DVMRP Route Exchange

DVMRP Route Exchange

DVMRP initially advertises directly connected networks. As other networks are learned through the route advertisement process and routes are added to the local DVMRP routing table, more routes may be advertised.

Unlike RIP route advertisements, DVMRP routes are sent in an abbreviated format, as shown in Figure 5-18. Route advertisements consist of three components: the netmask, the network, and the metric. The netmask is assumed to be of the form 255.x.x.x because the standard subnet masks for class A, B, and C addresses begins with 255. Because the first octet of every subnet mask is assumed to be 255, then the first octet does not need to be included in the route report. This is why the length of the netmask fields in Figure 5-18 is shown as only three bytes. For example, if the netmask in the route report has a value of 255.255.128, then the full netmask has the value 255.255.255.128.

Figure 5-18: DVMRP Route Report packet format

Another method used to reduce the size of the route report is to list one netmask for all networks having the same netmask, instead of listing a netmask for every network. If we are advertising networks 172.16.1.0/24 and 172.16.2.0/24, for example, then we could list the two networks, 172.16.1.0 and 172.16.2.0, and one netmask, 255.255.0 (remember the assumed 255 at the beginning of the netmask). For routing, we only need to know the network address that corresponds to the non-zero values of the netmask. To reduce the packet size further, only the portion of the network that corresponds to a non-zero value of the netmask needs to be reported. With a netmask of 255.255.255.0, we only need to report 172.16.1 and 172.16.2 for the networks mentioned previously. The metric parameter must be listed for each advertised network and the metric values will be explained shortly.

Looking back at Figure 5-18, it is not clear how to differentiate when one set of netmask-network-metric groups ends and another group begins. The delineation between groups is accomplished by setting the most significant bit of the last metric value for the last network in the group, which is equivalent to adding 128 to the metric.

Let’s look at an example. Assume a DVMRP router has the following routes in the local routing table.

Network

Netmask

Metric

156.26.1.0

255.255.255.0

1

144.223.0.0

255.255.0.0

2

12.0.0.0

255.0.0.0

3

191.56.3.0

255.255.255.0

4

130.10.10.0

255.255.0.0

5

188.44.0.0

255.255.0.0

6

The first step in determining the DVMRP route report format is to group the networks to be advertised according to their netmask.

Network

Netmask

Metric

Network Reported

12.0.0.0

255.0.0.0

3 + 128 = 131

12

144.223.0.0

255.255.0.0

2

144.223

130.10.10.0

255.255.0.0

5

130.10

188.44.0.0

255.255.0.0

6 + 128 = 134

188.44

156.26.1.0

255.255.255.0

1

156.26.1

191.56.3.0

255.255.255.0

4 + 128 = 132

191.56.3

Notice that 128 (the most significant bit set) has been added to the last metric of the last network in each group. With the route information listed above, the route report packet can be built and is shown in Figure 5-19.

Figure 5-19: Example DVMRP Route Report packet

One special case is that of the default route. The default route is represented by the mask-network pair 00 00 00/00 . The mask indicates a standard Class C address and normal processing indicates that the mask is 255.0.0.0. This case needs to be interpreted correctly, so the mask for the default route is 0.0.0.0 and not 255.0.0.0.

The processing of DVMRP route reports is much more complex than RIP route processing. The rules that follow dictate how a DVMRP router will treat the routes received in a route report:

1.   If the route is received from a neighbor, then accept it. If the route report is received from a router for which a two-way adjacency was not established (not a neighbor), then reject the route report.

2.   If the metric of a route in the report plus the metric of the receiving router is greater than or equal to infinity (32), then set the metric to infinity (32).

3.   If the metric of a route in the report is greater than or equal to infinity, then no change to the metric will be made (we will see why).

4.   If a route is not in the routing table (a new route) and the metric plus the metric of the receiving router is less than infinity (32), then the route is added to the routing table.

5.   If a route is in the routing table, then another set of rules comes into effect.

a.   If the metric is between but not equal to 32 (infinity) and 64 (2 x 32), then the sending router is informing the receiving router that it is dependent on the receiving router for multicast traffic from any source on that network. Another way of stating this is that the receiving router is on the shortest path back to any source on that network. Figure 5-20 illustrates this situation.

Figure 5-20: DVMRP poison-reverse used to indicate route dependency

In Figure 5-20, we assume that routers A and B have completed the neighbor discovery process and that they have formed a two-way adjacency. As part of its route report, router B says that it can reach network 172.16.2.0/24 with a metric of one (directly attached). In some cases, metrics can be assigned to an interface, but typically the metric is set to one, indicating that the network is one hop away. Router A installs this route in its routing table because this is a new route.

Router A also determines that traffic from any multicast source on network 172.16.2.0/24 has to pass through router B to get to router A. In this situation, router A will poison-reverse the route by adding infinity (32) to the metric and reporting the route back to router B. Router A has a metric of 2 for network 172.16.2.0/24  and the poison-reverse value is 34 (2 + 32). When router B receives this metric (34), then it knows that router A depends on it for multicast traffic from network 172.16.2.0/24. This information is important when pruning occurs.

The function of poison-reverse can easily be seen in the network of Figure 5-21. Router A advertises to routers B and C that it can reach the source in one hop. Routers B and C add one to the metric and advertise the metric as two. Router D then adds one to the metric and advertises the distance as three. Router E receives two advertisements for the source with metrics of two and three, chooses the smallest metric as the RPF interface, and poison-reverses the route. So when router E transmits its routing table to C, the metric is 35, indicating that router E is dependent on router C for traffic from the source.

Figure 5-21: DVMRP poison-reverse example

b.   If the metric plus the metric of the receiving router is greater than the metric of the route already in the routing table, then check the address of the sending router. If the address of the sending router is different than the address of the sending router for the route in the table, ignore the route. If the address of the sending router is the same as the address of the sending router for the route in the table, then replace the metric in the table for that route.

c.   If the metric plus the metric of the receiving router is less than the metric of the route in the table, then replace the route in the table. If the address of the sending router is different than the address of the sending router in the table, then poison-reverse the route.

d.   If the metric plus the metric of the receiving router is equal to the metric in the routing table and the address of the sender matches the address of the sender in the routing table, then refresh the route. If the address of the sender is not the same as the address of the sender in the routing table and the new sender’s IP address is lower, use this neighbor as the upstream router.

e.   If the metric of the received route is greater than or equal to 2 x 32 (64), then ignore the route.

Figure 5-22 illustrates rule 5b:

Figure 5-22: Illustration of rule 5b

1.   Router B sends a route report to router C advertising network 172.16.1.0/24.

2.   This is a new route for router C, so the route is installed in the routing table and the poison-reverse route is sent back to router B.

3.   Router A sends a route report containing the network 172.16.1.0/24 with the same metric being advertised by router B.

4.   Router C now selects router A as the upstream router for multicast traffic from network 172.16.1.0/24 because router A has a lower IP address than router B and sends a poison-reverse to router A for this network. An updated poison-reverse is also sent to router B (without the addition of infinity) to inform router B that router C is no longer dependent on router B for multicast traffic from network 172.16.1.0/24.

In the case of a multi-access network such as ethernet, only one router needs to forward multicast traffic onto the network. For the network in Figure 5-23, each router is a designated forwarder for a particular multicast source. The designated forwarder for each multicast source is the router that has the smallest metric back to the source. If two or more routers attached to a multi-access network have the same metric back to the source, then the router with the lowest IP address is elected designated forwarder.

Figure 5-23: DVMRP-designated router example

For network 172.16.1.0 in Figure 5-23, there are three multicast sources for which a designated forwarder needs to be elected. For source 1, the choices are router A or router B. Both have an identical metric back to source 1, so the IP address of the routers is used to break the tie. In this case, router A becomes the designated forwarder because it has a lower IP address. For source 2, router B is the designated forwarder because it is the only router attached to network 172.16.1.0/24 that has a path back to source 2. The same argument applies for router C and source 3. In this scenario, we have three designated forwarders on the multi-access network, one for each source.