Skip to content

Routers-and-Routing.md

Latest commit

 

History

History
180 lines (175 loc) · 8.54 KB

Routers-and-Routing.md

File metadata and controls

180 lines (175 loc) · 8.54 KB

How to know when to pick up a router versus a switch?

  • Routers tend to be WAN-centric
    • If you are connecting T1s, you might want a router
  • Switches tend to be LAN-centric
    • If you are connecting Ethernet, you might want a switch

Routing Tables

  • In Cisco routers, the routing table is called:
    • Route Information Base (RIB)
      • "show ip route"
        • Formatted view of the RIB
  • "Add link to Routing protocols down the line"
    • Each Routing Protocol has its own table of information
      • Metric value:
        • Is determined by the routing protocol from which the route was learned.
      • If the same route is learned from two sources within a single routing protocol.
        • One with the best metric will win
        • Or the one with the lowest administrative distance will win

Route Type-Administrative Distance Table

Route Type Administrative Distance
Connected Interface 0
Static route 1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5
External Border Gateway protocol (BGP) 20
Internal EIGRP 90
Interior Gateway Routing Protocol (IGRP) 100
Open Shortest Path First (OSPF) 110
Intermediate System - Intermediate System (IS-IS) 115
routing Information Protocol (RIP) 120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200
Unknown 255

  • When a packet arrives at a router,

    • Determines if the packet needs to be forwarded to another network.
      • If it does:
        • It check the RIB to see whether it contains a route to the destination network.
          • If there is a match; the packet is adjusted and forwarded out the proper interface.
          • If there is not a match; the packet is forwarded to the default gateway.
            • Gets dropped if there is no default gateway

IP Routing Table

R2# show ip route
Codes: 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, 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 11.0.0.1 to network 0.0.0.0

     172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
D       172.16.200.0/23 is a summary, 00:56:18, Null0
C       172.16.200.0/24 is directly connected, Loopback2
C       172.16.201.0/24 is directly connected, Serial0/0
C       172.16.202.0/24 is directly connected, Loopback3
C       172.16.100.0/23 is directly connected, Loopback4
D       172.16.101.0/24 [90/2172416] via 11.0.0.1, 00:53:07, FastEthernet0/1
C    10.0.0.0/8 is directly connected, FastEthernet0/0
C    11.0.0.0/8 is directly connected, FastEthernet0/1
     192.168.1.0/32 is subnetted, 1 subnets
D       192.168.1.11 [90/156160] via 11.0.0.1, 00:00:03, FastEthernet0/1
S*   0.0.0.0/0 [1/0] via 11.0.0.1
D    10.0.0.0/7 is a summary, 00:54:40, Null0
  • First block of code explains the codes listed down the left side of the routing table
Gateway of last resort is 11.0.0.1 to network 0.0.0.0
  • If there are two or more default gateways, they will all be listed
    • Packets will be equally balanced between the two links using per-packet load balancing
Gateway of last resort is not set
  • No default gateway(s) has been configured/learned
     172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
D       172.16.200.0/23 is a summary, 00:56:18, Null0
C       172.16.200.0/24 is directly connected, Loopback2
C       172.16.201.0/24 is directly connected, Serial0/0
C       172.16.202.0/24 is directly connected, Loopback3
C       172.16.100.0/23 is directly connected, Loopback4
D       172.16.101.0/24 [90/2172416] via 11.0.0.1, 00:53:07, FastEthernet0/1
C    10.0.0.0/8 is directly connected, FastEthernet0/0
C    11.0.0.0/8 is directly connected, FastEthernet0/1
     192.168.1.0/32 is subnetted, 1 subnets
D       192.168.1.11 [90/156160] via 11.0.0.1, 00:00:03, FastEthernet0/1
S*   0.0.0.0/0 [1/0] via 11.0.0.1
D    10.0.0.0/7 is a summary, 00:54:40, Null0

D       172.16.101.0/24 [90/2172416] via 11.0.0.1, 00:53:07, FastEthernet0/1
  • From left to right
  • Route code:
    • D in this case
      • Indicates the route was learned via EIGRP
  • Route itself
    • 172.16.101.0/24
      • Route to the subnet
  • Two numbers in the brackets
    • [90/2172416]
    • 90 (First number)
      • Administrative distance
    • 2172416 (Second number)
      • Metric for the route
        • Determined by the routing protocol
        • In this case EIGRP
  • Next hop the router needs to send packets to in order to reach this subnet
    • via 11.0.0.1
      • Packets destined for the subnet 172.16.101.0/24 should be forwarded to the IP address 11.0.0.1
  • Age of the route
    • (00:53:07)
  • Interface out which the router will forward the packet
    • FastEthernet0/1

Route Types

  • Host Route

    • Is a route to a host.
      • The route is not to a network
    • Has a subnet mask of 255.255.255.255 and a prefix length of /32
    • In the sample routing table:
      • Route to 192.168.1.11 is a host route
      • Denoted by the /32 at the end of the IP

  • Subnet

    • Portion of a major network
    • Used to determine the size of the subnet.
      • 10.10.10.0/24 (255.255.255.0) is a subnet
    • Are indented under their source major networks
      • The major network 172.16.0.0/16 has been subnetted
        • Has been subnetted under the rules of Variable Length Subnet Masks (VLSM)
          • Allows each subnet to have a different subnet mask

  • Summary

    • Is a single route that references a group of subnets
      • 10.10.0.0/16 (255.255.0.0) given that subnets with longer masks (10.10.10.0/24) exists
    • Used in a routing table to represent any group of routes
      • Cisco states that a summary is a group of subnets
        • Super net is a group of major networks
    • In our example:
      • `D 172.16.200.0/23 is a summary, 00:56:18, Null0
        • This one is technically a summary route
      • D 10.0.0.0/7 is a summary, 00:54:40, Null0
        • This one is technically a super net reported as a summary
    • Note the destinations of either summaries: Null0
      • Means that packets sent to this network will be dropped

  • Major Network

    • Any class-full network, along with its native mask
      • 10.0.0.0/8 (255.0.0.0) is a major network
    • In our example:
      • `C 10.0.0.0/8 is directly connected, FastEthernet0/0
      • `C 11.0.0.0/8 is directly connected, FastEthernet0/1
        • Note the /8
          • Referencing 10.0.0.0 with a prefix mask
            • Longer than /8 changes the route to a subnet
            • Shorter than /8 changes the route to a super net

  • Supernet

    • Is a single route that references a group of major networks.
      • 10.0.0.0/7 is a super net that references 10.0.0.0/8 and 11.0.0.0/8
    • In our example:
      • D 10.0.0.0/7 is a summary, 00:54:40, Null0
        • Has the same reference as the information above
        • Notice the destination is Null0
          • On a connected router, we will only see the summary
            • NOT the more specific routes:
          • D 10.0.0.0/7 [90/30720] via 11.0.0.2, 04:30:22, FastEthernet0/1

  • Default Route

    • Is shown as 0.0.0.0/0 (0.0.0.0)
      • Route of last resort
    • Used when no other route matches the destination IP address in a packet
    • In our example:
      • Gateway of last resort is 11.0.0.1 to network 0.0.0.0
      • S* 0.0.0.0/0 [1/0] via 11.0.0.1
        • Static route, as indicated by the S
          • The asterisk next to the S indicates that this route is a candidate for the default route
            • Can be more than one