Routing Flashcards
Routing Fundamentals
Traffic is routed to flow between subnets
Each subnet is its own broadcast domain
Routers are the layer 3 devices that separate broadcast domains, but multilayer switches are also used
Routing Decisions
Layer 3 to Layer 2 Mapping
Routers use ARP caches to map an IP to a given MAC
Make packet-forwarding decisions based upon their internal routing tables
Routing Tables
Table kept by the router to help determine which route entry is the best fit for the network
A route entry with the longest prefix is the most specific network
(10.1.1.0/24 is more specific than 10.0.0.0/8)
Sources of Routing Information
Directly Connected Routes
Learned by physical connection between routers
Static Routes
Manually configured by an admin
Default 0.0.0.0/0 is a special case
(If I don’t know where, then send out default static route)
Dynamic Routing Protocols
Learned by exchanging info between routers
Routing Loops: Split Horizon
Prevents a route learned on one interface from being advertised back out of that same interface
Routing Loops: Poison Reverse
Causes a route received on one interface to be advertised back out of that same interface with a metric considered to be too large
Routing Protocols (Interior/Exterior)
IGP (Interior Gateway Protocol)
Operate within an autonomous system
EGP (Exterior Gateway Protocol)
Operated between autonomous systems
Router Advertisement: Distance Vector
Sends full copy of routing table to its directly connected neighbors at regular intervals
Slow convergence time
(Time it takes for all routers to update their routing tables in response to a topology change)
Hold-down timers speeds up convergence
(Prevents updates for a specific period of time)
Uses hop count as a metric
Router Advertisement: Link State
Requires all routers to know about the paths that all other routers can reach in the network
Info is flooded throughout the link-state domain to ensure routers have synchronized info
Faster convergence time & uses cost or other factors as a metric
Each router constructs its own relative SPT (shortest-path tree) with itself as the root for all known routes in the network
RIP
Routing Information Protocol
IGP
Distance-vector protocol using hop count
15 max hops (16 = infinite)
Oldest dynamic routing protocol
Updates every 30 seconds
Easy to configure
Runs over UDP
OSPF
Open Shortest Path First
IGP
Link-state protocol using cost
Cost is based on link speed between routers
Supports VLSM
IS-IS
Intermediate System to Intermediate System
IGP
Link-state protocol using cost
Cost is based on link speed between routers
Just like OSPF, just not widely adopted
EIGRP
Enhanced Interior Gateway Routing Protocol
IGP
Advanced distance-vector protocol using bandwidth & delay making it a hybrid of distance-vector & link-state
Cisco-proprietary (Cisco-only networks)
BGP
Border Gateway Protocol
EGP
Path vector using the number of autonomous system hops instead of router hops
Widespread utilization
Runs the backbone of the internet
Does not converge quickly
Route Believability
If a network is using more than one routing protocol, how does it choose which one to use?
Some routing protocols are considered more believable than others, so routers use an index of believability called AD (administrative distance)
Lower the AD, the more believable
(Refer to chart on study guide)
