CCNA Exploration: Routing Protocols and Concepts Chapter 11 Case Study

Transcription

1 bjectives: Consolidate SPF knowledge. Describe the operation of SPF multi-area. Introduce the concept of route summarization with SPF. Introduce the concept of SPF optimization. Intro: Trevni Inc. needed to add 6 more networks under R3 and called for help when the network performance dropped considerably. Topology:

2 Scenario: As expected, Trevni growth made necessary adding 6 more networks under R3 but some of the networks are flapping (going up and down in short time intervals) because they are still being tested. Flapping interfaces are a problem to SPF because forces it to generate and flood extra LSAs to the other SPF routers. The other SPF routers, upon receipt of update LSAs, are forced to re-run SPF against their LSDBs. In Trevni s case, R3 is advertising the change of state of its new links (due to interface flapping) to the rest of the network and forcing extra SPF re-runs. The extra SPF calculation is impacting Trevni s routing performance. Step 1 Splitting SPF in areas Internal SPF routers within the same area must have the exact same SPF database. This leads SPF routers within the same area to run SPF against their own LSDB every time a link change happens within the area. To avoid this problem, 2 techniques are used together: SPF area segmentation and summary routes. When a specific route is part of a summary route and this specific route goes down, SPF does not declare the entire summary route as down because the other routes included into the summary might be still up. Because in SPF summarization can only be done by an ABR or an ASBR, you need to split Trevni s network into areas to create an ABR and then summarize R3 s new networks into Area 0. Even if a few of the new networks go down, because they all inside a summary route, R1 and R2 won t be aware of it and will keep the summary route into their routing tables. As shown on the topology above, R3 now has 6 more networks and you decide split Trevni s SPF domain in 2 areas. This will allow R4 (now an ABR) to summarize R3 s new networks into one single summary route and advertise it to Area 0, reducing the interface flapping negative impact in the network and improving performance. The areas will be Area 0 (backbone or transit area) and Area 1, a standard area. Area 0 will contain R1, R2 and all the networks connected to them while Area 1 will contain R3 and its networks. R4 will have links connected to Area 0 and to Area 1 which makes it the Area Border Router (ABR). Notice that because R4 also has an interface connected to environment external to the local SPF AS, it is also acting as an Autonomous System Border Router (ASBR). Question 1: How segmenting SPF area does improve Trevni network s routing performance? Answer: By splitting SPF domain into areas allows smaller SPF databases (LSDBs), smaller routing table and less SPF calculations. Since R1 and R2 belong to a different area than R3, only R3 will deal with its interface flapping while R1 and R2 be won t be aware of it.

3 You connect the console cable to R3 and change SPF configuration in it. You start by adding up the just installed networks to R3 s SPF process 1 (SPF process ID 1 instance is being currently spoke with R4) and assigning them to area 1. nce the new networks were added, you remove network 3 ( /24) and network 200 ( /30) from SPF 1 just to add them again back but assigned to area 1 rather than area 0. The commands issued into R3 are documented below for future reference: R3(config)# router ospf 1 R3(config-router)# no network area 0 R3(config-router)# no network area 0 R3(config-router)# network area 1 R3(config-router)# network area 1 R3(config-router)# network area 1 R3(config-router)# end Question 2: Even though the new networks have subnet mask /27, the SPF network command uses only one /24 subnet to add such network to SPF instance 1. Is this correct? What about the other five /27 new networks, don t they have to be added to SPF process ID 1? Answer: Yes, it is correct. The mask used in the SPF network command doesn t relate to summaries, it only specifies the address range to be added to SPF. The other networks were also added to SPF by the single network command with the /24 mask. Because a /24 mask includes all the 6 new networks, only one network command is necessary to add all 6 new networks. SPF summary routes are done differently. Still in R3, you issue sh ip ospf command to ensure SPF was well configured. Everything is ok. Because of the area change, you allow a few seconds to SPF recreate all the neighbor relationships based on the new area and switch the console cable to R4 (now the ABR) to check whether or not the changes were correctly advertised by R3. Below is the output of a sh ip route issued in R4: R4# sh ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, - SPF, IA - SPF inter area N1 - SPF NSSA external type 1, N2 - SPF NSSA external type 2 E1 - SPF external type 1, E2 - SPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - DR, P - periodic downloaded static route Gateway of last resort is to network C /24 is directly connected, Serial0/ /32 is subnetted, 6 subnets [110/782] via , 03:26:01, Serial0/3/ [110/782] via , 03:26:01, Serial0/3/ [110/782] via , 03:26:01, Serial0/3/ [110/782] via , 03:26:01, Serial0/3/ [110/782] via , 03:26:01, Serial0/3/ [110/782] via , 03:26:01, Serial0/3/1

4 /30 is subnetted, 1 subnets C is directly connected, Serial0/3/ /32 is subnetted, 1 subnets [110/2] via , 03:26:13, Vlan /32 is subnetted, 1 subnets [110/2] via , 03:26:13, Vlan20 C /24 is directly connected, Vlan /32 is subnetted, 1 subnets [110/782] via , 03:26:08, Serial0/3/1 S* /0 [1/0] via R4# R4 s routing table shows all the routes were correctly learnt from R3 via SPF and placed in R4 s routing table. You issue a few pings from R4 to different IP addresses under the new networks. All of them are successful. You switch to R1 and also check its routing table. R1 s routing table is as below: R1# sh ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, - SPF, IA - SPF inter area N1 - SPF NSSA external type 1, N2 - SPF NSSA external type 2 E1 - SPF external type 1, E2 - SPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - DR, P - periodic downloaded static route Gateway of last resort is to network /32 is subnetted, 6 subnets IA [110/783] via , 00:00:51, FastEthernet0/0 IA [110/783] via , 00:00:51, FastEthernet0/0 IA [110/783] via , 00:00:51, FastEthernet0/0 IA [110/783] via , 00:00:51, FastEthernet0/0 IA [110/783] via , 00:00:51, FastEthernet0/0 IA [110/783] via , 00:00:52, FastEthernet0/ /30 is subnetted, 1 subnets IA [110/782] via , 00:21:01, FastEthernet0/0 C /24 is directly connected, Loopback /32 is subnetted, 1 subnets [110/2] via , 00:21:04, FastEthernet0/0 C /24 is directly connected, FastEthernet0/ /32 is subnetted, 1 subnets IA [110/783] via , 00:21:04, FastEthernet0/0 *E /0 [110/1] via , 00:00:50, FastEthernet0/0 R1# Because R3 now is no longer into the same area as R1, all routes learnt from Area 1 are now flagged as SPF inter-area routes ( IA) and placed into R1 s routing table, as expected. SPF inter-area routes are advertised by the area s ABR using Type 3 LSAs. You issue a few pings from R1 to different IP addresses under R3 s just added networks and watch flow successfully. You switch to R2 and check its routing table: it looks a lot like R1 s which is expected. nce more, you issue a few pings from (from R2 this time) to different IP addresses under R3 s. All pings are successful, SPF area segmentation is complete.

5 Question 3: R1 s and R2 s have a route which was flagged as *E2. What is the meaning of this? Answer: This route is the default route learnt from R4. R4 was configured to advertise its default route but because the network used by R4 to reach networks external to Trevni s SPF domain (the internet) was not included into R4 s SPF s process, SPF advertises it as an External Route (*E2). The star means that route was also flagged as a default route. Note: SPF has 2 types of external routes: type 1 (E1) and type 2 (E2). E1 routes have their metric updated as they travel through the internal SPF routers while E2 routes keep their original metric untouched. SPF uses E2 routes by default. Since area segmentation is complete, R4 is now the ABR and can summarize inter-area routes. By summarizing R3 s networks, R4 will inject one single summary route into Area 0, reducing the size of R1 s and R2 s routing tables and hiding all the interface flapping from them. Question 4: How a long routing table does impact the routing process? Answer: The routers have to search all the way through routing table in order to find a match and forward packets. The longer the routing tables, the longer the router will take looking through it before forwarding the packet. Because R1 and R2 must forward packets to R4 in order to reach the new networks under R3 and R4 is the ABR connecting area 0 to area 1, you decide to configure R4 to summarize the new networks into one single summary route and inject this single summary route into area 0. You switch back to R4 and configure the summarization. The commands are listed below: R4(config)# router ospf 1 R4(config-router)# area 1 range R1(config-router)# end You check R4 s routing table once more and notice its routing table didn t change much, as you expected. Since R4 is the ABR for area 1, it installs all the routes learnt from R3 into its routing table as internal routes. (as an ABR, R4 is part of area 0 and area 1 simultaneously) You switch the console cable to R1 and analyze its routing table. R1 s routing table, after the R4 was configured to summarize R3 s routes, is shown below: R1#sh ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, - SPF, IA - SPF inter area N1 - SPF NSSA external type 1, N2 - SPF NSSA external type 2 E1 - SPF external type 1, E2 - SPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route

6 o - DR, P - periodic downloaded static route Gateway of last resort is to network IA /24 [110/783] via , 00:02:16, FastEthernet0/ /30 is subnetted, 1 subnets IA [110/782] via , 00:39:48, FastEthernet0/0 C /24 is directly connected, Loopback /32 is subnetted, 1 subnets [110/2] via , 00:39:48, FastEthernet0/0 C /24 is directly connected, FastEthernet0/ /32 is subnetted, 1 subnets IA [110/783] via , 00:39:49, FastEthernet0/0 *E /0 [110/1] via , 00:02:12, FastEthernet0/0 R1# R1 s routing table clearly shows the difference: Because of the single summary sent by R4, R1 now replaces all 6 routes by only one single summary route. This single summary route includes all the new 6 routes advertised by R3. You check R2 s routing table and notice it also installed the single summary route /24 pointing to R4. Question 4: Why, in R4 s routing table, the summary route points to null0? Answer: Cisco SPF implementation automatically creates a summary route point to null0 into the router which generates the summary (in this case the ABR). This is done to keep packets to inexistent networks from be forwarded, by the ABR, via default route. As an example, suppose R1 pings /27. Because the summary route advertised by R4 includes the address pinged by R1, R1 would forward such pings to R4 but since the network /27 is not assigned to any interfaces (it is an inexistent network), R4 would have no route to it. R4 would wrongly forward such packets to its default route in a try to reach the destination. In other hand, if the destination network does exist, R4 would have specifically heard of it from R3 and would have installed a more specific route to it into its own routing table. R4, based on the longest match rule, would use the more specific route to forward packets rather the summary pointing to null0. After some monitoring, you conclude R1 and R2 are no longer performing unnecessary SPF calculations and have shorter and optimized LSDBs and Routing Tables. The routing processes on those 2 routers run faster and the overall performance of Trevni Inc. network has improved.