6: Routing
Working at a Small to Medium Business
Basics
• Routers can be used to break network
smaller
– Subnet
• To go from router to router
– Routing table
– Finds route to other networks
– Statically set
– Dynamically learned
The Routing Table
• Router looks at destination IP & SM
– ANDing the destination IP & SM
• Result is a network #
• Looks in table for the match & forwards it out that
interface
• No match= default route, if set
• Routing Table has list of networks & paths
The Routing Table
Configuring the Static Route
• ip route destination_network subnet_mask next hop ip or outgoing int
• R1(config) #ip route 192.168.16.0 255.255.255.0 192.168.15.1
PT 6.1.1.5
What would be the
static route on R2 to
reach the ladies’
network?
Dynamic Routing Protocols
• Maintain tables when changes occur
– Bad cables, interfaces go down, better route
learned
– Best route to a network in table
– Removes routes when no longer valid
• When all routers agree on topology=
converged
• Two routers can exchange these tables as
long as they use the same protocol
Distance Vector Routing
• Passes updates every so often to connected
neighbors
• Distance & Direction
– Metric (hops, speed, reliability, etc)
Distance Vector Routing & Lab
• Interface that leads to the connected
network has a distance (metric) of 0
• Neighbors update each other & add on how
far away it is
Star is 2
away
Star is 0
away
Star is 1
away
Star is 2
away
Practice
• 6.1.2.4
• 6.1.2.5 Lab
RIP- Routing Information
Protocol
• DV
• Metric is Hops
– Only15 Max; 16 is unreachable (D)
• Updates every 30 seconds by default
– Sends entire routing table (D)
– If change, update sent immediately (triggered)
– Slow to converge whole network (D)
• Administrative Distance is 120
RIP
• RIPv1
– Doesn’t send subnet mask in updates
– Classfull subnetting
• RIPv2
– Classless subnetting
RIP
EIGRP
• Enhanced Interior Gateway Routing
Protocol
– Cisco proprietary DV (mix LS & DV)
– AD of 90
– Many metrics (bandwidth, delay, load, reliability)
– Up to 224 hops
– Routing Table, Neighbor Table, Topology Table
– Updates on start of router & only when a
change happens
– VLSM Support
EIGRP Tables
• Neighbor Table
– Has info about neighbor direct connect routers
• Topology Table
– Built from advertisements of its neighbors
– Contains ALL routes advertised by neighbor routers
– DUAL calculates the shortest path to a destination and
installs it into the routing table
– Is able to find the best alternate path quickly when a
network change occurs
– If no alternate route exists it asks its neighbors to find a
new path to the destination
Link State Routing
• Knows routes further away
• LSA
• Topological database
– Info from LSAs
• SPF
– Each change causes new calc & database
update
– Map of network from point of view of the router
– Info in tree is used to build the routing table
OSPF
•
•
•
•
AD of 110
Metric is cost
Uses SPF
Sends updates only when the topology
changes
– Does not send periodic updates of the entire
routing table
• Fast convergence
• Supports VLSM
• Provides route authentication
Which to Use??????
DV vs. Link State
Summary of Routing Protocols
Protocol
RIP v1
DV or LS
DV
RIP v2
DV
EIGRP
DVHybrid
OSPF
LS
AD
Metric
Features
Hops
15 hops
max
120
Hops
15 max;
VLSM
90
Bandwidth,
Load, Delay,
Reliability
Cisco’s;
VLSM
Cost
Multivendor;
VLSM
120
110
Routing
• May be multiple routes to same path
– Use AD (metrics) to pick best path
– Lowest AD is best
Configure RIP
• Router(config)#router rip
• Router (config-router)#version 2
• Router(config-router)#network network-number
Verifying RIP
• Ping
• Show ip route
• Show ip protocols
•PT 6.1.5.3
•Lab 6.1.5.4
Show ip protocols output
Show ip route output
Administrative Distance/metric; lower AD to a network=more trustworthy route
Problems with RIP
Autonomous Systems
• A network under one administration
– An ISP & its customers
• Large businesses have their own AS
– Each has unique AS#
– Manages all routes to it
Activity
Routing Between AS
• IGP runs within your network & between
your routers
– RIP, EIGRP, OSPF
• EGP runs between different AS
– Most common is BGP
– Runs on border gateway routers
• The router at the end of an AS
– Finds the best path between AS
AS- Link
Routing Across Internet
Help from the ISP
• Keep the Internet connection available
• ISP provides backup routes & routers
• ISP advertises routes to other AS
– If route fails, sends an update with a backup
route
ISP Handling of Traffic- Link
• Local traffic
– Stays within the AS
• Transit traffic
– Outside of AS to go in/through AS
– Like detouring traffic from Rt.55 onto Tanyard
Rd.
• Could cause overflow of traffic
• Can’t handle that much
• AS may or may not allow this traffic
Configure the Border Gateway
• Static Route to ISP
• If router participates in the AS, configure
BGP
Lab 6.2.5
on PT
Review
6: Routing
Working at a Small to Medium Business