Looping


The information of a distance vector algorithm is said to be based on rumors. This is also known as routing by rumor. Because RIP is a distance vector protocol, it builds its routing table based on the information received from its directly attached neighbors after checking if the information received is genuine and error-free. To understand the looping characteristics of the RIP routing protocol and its workaround, it is important to understand the routing procedure of the RIP protocol. So you may understand the RIP routing behavior, its topology is shown in Figure 6.8.

click to expand
Figure 6.8: Scenario to describe the time required to build a consistent routing table across four routers.

In the topology shown in Figure 6.8, assume that the router link from Router1 to Router4 has just been activated. Therefore, none of the routers have any information about the network topology to which they are connected or the next hop router to which they should forward the traffic. The only information they possess at this point is about the directly connected networks.

Table 6.5 shows the details of routing calculation for the RIP routing protocol.

Table 6.5: Routing Details for RIP

Situation

Router

Routes

Metric

Before any update, just after the router startup

Router1

11.0.1.0 -> Directly connected

11.0.2.0 -> Directly connected

192.168.1.0->Directly connected

Router2

192.168.1.0->Directly connected

192.168.2.0->Directly connected

Router3

192.168.2.0->Directly connected

10.0.1.0 -> Directly connected

10.0.2.0 -> Directly connected

10.0.3.0 -> Directly connected

10.0.4.0 -> Directly connected

Router4

10.0.4.0 -> Directly connected

10.0.5.0 -> Directly connected

Just after receiving the first update

Router1

11.0.2.0 -> Directly connected

11.0.2.0 -> Directly connected

192.168.1.0->Directly connected

1

192.168.2.0 -> via 192.168.1.1

Router2

192.168.1.0->Directly connected

192.168.2.0->Directly connected

1

11.0.0.0 -> via 192.168.1.2

1

10.0.0.0 -> via 192.168.2.2

192.168.2.0->Directly connected

10.0.1.0 -> Directly connected

10.0.2.0 -> Directly connected

Router3

10.0.3.0 -> Directly connected

10.0.4.0 -> Directly connected

192.168.1.0 ->via 192.168.2.1

1

10.0.5.0 -> via 10.0.4.2

1

10.0.4.0 -> Directly connected

10.0.5.0 -> Directly connected

Router4

192.168.2.0 -> via 10.0.4.1

1

10.0.1.0 -> via 10.0.4.1

1

10.0.2.0 -> via 10.0.4.1

1

10.0.3.0 -> via 10.0.4.1

1

Just after receiving the second update

Router1

11.0.1.0 -> Directly connected

11.0.2.0 -> Directly connected

192.168.1.0->Directly connected

192.168.2.0 -> via 192.168.1.1

1

10.0.0.0 -> via 192.168.1.1

2

Router2

192.168.1.0->Directly connected

192.168.2.0->Directly connected

11.0.0.0 -> via 192.168.1.2

1

10.0.0.0 -> via 192.168.2.2

1

192.168.2.0->Directly connected

10.0.1.0 -> Directly connected

10.0.2.0 -> Directly connected

Router3

10.0.3.0 -> Directly connected

10.0.4.0 -> Directly connected

192.168.1.0 ->via 192.168.2.1

1

10.0.5.0 -> via 10.0.4.2

1

11.0.0.0 -> via 192.168.2.1

2

Router4

10.0.4.0 -> Directly connected

10.0.5.0 -> Directly connected

192.168.2.0 -> via 10.0.4.1

1

10.0.1.0 -> via 10.0.4.1

1

10.0.2.0 -> via 10.0.4.1

1

10.0.3.0 -> via 10.0.4.1

1

Just after receiving the third update

Router1

11.0.1.0 -> Directly connected

11.0.2.0 -> Directly connected

192.168.1.0->Directly connected

192.168.2.0 -> via 192.168.1.1

1

10.0.0.0 -> via 192.168.1.1

2

Router2

192.168.1.0->Directly connected

192.168.2.0->Directly connected

11.0.0.0 -> via 192.168.1.2

1

10.0.0.0 -> via 192.168.2.2

1

Router3

192.168.2.0->Directly connected

10.0.1.0 -> Directly connected

10.0.2.0 -> Directly connected

10.0.3.0 -> Directly connected

10.0.4.0 -> Directly connected

192.168.1.0 ->via 192.168.2.1

1

10.0.5.0 -> via 10.0.4.2

1

11.0.0.0 -> via 192.168.2.1

2

Router4

10.0.4.0 -> Directly connected

10.0.5.0 -> Directly connected

192.168.2.0 -> via 10.0.4.1

1

10.0.1.0 -> via 10.0.4.1

1

10.0.2.0 -> via 10.0.4.1

1

10.0.3.0 -> via 10.0.4.1

1

11.0.0.0 -> via 10.0.4.1

3
  • Hop count

  • Split horizon

  • Route poisoning

  • Triggered updates

  • Hold-down timers

Hop Count

Also referred to as counting to infinity, hop count sets the maximum number of routers that a particular path can cross. This parameter helps control the endless routing loops in the network. According to this control, the maximum number of hop counts a path can cross is 15, and a hop count of 16 and onwards is called an invalid path or an unreachable path.

Split Horizon

According to the distance vector functionalities, every router sends its entire routing table information as an update to the directly connected neighbors. The split horizon deviates from this routine activity by restricting the routers from sending information about the network interface from which it was learned. The route entry pointing back to the router from which that particular route was learned is known as the reverse route, which becomes the major cause for maturing of routing loops. The split horizon eliminates the existence of reverse routes and helps in avoiding routing loops. There are two types of split horizon methods:

  • Simple split horizon

  • Split horizon with poisoned reverse

  • Simple Split Horizon

    This is defined as sending updates out of an interface, excluding the networks that were learned from the updates received on that interface. For example, as per Figure 6.8, when Router3 sends its routing update to Router4 via its interface fa0/0, the information sent will be as shown in Table 6.6.

    Table 6.6: Routing Update Information for Router3

    Routes

    Metric

    Status

    192.168.2.0 -> Directly connected

    Will be sent

    10.0.1.0 -> Directly connected

    Will be sent

    10.0.2.0 -> Directly connected

    Will be sent

    10.0.3.0 -> Directly connected

    Will be sent

    10.0.4.0 -> Directly connected

    1

    Will not be sent

    192.168.1.0 ->via 192.168.2.1

    1

    Will be sent

    10.0.5.0 -> via 10.0.4.2

    2

    Will not be sent

    11.0.0.0 -> via 192.168.2.1

    Will be sent

The update packet broadcasted from Router4 on its interface fa0/0 will not carry any information about the networks learned from Router3. This is shown in Table 6.7.

Table 6.7 : Routing Update Information for Router4

Routes

Metric

Status

10.0.4.0 -> Directly connected

Will not be sent

10.0.5.0 -> Directly connected

Will be sent

192.168.2.0 -> via 10.0.4.1

1

Will not be sent

10.0.1.0 -> via 10.0.4.1

1

Will not be sent

10.0.2.0 -> via 10.0.4.1

1

Will not be sent

10.0.3.0 -> via 10.0.4.1

1

Will not be sent

11.0.0.0 -> via 10.0.4.1

3

Will not be sent

To check the split horizon configuration status for an interface, look at the highlighted line in the output of the command show ip interface fa0/0 from Router4, as shown in Listing 6.52.

Listing 6.52 Output of show ip interface fa0/0 Command from Router4

start example
Router4#show ip interface fa0/0 FastEthernet0/0 is up, line protocol is up   Internet address is 10.4.0.2/16   Broadcast address is 255.255.0.0   MTU 1500 bytes,   Helper address is not set   Directed broadcast forwarding is disabled   Outgoing access list is not set   Inbound  access list is not set   Proxy ARP Is Enabled   Security Level Is Default   Split horizon Is Enabled   ICMP redirects are always sent   ICMP unreachables are always sent   ICMP mask replies are never sent   IP fast switching is enabled   IP fast switching on the same interface is enabled   IP Null turbo vector   IP multicast fast switching is enabled   IP multicast distributed fast switching is disabled   router Discovery Is disabled   IP output packet accounting is disabled   IP access violation accounting is disabled   TCP/IP header compression is disabled   RTP/IP header compression is disabled   Probe proxy name replies are disabled   Policy routing Is disabled   Network address translation is disabled   WCCP Redirect outbound is disabled   WCCP Redirect exclude is disabled   BGP Policy Mapping is disabled
end example

To enable and disable the split horizon, use the command shown in Listing 6.53 from the interface configuration mode.

Listing 6.53 Command to Enable and Disable Split Horizon

start example
Router#conf t Router(config)#int fa0/0 Router(config-if)#ip split-horizon     =  Enables  Router(config-if)#no ip split-horizon  =  Disables
end example




Cisco IP Routing Protocols(c) Trouble Shooting Techniques
Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)
ISBN: 1584503416
EAN: 2147483647
Year: 2006
Pages: 130

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