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Gateways and routing

Configuring gateways, static and dynamic routing and OSPF areas on a Cisco device.

A route of a packet is normally chosen depending on the default distance value table.

Default gateway​

ip default-gateway 192.168.0.254
ipv6 default-gateway fe80::1

Default gateways don't work on routers when IP routing is active. One would have to use a wildcard route instead.

Static Routing​

! Enable routing
! By default, a router would only forward packets that are directly targeted at one of its interfaces
ip routing
ipv6 unicast-routing

! Example routes
ip route 192.168.5.0 255.255.255.0 192.168.0.1
ipv6 route 2001:db8:acad:5::/64 fe80::7

Wildcard routes​

ip route 0.0.0.0 0.0.0.0 192.168.0.254
ipv6 route ::/64 fe80::1

! Specify outgoing interface
ip route 0.0.0.0 0.0.0.0 gi0/0/0

! Floating static route (higher administrative distance of 5)
ip route 0.0.0.0 0.0.0.0 gi0/0/1 5

Dynamic routing​

OSPF​

Terminology​

  • OSPF area: a routing domain under which routes are propagated
  • SPF algorithm: shortest path first algorithm (using Dijkstra's)
  • adjacency database: list of neighbor routers
  • link-state database (LSDB): database describing the whole topology (the same for routers in the same area)
  • forwarding database (aka routing table): generated list of next-hop routes (from the link-state db with the SPF algorithm)
  • loopback interfaces: can be configured with an IP address for higher availability, explained in the administration chapter
  • router id: id for picking DR and BDR, can be the following (ordered by priority)
    • configured router id (highest priority)
    • loopback ip address
    • highest ip address of all interfaces
  • DR: designated router, elected for each segment (Layer 2 broadcast domain) to reduce OSPF packets
  • BDR: backup designated router
  • cost: the cost on an interface is used by the SPF algorithm to pick the best path
  • reference-bandwidth: default cost = interface-bandwidth / reference-bandwidth (rounded up to an integer)

Packet types​

  • hello: to create the adjacency db
  • database desc (DBD): describes a link-state db, so other routers know what they are missing
  • link state
    • request (LSR): request a specific missing link state advertisement (LSA)
    • update (LSU): contains the LSAs with data
    • ack (LSAck): acknowledge the receipt

Configuration​

interface loopback 0
  ip address 2.2.2.2 255.255.255.255
  no shutdown

! In OSPF, the process-id is local to the device for identification
! router ospf <process-id>
router ospf 10
  router-id 1.1.1.1

  ! Disable OSPF packet propagation on an interface (i.e. on loopbacks)
  passive-interface lo0

  ! If configured, should be on all devices in the network
  auto-cost reference-bandwidth 1000

  ! Propagates default static routes to OSPF
  default-information originate

  ! * Either add a network to OSPF here
  network 192.168.42.0 0.0.0.255 area 0

interface gi0/0/0
  ! * Or add the network from the interface directly
  ip ospf 10 area 0

  ! Optional settings
  ip ospf cost 10
  ip ospf hello-interval 5
  ip ospf dead-interval 20

  no shutdown
  ip address 192.168.42.254 255.255.255.0

EIGRP​

Terminology​

  • Autonomous system (AS): a routing domain under which routes are propagated (like the OSPF area)
  • metric weights: numbers that change how the shortest route is calculated
  • router id: the router id is picked the same way as in OSPF

Configuration​

interface loopback 0
  ip address 2.2.2.2 255.255.255.255
  no shutdown

! Whe autonomous-system-no identifies the AS domain
! router eigrp <autonomous-system-no>
router eigrp 100
  eigrp router-id 1.1.1.1

  ! Disable OSPF packet propagation on an interface (i.e. to an ISP)
  passive-interface gi1/0/1

  ! Weights have to be the same on all devices in the AS domain
  ! metric weights <type> <bandwidth> <load> <delay> <reliability> <MTU>
  metric weights 0 2 0 1 0 0

  network 192.168.42.0 0.0.0.255

interface gi0/0/0
  no shutdown
  ip address 192.168.42.254 255.255.255.0

Redistributing routes​

The redistribute command is useful to share routes from one routing protocol to another.

Example: bridging OSPF and EIGRP​

router eigrp 100
  redistribute ospf 1

router ospf 1
  redistribute eigrp 100 subnets

Evaluation​

#
! Viewing routing tables
show ip route
show ip route static
show ipv6 route
show ipv6 route static

! Tracing the route to a device
traceroute 192.168.5.3

! OSPF
show ip protocols | include Router ID
show ip ospf neighbor
show ip ospf database
show ip ospf interface gi0/0/0