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Chapter 8 - PIM-DVMRP Networks

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Route Exchange
Which unicast routes from the local routing table are reported to the DVMRP neighbor? By default, only the directly connected routes are reported. For example, in Figure 8-5, we have a PIM-DM-enabled router connected through a DVMRP tunnel to an MBONE DVMRP router. The configuration for the PIM router is given below.
Figure 8-5: Connecting to the MBONE with a DVMRP tunnel
interface Ethernet 0
ip address 10.1.1.1 255.255.255.0
ip pim dense mode
interface Serial 0
ip address 10.1.2.1 255.255.255.0
ip pim dense mode
interface Tunnel 0
ip unnumbered Ethernet 0
ip pim dense-mode
tunnel source Ethernet 0
tunnel destination 10.1.1.2
tunnel mode dvmrp
The routing table for the PIM router contains the directly connected routes and any routes learned through a dynamic unicast IP routing protocol. Assume that for now the unicast routing table contains only the directly connected routes and that the DVMRP route advertises two routes:
144.223.136.0/24
Metric = 5
156.26.31.0/24
Metric = 7
When the PIM router receives the routes, the metric is increased by one and the routes are placed in the local DVMRP routing table, which contains
144.223.136.0/24
Metric = 6
156.26.31.0/24
metric = 8
These routes are then reported back to the DVMRP router and are oisoned-reversed. The routes from the local DVMRP table sent in the route report are
144.223.136.0/24
metric 38
156.26.31.0/24
metric 40
The routes that are reported from the unicast routing table to the DVMRP router are
10.1.1.0/24
Metric = 1
10.1.2.0/24
Metric = 1
Notice that a default metric of one hop is used for the routes reported from the unicast routing table. How do we advertise non-connected networks from the unicast routing table? The answer is with the following interface command on the tunnel interface:
ip dvmrp metric metric [list access-list] {[protocol process-id] | dvmrp]
ip dvmrp metric metric route-map map-name
metric
Metric to be used for the routes in the DVMRP route report. The value can be between 0 and 32. A value of 0 prevents a route or routes from being advertised. A value of 32 indicates infinity or unreachable.
list access list
Optional. A standard IP access list can be used to control which routes are reported.
protocol
Optional. Unicast routing protocol name (rip, igrp, eigrp, ospf, bgp, isis, static, or dvmrp).
process-id
Optional. Unicast routing protocol process ID.
dvrmp
Optional. Allows routes in the DVMRP routing table to be filtered or have their metric adjusted.
route-map
Filter the unicast routes that are reported using a route map.
map-name
ip dvmrp metric <metric>
The configuration for the DVMRP tunnel would be
interface Tunnel 0
ip unnumbered Ethernet 0
ip pim dense-mode
ip dvmrp metric 1
tunnel source Ethernet 0
tunnel destination 10.1.1.2
tunnel mode dvmrp
What we have done is make a very serious mistake. The dvmrp metric command applies to every route in the unicast routing table. This is not too serious, however, if the unicast routing table is small. If the table is large, on the order of thousands of routes, then all these routes will be injected in the DVMRP router and the MBONE. When something like this occurs, we usually need a rule to remind us not to do it:
When using the command ip dvmrp metric, always use an access list.
Another good rule when connecting PIM and DVMRP is to always use a tunnel, because a tunnel gives us the maximum DVMRP capability.
If we have the routes 172.16.1.0/24 and 202.5.6.0/24 in our routing table, for example, and we only want to advertise the 172.16.1.0 network, then we could use the access list shown below:
access-list 1 permit 172.16.1.0 0.0.0.255
access-list 1 deny any
The modified tunnel configuration would now contain
interface Tunnel 0
ip unnumbered Ethernet 0
ip pim dense-mode
ip dvmrp metric 1 list 1
tunnel source Ethernet 0
tunnel destination 10.1.1.2
tunnel mode dvmrp
access-list 1 permit 172.16.1.0 0.0.0.255
access-list 1 deny any
If the value of the metric is 0, then this means the indicated routes will not be advertised. Let’s look at some examples to illustrate some of the permutations of this command.
ip dvmrp metric 0
Do not advertise any of the routes in the unicast routing table. The same effect can be achieved by not even using this command.
ip dvmrp metric 0 list 1
Denies routes in list 1 but advertises others with a metric of one.
ip dvmrp metric 1 eigrp 100
Advertises EIGRP routes in the routing table with a metric of one.
ip dvmrp metric 0 dvmrp
If your network has more than one PIM-DVMRP boundary router, then you may want to prevent DVRMP routes learned from one border from being advertised back into the MBONE by another boundary router. This form of the command will prevent that from happening.

 


 
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