Lab 21: Default Routing, Filtering, and Unequal-Cost Load Sharing in IGRP Networks-Part I

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Tuning, Redistribution, and Control of RIP Updates

RIP offers several parameters for tuning timers, controlling broadcasts, and controlling routes. The following is a list of some of the common parameters adjustable within RIP:

  • Router(config-router) timers basic update invalid holddown flush ” This allows the user to set the update, invalid, hold-down, and flush timers for RIP.

  • Router(config-router) passive-interface interface_name ” This command prevents the sending of routing updates on a interface; however, the router still listens to updates received from that interface.

  • Router(config-router) neighbor ip-address ” This command defines a RIP neighbor to exchange unicast updates with. This command should be used in conjunction with the passive-interface command.

  • Router(config-router) offset-list [ access_list_0-99 { in out } offset [ metric _ offset_1-16 ] ” Use this command to increase the value of the routing metrics. The metric offset cannot exceed 16.

The following commands are not exclusive to RIP and can be used with other routing protocols:

  • Router(config-router) distribute-list [ 1-199 ] [ in out ] [ interface ] ” Use this command to call a standard or extended access list to filter inbound or outbound routing updates.

  • Router(config-router) distance [ 1-255 ] adjacent_neighbors_ip_address wildcard_mask [ access_list_0-99 ] ” Use this command to change the administrative distance of routes received from a neighbor. If the IP address and wildcard mask are omitted, all routes for that protocol will be set to the distance value.

  • Router(config-router) redistribute [ connected, static, bgp, igrp, eigrp,ospf, isis ] { metric } { route-map } ” Use this command to redistribute other routing protocols into RIP. A route map may be added for additional route control. This sets the metric to be used for the redistribution of this specific protocol and the specified autonomous system, if the protocol uses one. The other option for setting the redistribute metrics is to use the default-metric command discussed in the item that follows . Whenever redistributing routes, remember that IP needs a route to and from a destination. Many times, mutual redistribution might be required to give IP a path to and from its destination.

  • Router(config-router) default-metric [ 1-16 ] ” Use this command to set the default metric of all routes redistributed into RIP. You must supply a default metric whenever redistributing.

To see how some of these concepts work, apply some to the existing lab. Figure 9-3 changes the subnet between rogue and cyclops to 128.200.3.16/29. Instead of RIP on this segment, run EIGRP as the routing protocol. Redistribute between EIGRP and RIP on the rogue router. As Figure 9-3 shows, another router, storm, has been integrated into the network. Instead of running broadcast updates on the Token Ring segment between the storm and wolverine routers, run unicast updates.

Figure 9-3. Route Redistribution and Unicast Example

graphics/09fig03.gif

To configure RIP unicast between storm and wolverine, you must first use the passive-interface command to prevent RIP broadcasts from entering the Token Ring network. Next , add a neighbor statement to point to the router in which RIP updates are to be sent. Example 9-8 lists the RIP configuration of wolverine and storm.

Example 9-8 RIP Unicast Configuration
  hostname wolverine   !   router rip   passive-interface TokenRing1   network 128.200.0.0   neighbor 128.200.10.2  _______________________________________________________________________  hostname storm   !   router rip   passive-interface TokenRing0   network 128.200.0.0   neighbor 128.200.10.1  

The next step is to perform mutual redistribution between EIGRP and RIP on the rogue router. Because there is only one redistribution point in the network, there is no need to perform any route filtering when using mutual redistribution. To redistribute between EIGRP and RIP, use the redistribution command, along with adding a default metric. The default metric chosen for RIP is 2, whereas 10000 1000 254 1 1500 is the default metric for EIGRP. Because the routing domains overlap (which means that RIP broadcasts will be heard on cyclops), use the passive-interface command to prevent excessive broadcasts from occurring. Example 9-9 lists the configuration of EIGRP and RIP on the rogue router.

Example 9-9 EIGRP and RIP Configuration on rogue
  router eigrp 2001   redistribute rip   passive-interface Ethernet0/0   network 128.200.0.0   default-metric 10000 1000 254 1 1500   no auto-summary   !   router rip   redistribute eigrp 2001   passive-interface Ethernet0/1   network 128.200.0.0   default-metric 2  

At this point, the network is close to being complete. However, if you observe the routing table on storm, you see that it does not have a route to 128.200.3.16/29, as illustrated in Figure 9-4.

Figure 9-4. Route Table Discrepancies: RIP/EIGRP Network Before Summarization

graphics/09fig04.gif

Because the Ethernet segment between rogue and cyclops is on a 29-bit boundary, and because the segment between rogue and wolverine is on a 24-bit boundary, the rogue router will not forward any 29-bit network updates out its E0/0 port. Performing a debug ip rip command on the rogue router as done in Example 9-10 substantiates this.

Example 9-10 debug ip rip Command Output from rogue
 rogue#  debug ip rip  RIP protocol debugging is on  RIP: sending v1 update to 255.255.255.255 via Ethernet0/0 (128.200.1.2)   suppressing null update   graphics/u2190.gif no 128.200.3.x route  RIP: received v1 update from 128.200.1.1 on Ethernet0/0      128.200.10.0 in 1 hops      128.200.2.0 in 1 hops RIP: sending v1 update to 255.255.255.255 via Ethernet0/0 (128.200.1.2)    suppressing null update RIP: received v1 update from 128.200.1.1 on Ethernet0/0      128.200.10.0 in 1 hops      128.200.2.0 in 1 hops rogue# 

To allow full IP reachability between cyclops and storm, the route between them must be summarized on a 24-bit boundary. To accomplish this, use the following command on the cyclops router:

 cyclops(config-if)#  ip summary-address eigrp 2001 128.200.3.0 255.255.255.0  

Observing the routing table on rogue, as in Figure 9-5, an EIGRP route 128.200.3.0/24 now exists. Because the route has a 24-bit mask, rogue can forward it out its E0/0 port, where storm will eventually receive the route.

Figure 9-5. Route Table Discrepancies Resolved: RIP/EIGRP Network After Summarization

graphics/09fig05.gif

CAUTION

Caution should be used whenever redistributing one routing protocol into another. Potential routing loops can occur if there are two or more redistribution points within the network. When multiple redistribution points are in the network, distance vector protocols are vulnerable to loops. Use the route maps with a well-thought-out IP addressing scheme to control loops .


TIP

Whenever you are working with routing protocols that don't carry a subnet mask within the routing update, such as RIP-1 and IGRP, be extremely careful to preserve a consistent bit mask throughout the entire internetwork. For example, a RIP domain might be operating on 24-bit boundary, whereas an OSPF domain might have some LAN networks at the 24-bit boundary, but its entire WAN is using a 30-bit boundary. The RIP domain will have no problem reaching the LANs because of the bit boundary match; however, it will not be capable of reaching the WAN interfaces. To allow full reachability between the two domains, the OSPF WAN networks that reside on a 30-bit boundary must be summarized to a 24-bit boundary before redistribution into RIP.


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CCIE Practical Studies, Volume I
CCIE Practical Studies, Volume I
ISBN: 1587200023
EAN: 2147483647
Year: 2001
Pages: 283
Authors: Karl Solie

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