1 Cabrillo College Building Scalable Cisco Networks Rick Graziani, Instructor with Mark McGregor March 22, 2001

2 Nice to know commands... line con 0 logging synchronous exec-timeout 0 0 interface fa 0 ip add no keepalive (line protocol now up) no shutdown interface serial 0 clock rate (DCE interface) show ip interface brief

3 Routing Viewing the routing table n show ip route Routes enter the routing table via: n static routing - Administrator manually defines routes to a destination network. n dynamic routing - Routers follow rules defined by a routing protocol to exchange routing information and independently select the best path.

4 Static Routing n Configuring static routes n More later when we discuss default routing and floating static routes

5 Advantages and Disadvantages of Static Routing Advantages n Low processor overhead n No bandwidth utilization –Secure operation don’t inadvertently advertise networking information to an untrusted source n Predictability (precise control) Disadvantages n High-maintenance configuration n No adaptability (except for floating static routes)

6 Advantages and Disadvantages of Dynamic Routing Advantages n High degree of adaptability n Low-maintenance configuration Disadvantages n Increased processor overhead n High bandwidth utilization

7 Configuring Static Routing ip route prefix mask {address | interface} [distance] [tag tag] [permanent] n prefixIP route prefix for the destination. n maskPrefix mask for the destination. n addressIP address of the “next hop” that can be used to reach that network. n interfaceNetwork interface to use. n distance(Optional) An administrative distance. n tag tag(Optional) Tag value that can be used as a "match" value for controlling redistribution via route maps. (later) n Permanent (Optional) Specifies that the route will not be removed, even if the interface shuts down.

8 McGregor p.118 network

9 No static or dynamic routing yet, only interfaces have been configured. RTB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C /24 is directly connected, Serial0 C /24 is directly connected, Serial1 C /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0 RTB#

10 Static route options Configured using: n next-hop interface n exit interface

11 RTB(config)#ip route RTB(config)#ip route s2 RTB#show ip route C /24 is directly connected, Serial0 S /24 [1/0] via S /24 is directly connected, Serial2 C /24 is directly connected, Serial1 C /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0 RTB# n Notice that the static route using the exit interface shows the route as a “directly connected” even though it is not. n What is the difference between these two?

12 RTB(config)#ip route RTB(config)#ip route s2 RTB#show ip route C /24 is directly connected, Serial0 S /24 [1/0] via S /24 is directly connected, Serial2 C /24 is directly connected, Serial1 n When using a routing protocol such as RIP or IGRP (lets say between RTA and RTB), static routes that show as “directly connected” will automatically be advertised to other routers as long as the appropriate network command (network ) has been issued. n The next-hop static route won’t be advertised without additional configuration (redistribute static - later).

13 Static routes n What happens to the static route when an interface goes down?

14 RTA and RTC interfaces to RTB are down... When an interface goes down, all static routes mapped to that interface are removed from the IP routing table, along with the directly connected networks. RTB#show ip route C /24 is directly connected, Serial0 C /24 is directly connected, FastEthernet0 n If the interface comes back up the routes are returned, along with the directly connected networks. C /24 is directly connected, Serial0 S /24 [1/0] via S /24 is directly connected, Serial2 C /24 is directly connected, Serial1 C /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0

15 Static route and permanent option n The permanent option specifies that the route will not be removed, even if the interface shuts down.

16 Static routes with the permanent option: ip route Serial2 permanent ip route permanent RTB#show ip route C /24 is directly connected, Serial0 S /24 [1/0] via S /24 is directly connected, Serial2 C /24 is directly connected, Serial1 C /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0

17 n What does the routing table look like when the interfaces go down? n Static routes are still in the routing table, but can you really get to those routes? - No. Notice that the directly connected networks of the downed interfaces are no longer in the routing table. RTB#show ip route C /24 is directly connected, Serial0 S /24 [1/0] via S /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0

18 Static routes and recursive table lookups n Extra information - not exam knowledge n All routes do not necessarily have to point to the next-hop router, as long as the router has a route to the “next-hop” ip address. n This is not usually good design practice because it costs the router processor time to perform multiple route table lookups. n Here is a simple example.

19 Configuring static routes using recursive table lookups: ip route ip route RTA# show ip route C /24 is directly connected, FastEthernet0 S /24 [1/0] via C /24 is directly connected, Serial0 S /24 [1/0] via RTA#ping Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds: !!!!!

20 Dynamic Routing Protocols n Note: The material on IPX and Appletalk is FYI only regarding my exams or the CCNP exam (and now CCIE) n Read this section for now, but all of this material will be covered in much more detail in the coming weeks. n We will concentrate on default routing and and floating static routes in this section

21 Default Routing n Quad-zero routes n Ip default-network command

22 Quad-Zero Routes ip route [next-hop-ip-add|exit-inter] n To the Cisco IOS this route has a special meaning as the “route of last resort” n Using our network diagram, and running RIP on all router except between RTB and ISP, lets look at the effect of a quad-zero default route

23 RTB(config)#ip route RTB#show ip route Gateway of last resort is to network C /24 is directly connected, Serial0 R /24 [120/1] via , 00:00:25, Serial1 R /24 [120/1] via , 00:00:24, Serial2 C /24 is directly connected, Serial1 C /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0 S* /0 [1/0] via

24 Default Routing and RIP n Cisco IOS 12.0 and earlier: RIP will propagate the quad-zero default route automatically. n Cisco IOS 12.1 and later: RIP will not propagate the qua-zero default route automatically. –You must use the default-information originate command or redistribute static command (later) –Using default-information originate will not break < 12.0 n Normally, only add one quad-zero route or the routers will load- balance to wrong destinations. (actual example) RTB: ip route router rip network network network default-information originate

25 RTA#show ip route Gateway of last resort is to network C /24 is directly connected, FastEthernet0 R /24 [120/2] via , 00:00:03, Serial0 C /24 is directly connected, Serial0 R /24 [120/1] via , 00:00:03, Serial0 R /24 [120/1] via , 00:00:03, Serial0 R* /0 [120/1] via , 00:00:03, Serial0

26 Ip default-network command n Primarily used with IGRP n Can be used with RIP but quad-zero is usually preferred n EIGRP supports both the quad-zero and ip default-network commands. (More later)

27 IGRP and ip default-network n All routers running IGRP including between RTB and ISP. n IGRP does not recognize the /0 route and will not include it in its updates. n We must use the ip default-network command (which is propagated) n RTB must also have a /0 route to forward traffic to a default route once it reaches RTB. Must include network between RTB and ISP in RTB’s config: router igrp 24 network ip route ip default-network

28 RTB’s configuration router igrp 24 network network network network (replace with loopback!) ip default-network (use loopback) ip route n Using a loopback address on RTB is actually better, so that IGRP updates do not need to be sent out the net. Will need to include loopback address as a network statement and as the default-network. n Quad-zero route is not propagated but used when the packet arrives at RTB. n “A router that is generating the default for a network also can need a default of its own. One way of doing this is to specify a static route to the network through the appropriate device.” CCO ip default-network

29 RTB(config)# ip default-network RTB#show ip route Gateway of last resort is not set C* /24 is directly connected, Serial0 I /24 [100/80135] via , 00:00:59, Serial1 I /24 [100/80135] via , 00:00:22, Serial2 C /24 is directly connected, Serial1 C /24 is directly connected, Serial2 C /24 is directly connected, FastEthernet0 S* /0 [1/0] via RTB#

30 RTA now has a default route... RTA#show ip route Gateway of last resort is to network I* /24 [100/82125] via , 00:00:53, Serial0 C /24 is directly connected, FastEthernet0 I /24 [100/82135] via , 00:00:53, Serial0 C /24 is directly connected, Serial0 I /24 [100/82125] via , 00:00:53, Serial0 I /24 [100/80135] via , 00:00:53, Serial0 RTA#

31 IGRP, ip default-network and ip unnumbered n See Sem5 book for examples pp n Not significant material for exam

32 RIP and ip default-network n All routers running RIP except between RTB and ISP n RTB must also have a /0 route to forward traffic to a default route once it reaches RTB. Do not include network between RTB and ISP in RTB’s config. router rip network network network (assuming 12.1 with no default-information originate, so quad-zero route no propagated.) ! ip route ip default-network

33 n RIP propagates ip default-networks as /0 routes. n In other words, the command ip default-network will send neighbors (RTA) a route to /0 not a flagged route to n Why would you use ip default-network with RIP. n Due to our routers running IOS 12.0 and not 12.1 or greater, I can’t currently show you output for this configuration. With 12.0, the /0 route will automatically be propagated and confuse the output. Check CCO if you would like to see more information on RIP and ip default-network.

34 Floating static routes n Most of this information is included in one of the chapter 3 labs. n Floating static routes are static rotes configured with an administrative distance value that is greater than that of the primary route (or routes), whether it is another static route or a dynamic route. n Floating static routes are basically fallback or backup routes, that do not appear in the routing table unless the primary route fails.

35 Administrative Distance Values Connected interface 0 Static route 1 EIGRP summary route 5 External BGP 20 Internal EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EGP 140 External EIGRP 170 Internal BGP 200 Unknown 255

36 Examples of primary static and floating static routes: ip route s0 ip route s1 5 ip route ip route ip route s0 ip route s1 5 ip route s2 10 ip route s3 15

37 Examples of primary dynamic and floating static routes: n RTA’s primary route to network is learned via RIP from RTB. n RTA also has a floating static route to this same network. n Assume RTA’s s1 interface is another route over a non-RIP link. (There are several different scenarios we can use here.) n If RIP fails or the s0 interface fails, the floating static route will enter the routing table. router rip network network ip route s1 130

38 ip default-gateway n “The ip default-gateway command differs from the other two commands in that it should only be used when ip routing is disabled on the Cisco router.” n “For instance, if the router is a host in the IP world, you can use this command to define a default gateway for it. “ n “You might also use this command when your low end Cisco router is in boot mode in order to TFTP a Cisco IOS ® Software image to the router. In boot mode, the router doesn't have ip routing enabled.” n CCO