CIT 384: Network AdministrationSlide #1 CIT 384: Network Administration Routing ][

CIT 384: Network Administration Slide #1

CIT 384: Network Administration

Routing ][


Topics

1. Switch Forwarding Process

2. IP Routing Process

3. DNS

4. Connected and Static Routes

5. Classful Routing

6. VLSM

7. Summarization


Switch Forwarding Process1. Determine the VLAN in which the frame should be forwarded as

follows:a. If frame arries on an access interface, use the interface’s access VLAN.b. If frame arrives on a trunk interface, use VLAN listed in trunking header.

2. If incoming interface is in STP Learning or Forwarding State in that VLAN, add source MAC address to MAC address table with incoming interface and VLAN ID.

3. If incoming interface is not in an STP Forwarding State for that VLAN, discard the frame.

4. Look for destination MAC address of frame in MAC address table for VLAN identified in step 1. If address is

a. Found: Forward frame out the only interface listed in the matched address table entry.

b. Not found: Flood frame out all other access ports in same VLAN and out all trunk ports that support this VLAN.


IP Routing Process: Edge Hosts

1. Compare destination IP of packet to local subnet and netmask.

2. If destination on same subnet as host, send packet directly to host. ARP is used to find destination MAC address.

3. If destination host not on same subnet, then send packet directly to gateway router. ARP is used to find router’s MAC address.


IP Routing Process: Routers1. Check errors using data link FCS. If error, discard frame.2. Check frame’s destination MAC address, and process

only if directed to this router or to a broadcast/multicast address.

3. Discard frame’s old data-link header and trailer, leaving IP packet.

4. Compare destination IP address to routing table, finding route that matches destination address. Route identifies outgoing interface of router and possibly next-hop router.

5. Determine destination MAC address.6. Encapsulate packet inside new data link header and

trailer, appropriate for outgoing interface, and fwd packet.


IP Routing Example

PC1 sends packet to PC3 (172.16.3.3)– PC1 configuration

• ifconfig eth0 172.16.1.1 netmask 255.255.255.0

• route add default gw 172.16.1.251

– Compares 172.16.3.3 to local net:• Network address: 172.16.1.0

• Subnet mask: 255.255.255.0

– Since 172.16.3.3 is not in address range,• Sends to default route 172.16.1.251



IP Routing Example: R1

1. R1 checks FCS, finds no errors.2. R1 finds its own fa0/0 MAC as dest MAC, so it

will process the frame.3. R1 discards old data-link header and trailer.4. R1 compares dest IP (172.16.3.3) to routing table,

finding a route for 172.16.3.0 with netmask 255.255.255.0.

5. R1 looks up MAC address of route destination (172.16.2.252) in ARP table.

6. R1 encapsulates IP packet in new Ethernet frame, with R1’s fa0/1 MAC address as destination.


IP Routing Example: R2

1. R2 checks FCS, finds no errors.2. R2 finds own fa0/0 MAC address as destination

MAC, so it will process frame.3. R2 discards old data-link header and trailer.4. R2 compares destination IP (172.16.3.3) to

routing table, finding matching route with outgoing interface fa0/1.

5. R2 looks up destination IP’s MAC address.6. R2 encapsulates IP packet in new Ethernet frame,

with R2’s fa0/1 MAC as source and PC3’s MAC as destination. R2 sends the frame.


Routing Example with PPP


Matching the Most Specific Route

R1#show ip route rip

172.16.0.0/16 is variably subnetted, 5 subnets, 4 masksR 172.16.1.1/32 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1R 172.16.1.0/24 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0R 172.16.0.0/22 [120/1] via 172.16.25.2, 00:00:04, Serial0/1/1R 172.16.0.0/16 [120/2] via 172.16.25.129, 00:00:09, Serial0/1/0R 0.0.0.0/0 [120/3] via 172.16.25.129, 00:00:09, Serial0/1/0R1#show ip route 172.16.4.3

Routing entry for 172.16.0.0/16 Known via "rip", distance 120, metric 2 Redistributing via rip Last update from 172.16.25.129 on Serial0/1/0, 00:00:19 ago Routing Descriptor Blocks: * 172.16.25.129, from 172.16.25.129, 00:00:19 ago, via Serial0/1/0 Route metric is 2, traffic share count is


Ranges defined by each route

172.16.1.1

172.16.1.0 – 172.16.1.255

172.16.0.0 – 172.16.3.255

172.16.0.0 – 172.16.255.255

0.0.0.0 – 255.255.255.255


Matching the Most Specific Route

172.16.1.1: Matches all 5 routesLongest prefix is /32, route to 172.16.1.1/32

172.16.1.2: Matches last 4 routesLongest prefix is /24, route to 172.16.1.0/24




Why DNS?

• HOSTS.TXT original flat text file mapping

• Problems– Load: everyone had to d/l from one server.– Collisions: no two hosts can have the same

name in a flat namespace– Consistency: by the time a host file had reached

every host, it was already out of date when compared to the master copy on the server.


DNS

Domain Name System– Released in 1984 with RFCs 882 and 883.

– Nameservers provide name/IP mappings.

– Resolvers on clients query nameservers.

Distributed database of name/IP mappings.– Tree structure prevents collisions.

– Each domain controls its own nameservers, which contain db for names in that domain.

– Nameservers can query other-domain nameservers when resolvers ask for names outside the local domain.


The DNS Namespace


Domains are Subtrees

• Domains are subtrees of the DNS tree.– Similar to subtrees

of directory tree.– Does not indicate

anything about subnet addresses.

– Can be administered independently.


Delegation of Domains• Top Level Domains

delegate subdomains to organizations.

• Organizations can delegate subdomains of their domain too.


Name Resolution Process1. Resolver on host translates

hostnames for apps.2. Resolver asks nameserver to

translate for it.3. Nameserver asks root server,

with refers it to au svr.4. Au server refers to gov.au

server.5. Gov.au server refers to

gbrmpa.gov.au server.6. Gbrmpa.gov.au server returns

IP address to nameserver.7. Nameserver may cache

translation for future use.8. Nameserver sends IP address

to resolver.


Mapping Addresses to Names

Use in-addr.arpa domain.– Nodes are labeled by

an octet of IP address.

– in-addr has 256 subtrees, each representing the first octet of an IP address.

– Four-level nodes map back to hostnames.


Maximum Transmission Unit (MTU)

router(config)# interface fa0/1router(config-if)# mtu 1000router(config-if)# end


Connected Routes

Routes to networks where rtr has interface.– Router determines these routes from interface IP

address and subnet.– Listed in route table with a C prefix.

How to deliver packets to non-connected nets?– Needs routes for those subnets, or– A default route for all other networks.


show ip routeRouter-251>show ip routeCodes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF 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 - ODR, P - periodic downloaded static route

Gateway of last resort is 10.11.241.1 to network 0.0.0.0

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksC 10.11.241.0/24 is directly connected, FastEthernet0/0C 10.251.0.0/16 is directly connected, FastEthernet0/1S* 0.0.0.0/0 [1/0] via 10.11.241.1


show ip route addressRouter-251>show ip route 10.11.241.9Routing entry for 10.11.241.0/24 Known via "connected", distance 0, metric 0 (connected, via

interface) Routing Descriptor Blocks: * directly connected, via FastEthernet0/0 Route metric is 0, traffic share count is 1

Router-251>show ip route 144.144.144.144% Network not in table


Static Routes

Routes entered by network administrator.– Listed in route table with S prefix.– Easy to understand network behavior.– Avoids problems with routing protocols.– Requires extensive planning for large networks.– Does not adapt to changing networks.– Does not scale to large networks.


Creating a Static Route

ip route network subnet destinationip route 10.1.0.0 255.255.0.0 10.1.0.1

ip route 10.6.9.0 255.255.255.0 10.6.9.95

ip route 10.2.0.0 255.255.0.0 fa0/1


Default RoutesWhat happens if no route matches an IP?

– Packet is discarded by router.– Unless there is a default route.

Default route matches all destination addrs.– Listed with S* prefix in routing table.

Configuring a default routeip route 0.0.0.0 0.0.0.0 10.9.3.1

Configuring a default route for routing protocolsip default-network 10.0.0.0(a route to 10.x.y.z must already exist)


Classful and Classless Routing

Classless routing: When a packet’s destination only matches default route, forward packet using default route.

ip classlessClassful routing: When a packet’s destination

only matches default route, only use the default route if this router does not know any routes in the classful network in which the destination IP address resides.

no ip classless


Classful Routing

R3 will use its default route only if it does not have another route for that address class; a single class-B 168.13 route will prevent use of the default route for any 168.13 network.


Classful and Classless RoutingR3#show ip routeGateway of last resort is 168.13.100.1 to network 0.0.0.0 168.13.0.0/24 is subnetted, 4 subnetsR 168.13.1.0 [120/1] via 168.13.100.1, 00:00:13, Serial0.1C 168.13.3.0 is directly connected, Ethernet0R 168.13.2.0 [120/1] via 168.13.100.2, 00:00:06, Serial0.1C 168.13.100.0 is directly connected, Serial0.1R3#ping 10.1.1.1Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-trip min/avg/max = 84/89/114 msR3#ping 168.13.200.1Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 168.13.200.1, timeout is 2 seconds:.....Success rate is 0 percent (0/5)


Extended Ping

• Ping comes from interface closest to dest.

• Extended ping is configurable.– Alter source address to use different interface.– Can also modify

• Number of pings sent.

• Size of ping packets.

• Data included.

• Don’t fragment flag.

• Type of Service


Extended PingAlbuquerque#show ip route10.0.0.0/24 is subnetted, 5 subnetsS 10.1.3.0 [1/0] via 10.1.130.253S 10.1.2.0 [1/0] via 10.1.128.252C 10.1.1.0 is directly connected, Ethernet0C 10.1.130.0 is directly connected, Serial1C 10.1.128.0 is directly connected, Serial0Albuquerque#ping 10.1.2.252Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 10.1.2.252, timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 msAlbuquerque#pingProtocol [ip]:Target IP address: 10.1.2.252 (Ethernet0 on Yosemite)Extended commands [n]: ySource address or interface: 10.1.1.251 (Ethernet0)Data pattern [0xABCD]:Sending 5, 100-byte ICMP Echos to 10.1.2.252, timeout is 2 seconds:. . . . .Success rate is 0 percent (0/5)


VLSM

Variable Length Subnet Masking– Using more than one subnet mask in a single

classful network.– Not supported by old protocols RIPv1, IGRP.


Overlapping Subnets

Subnets must not have overlapping addresses. Results in overlapping router table entries. Routing is not predictable in such a situation. Some hosts may only be accessible from some parts of

the network.

Determining overlap1. Calculate subnet number and broadcast address of

each subnet; this gives you the address range.

2. Compare address ranges of all subnets, looking for overlaps.


Overlapping Subnets Example

Subnet Subnet Number 1st Address Last Address Broadcast

R1 LAN 172.16.2.0 172.16.2.1 172.16.3.254 172.16.3.255

R2 LAN 172.16.4.0 172.16.4.1 172.16.5.254 172.16.5.255

R3 LAN 172.16.5.0 172.16.5.1 172.16.5.254 172.16.5.255

R1-R2 172.16.9.0 172.16.9.1 172.16.9.2 172.16.9.3

R1-R3 172.16.9.4 172.16.9.5 172.16.9.6 172.16.9.7


Designing a Subnetting Scheme

• Three subnets with prefix /24 (255.255.255.0)• Three subnets with prefix /26 (255.255.255.192)• Four subnets with prefix /30 (255.255.255.252)


Route Summarization

Combining routes to same destination.– Routes must be to adjacent subnets.– Reduces routing table size, improves perf.

Manual– Admin does with ip summary-address

Automatic– Handled by routing protocol.– Can break routing in discontiguous networks.


Creating Summary RoutesSeville(config)#interface serial 0/0Seville(config-if)#ip summary-address eigrp 1 10.3.0.0 255.255.0.0Seville#show ip routeCodes: C - connected, S - static, D - EIGRP, EX - EIGRP external ...Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 9 subnets, 3 masksD 10.2.0.0/16 [90/2684416] via 10.1.6.1, 00:00:36, Serial0/0D 10.3.0.0/16 is a summary, 00:00:38, Null0D 10.1.1.0/24 [90/2195456] via 10.1.6.1, 00:00:36, Serial0/0C 10.3.5.0/24 is directly connected, Loopback5C 10.3.4.0/24 is directly connected, FastEthernet0/0C 10.1.6.0/30 is directly connected, Serial0/0C 10.3.7.0/24 is directly connected, Loopback7D 10.1.4.0/30 [90/2681856] via 10.1.6.1, 00:00:36, Serial0/0C 10.3.6.0/24 is directly connected, Loopback


Creating a Manual Summary

1. List all subnets you want to summarize.

2. Find first N bits of subnet numbers that are the same for all subnets.

3. Summary route subnet number is the in-common bits followed by all 0s.

4. Summary route subnet mask is N 1s followed by 32-N 0s.

5. Check work by comparing IP address range of summary subnet with summarized subnet ranges.


Manual Summary Example

0000 1010 0000 0010 0000 0 001 0000 0000 10.2.1.0

0000 1010 0000 0010 0000 0 010 0000 0000 10.2.2.0

0000 1010 0000 0010 0000 0 011 0000 0000 10.2.3.0

0000 1010 0000 0010 0000 0 100 0000 0000 10.2.4.0

Subnet Address: 0’s replace non-shared bits

0000 1010 0000 0010 0000 0 000 0000 0000 10.2.0.0

Broadcast Address: 1’s replace subnet address bits

1111 1111 1111 1111 1111 1 000 0000 0000 255.255.248.0


Autosummarization

Classful routing protocols use static masks.– Cannot use route to 10.3.4.0/24.

– Can only use 10.0.0.0/8 route.

Albuquerque will send all 10 net traffic to Seville.


Discontiguous Networks

Albuquerque ends up with 2 10.0.0.0/8 routes– Balances packets between S0/0 and S0/1– Applications lose ~50% of packets through A.


Discontiguous NetworksAlbuquerque#show ip routeCodes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static routeGateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnetsC 172.16.1.0 is directly connected, Ethernet0/0C 172.16.2.0 is directly connected, Serial0/0C 172.16.3.0 is directly connected, Serial0/1R 10.0.0.0/8 [120/1] via 172.16.3.3, 00:00:13, Serial0/1 [120/1] via 172.16.2.2, 00:00:04, Serial0/0


Autosummarization Support

• RIPv1 always uses autosummarization.

• RIPv2 and EIGRP use autosummary default– Turn off with no auto-summary

• OSPF does not use autosummarization.


References

1. Paul Albitz and Cricket Liu, DNS and BIND, 5th edition, O’Reilly, 2006.

2. James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005.

3. Cisco, Cisco Connection Documentation, http://www.cisco.com/univercd/home/home.htm

4. Cisco, Internetworking Basics, http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

5. Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.

http://www.cisco.com/univercd/home/home.htm

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

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