Note that routers are more scalable than switches since each routing table

Note that routers are more scalable than switches

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Note that routers are more scalable than switches, since each routing table entry is for an entire subnet of devices, while each switch forwarding table entry is for one individual device Routing table can have a default route. If a packet destination doesn’t match any other routes in the table, it is forwarded over the default route. 8
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IT 263 & TDC 405 ‐ Autumn 2018 17 Oct 2018 Dr. Oteafy © 2018 5 Routing Table How do we keep addresses current? Neighboring routers periodically exchange Router Table Update messages to keep their tables up-to- date. 9 What’s in a Routing Table? Each entry in a routing table contains the following: o Destination IP Subnet – this identifies a group of IP addresses. o Outgoing Interface – the router interface the packet should be sent out to forward it towards the destination IP subnet. o Next Hop – the IP address of the next router this packet needs to go through, if any. o There is some other stuff in the routing table, but this is all we will cover in this course. 10
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IT 263 & TDC 405 ‐ Autumn 2018 17 Oct 2018 Dr. Oteafy © 2018 6 Router forwarding operation For each arriving packet: o Find the Destination IP Subnet in the routing table that contains the IP destination address in the packet header o Send packet out the corresponding Outgoing Interface to the Next Hop router. 11 Routing Table example 12 Destination IP Subnet Outgoing Interface Next Hop 140.192.15.0 / 24 Fa0/0 -- 140.192.16.0 / 24 Fa0/1 -- 0.0.0.0 / 0 S0/0 210.1.33.43 Note: “0.0.0.0/0” is the IP subnet representing the default route. If the IP destination address in an arriving packet does not match any other entries in the routing table, then this route will be taken.
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IT 263 & TDC 405 ‐ Autumn 2018 17 Oct 2018 Dr. Oteafy © 2018 7 Keeping Routing Tables Up-to-Date Initially, a network administrator specifies an IP address and subnet mask for each router interface. o This creates routing table entries for directly connected subnets. In static routing , the network administrator manually enters routes for remote subnets into the routing table. In dynamic routing , the router exchanges Routing Protocol messages with all neighbor routers. o Each router creates and updates remote subnet routes in its routing table based on information in Routing Protocol messages. o Details of Routing Protocols (RIP, OSPF, EIGRP, BGP) are beyond the scope of IT 263. 13 Cisco Routing Table Format IOS Command: show ip route 14 IP Subnet Route Metrics (not covered here) Next Hop Outgoing Interface Code (not covered here) Time since last route update received
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IT 263 & TDC 405 ‐ Autumn 2018 17 Oct 2018 Dr. Oteafy © 2018 8 Important note Addresses! 15 IP vs. MAC Addresses IP addresses are global. o The IP address of a device can be used by any other device on the entire Internet to communicates with it. o IP addresses are easily discoverable via DNS lookup, hyperlinks or scanning.
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