer than /22). If you type the following command, "show ip route eigrp" you can see all three routes as follows:

R1#show ip route eigrp
D    192.110.88.0/24 [90/409600] via 192.168.0.1, 00:05:04, FastEthernet0/0
D    192.110.88.0/22 is a summary, 00:00:44, Null0
D    192.218.96.0/22 [90/409600] via 192.168.0.1, 00:05:04, FastEthernet0/0


R1#show ip route 192.218.96.0
Routing entry for 192.218.96.0/22, supernet
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via eigrp 1
  Last update from 192.168.0.1 on FastEthernet0/0, 00:05:37 ago
  Routing Descriptor Blocks:
  * 192.168.0.1, from 192.168.0.1, 00:05:37 ago, via FastEthernet0/0
      Route metric is 409600, traffic share count is 1
      Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

R1#show ip route 192.110.88.0
Routing entry for 192.110.88.0/24
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via eigrp 1
  Last update from 192.168.0.1 on FastEthernet0/0, 00:05:51 ago
  Routing Descriptor Blocks:
  * 192.168.0.1, from 192.168.0.1, 00:05:51 ago, via FastEthernet0/0
      Route metric is 409600, traffic share count is 1
      Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

R1#

Also,

If you type the following command, "show ip eigrp topology" you can see both networks in the topology table.

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(192.168.0.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 192.168.0.0/30, 1 successors, FD is 281600
        via Connected, FastEthernet0/0
P 192.168.1.0/30, 1 successors, FD is 281600
        via Connected, FastEthernet0/1
P 192.218.96.0/22, 1 successors, FD is 409600
        via 192.168.0.1 (409600/128256), FastEthernet0/0
P 192.110.88.0/22, 1 successors, FD is 409600
        via Summary (409600/0), Null0
P 192.110.88.0/24, 1 successors, FD is 409600
        via 192.168.0.1 (409600/128256), FastEthernet0/0

R1#

It's just the way IOS displays the routes for you... A longer prefix match is preferred over a lower administrative distance (/24 over /22 is preferred over AD 90 vs AD 5).

-Jeremy

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Correct Answer
Jeremy Buck Fri, 08/27/2010 - 14:38

mguzman4158,

I just set this up on a few routers and the reason why you see the /24 is because IOS uses a longest-prefix match (/24 is longer than /22). If you type the following command, "show ip route eigrp" you can see all three routes as follows:

R1#show ip route eigrp
D    192.110.88.0/24 [90/409600] via 192.168.0.1, 00:05:04, FastEthernet0/0
D    192.110.88.0/22 is a summary, 00:00:44, Null0
D    192.218.96.0/22 [90/409600] via 192.168.0.1, 00:05:04, FastEthernet0/0


R1#show ip route 192.218.96.0
Routing entry for 192.218.96.0/22, supernet
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via eigrp 1
  Last update from 192.168.0.1 on FastEthernet0/0, 00:05:37 ago
  Routing Descriptor Blocks:
  * 192.168.0.1, from 192.168.0.1, 00:05:37 ago, via FastEthernet0/0
      Route metric is 409600, traffic share count is 1
      Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

R1#show ip route 192.110.88.0
Routing entry for 192.110.88.0/24
  Known via "eigrp 1", distance 90, metric 409600, type internal
  Redistributing via eigrp 1
  Last update from 192.168.0.1 on FastEthernet0/0, 00:05:51 ago
  Routing Descriptor Blocks:
  * 192.168.0.1, from 192.168.0.1, 00:05:51 ago, via FastEthernet0/0
      Route metric is 409600, traffic share count is 1
      Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

R1#

Also,

If you type the following command, "show ip eigrp topology" you can see both networks in the topology table.

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(192.168.0.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 192.168.0.0/30, 1 successors, FD is 281600
        via Connected, FastEthernet0/0
P 192.168.1.0/30, 1 successors, FD is 281600
        via Connected, FastEthernet0/1
P 192.218.96.0/22, 1 successors, FD is 409600
        via 192.168.0.1 (409600/128256), FastEthernet0/0
P 192.110.88.0/22, 1 successors, FD is 409600
        via Summary (409600/0), Null0
P 192.110.88.0/24, 1 successors, FD is 409600
        via 192.168.0.1 (409600/128256), FastEthernet0/0

R1#

It's just the way IOS displays the routes for you... A longer prefix match is preferred over a lower administrative distance (/24 over /22 is preferred over AD 90 vs AD 5).

-Jeremy

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