OSPF equal cost - loops

[Steffen Frederiksen]

Hi

Please see attached diagram.

When I traceroute from the Cisco 892 router (on the top) to an 10.31.48.0 address the packages is looped between the two cisco 2821 routers two times before the package is delivered.

Type escape sequence to abort.
Tracing the route to 10.31.48.2
VRF info: (vrf in name/id, vrf out name/id)
  1 172.20.1.2 0 msec
    172.20.1.1 0 msec
    172.20.1.2 0 msec
  2 10.31.45.2 12 msec
    10.31.45.7 4 msec
    10.31.45.2 4 msec
 

The package should go from the 172.20.1.2 directly to 10.31.45.2 or 10.31.45.7 and then to the end host. The traceroute shows that the package is bounced two times.

Any suggestions?

Best Regards, Steffen.

[Bilal Nawaz]

Hello Steffen,

This is load balanced via equal cost routes rather than looping.

[Steffen Frederiksen]

So its expexted behavior that the package bouches between the equal cost routers?

[Bilal Nawaz · Accepted solution]

If it was a loop, your ping would not work. The output you get is as a result of the probes used in a traceroute. This link seems to give a reasonable explanation:http://www.dasblinkenlichten.com/why-does-my-traceroute-look-like-that/

Just so you can see, run an extended ping on the 892 like below and share output here.

Cisco-892#ping           
Protocol [ip]: 
Target IP address: 10.31.48.1
Repeat count [5]: 
Datagram size [100]: 
Timeout in seconds [2]: 
Extended commands [n]: y
Source address or interface: 
Type of service [0]: 
Set DF bit in IP header? [no]: 
Validate reply data? [no]: 
Data pattern [0xABCD]: 
Loose, Strict, Record, Timestamp, Verbose[none]: r
Number of hops [ 9 ]: 3
Loose, Strict, Record, Timestamp, Verbose[RV]: 
Sweep range of sizes [n]: 

[Steffen Frederiksen]

Output here

Vonsild-892#ping
Protocol [ip]:
Target IP address: 10.31.48.2
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface: 10.31.103.1
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]: r
Number of hops [ 9 ]: 3
Loose, Strict, Record, Timestamp, Verbose[RV]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.31.48.2, timeout is 2 seconds:
Packet sent with a source address of 10.31.103.1
Packet has IP options:  Total option bytes= 15, padded length=16
 Record route: <*>
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)

Reply to request 0 (4 ms).  Received packet has options
 Total option bytes= 16, padded length=16
 Record route:
   (172.20.1.103)
   (10.31.45.217)
   (10.31.48.2)
   <*>
 End of list

Reply to request 1 (4 ms).  Received packet has options
 Total option bytes= 16, padded length=16
 Record route:
   (172.20.1.103)
   (10.31.45.219)
   (10.31.48.7)
   <*>
 End of list

Reply to request 2 (8 ms).  Received packet has options
 Total option bytes= 16, padded length=16
 Record route:
   (172.20.1.103)
   (10.31.45.217)
   (10.31.48.7)
   <*>
 End of list

Reply to request 3 (4 ms).  Received packet has options
 Total option bytes= 16, padded length=16
 Record route:
   (172.20.1.103)
   (10.31.45.219)
   (10.31.48.2)
   <*>
 End of list

Reply to request 4 (1 ms).  Received packet has options
 Total option bytes= 16, padded length=16

[Steffen Frederiksen]

http://www.dasblinkenlichten.com/why-does-my-traceroute-look-like-that/

Makes sence!!!

[Bilal Nawaz]

Glad that helped. You can also see in the extended ping output, path goes via .217 and then .291