Designing Implementing an OSPF Network | OSPF Network Design Solutions

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Tuning OSPF Timers

Because all routers are Cisco and running the same version of code, there is no reason to tune individual HELLO, DEAD, or RETRANSMIT timers. Cisco’s default WAN values of 10, 40 and 120, respectively, will provide fast convergence times and ensure consistency across all routers:

    TENN#         interface Ethernet 0         description LAN connection to campus backbone         ip address 172.17.10.240 255.255.255.0         !         interface serial 0.1 point-to-point         description PVC to Cumberland router         ip address 172.17.95.1 255.255.255.252         ip ospf authentication-key watchtower         ipx network 179500         frame-relay interface-dlci 401 broadcast         !         interface serial 0.2 point-to-point         description PVC to west LA router         ip address 172.17.95.5 255.255.255.252         ip ospf authentication-key watchtower         ipx network 179504         frame-relay interface-dlci 401 broadcast         !         router ospf 5775         network 172.17.95.0 0.0.0.255 area 64         area 64 authentication

Strategizing Route Redistribution

Redistribution of routes between the OSPF and IGRP domains will be done at the frame-relay hub router (ASBR) in Tennessee. To learn of routes from both domains, the hub router must run both an OSPF and IGRP routing process. Redistribution of routes must address all of the issues detailed in the sections that follow.

Campus Routing to Frame Relay WAN

This section discusses how the existing campus routers will dynamically learn about the new Frame Relay networks, specifically examining the following issues:

• OSPF Route Redistribution into IGRP

• Static Route aggregation and redistribution into IGRP

• OSPF Route aggregation and redistribution into IGRP

• Testing of OSPF route redistribution into IGRP

Redistribution of OSPF routes into the IGRP process will cause the hub router to send IGRP advertisements of all /24 subnets known to OSPF. This will allow all spoke router LAN subnets to be learned by IGRP routers.

Use the “internal” keyword when performing this redistribution on the Hub Cisco router to allow only OSPF “internal” routes to be redistributed into IGRP. This will prevent a possible router loop in the future if more routers are installed and running “two-way” OSPF/IGRP redistribution. (All of the frame relay LAN/WAN networks will be known as OSPF “internal” routes because they originated from this same domain.)

The WAN subnets cannot be redistributed into IGRP this simply, however, due to the “classless” IP subnetting scheme of /30. IGRP only supports “classful” subnetting, and routers would ignore all /30 subnets when redistributing. Although this would not affect host-to-host IP connectivity, it could potentially cause a problem with network management tools, subsequently causing routing holes when accessing the router’s WAN IP address directly to/from frame-relay.

Two possible strategies for handling the WAN link advertisements into IGRP are possible: static route aggregation and redistribution into IGRP and OSPF Route aggregation and redistribution into IGRP.

With static route aggregation and redistribution into IGRP, you must represent all /30 WAN subnets into an aggregate 24-bit summary and then redistribute them because only /24 prefixed routes will be announced into IGRP. Configure a static route on the Hub router for 172.17.95.0 255.255.255.0, with the next hop as the “Null 0” interface (a.k.a. the hub router). Now, redistribute static routes into IGRP and all IGRP routers will be able to route traffic to these WAN links. Control redistribution of routes to just the 172.17.95.0/24 network by defining an access list that will only allow redistribution of this route. Defining an access list may prevent future routing problems if additional static routes are added to the hub router, which the campus need not know about through IGRP. The following configuration demonstrates how to control redistribution of routes.

    TENN#    router igrp 10    network 172.17.0.0    passive-interface serial0.1:0.30    default-metric 10000 100 255 1 1500    redistribute ospf 5774 match internal    redistribute static    distribute-list 3 out static    ip route 172.17.95.0 255.255.255.0 null0    access-list 3 permit 172.17.95.0

The passive-interface command stops IGRP updates from being broadcasted unnecessarily across all wan PVCs.

The default-metric command assigns IGRP metrics to routes known from all other route sources (in this case static routes) that need redistribution into IGRP.

Notes:
IGRP uses Bandwidth, Delay, Reliability, Load, and MTU components to calculate route metrics across specific interfaces. The values 10000 100 255 1 1500 are defaults for 10MB Ethernet.

An alternative to static route aggregation of the WAN subnets would be to employ OSPF route aggregation and redistribution into IGRP to accomplish this task. This is the preferred solution, and the one chosen for this case study, as OSPF is already currently being redistributed into the IGRP process in order to propagate the LAN subnets.

To accomplish OSPF route summarization, the Hub router will need to be configured as an ABR. This is required because OSPF inter-area summarization can only be accomplished at area boundaries towards the backbone. You can accomplish this by adding the Ethernet interface into OSPF area 0. Now that the hub router (TENN) is an ABR, you can summarize the WAN subnets as one /24 network (172.17.95.0/24). This network falls on the established 24-bit boundary and will be redistributed into IGRP and understood by all interior IGRP-speaking routers as shown in the following configuration example.

    TENN#    router igrp 10    network 172.17.0.0    passive-interface serial0.1:0.30    default-metric 10000 100 255 1 1500    redistribute ospf 5775 match internal    router ospf 5775    summary-address 172.17.95.0 255.255.255.0    network 172.17.95.0 0.0.0.255 area 64    network 172.17.10.240 0.0.0.0 area 0    area 64 range 172.17.95.0 255.255.255.0    area 64 authentication

To test the OSPF route redistribution into IGRP, you can display the routing table of any IGRP internal router, which will indicate the success or failure of the redistribution of OSPF routes into IGRP. If problems arise, debugging on IGRP transactions on the ASBR (hub) router may provide information as to what is going wrong.

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