Lesson 7: Configuring and Troubleshooting Routers Flashcards
Define a Router
An intermediate system with links and network topology information to facilitate forwarding packets
Where is routing information stored in a router?
In the routing table
What does a protocol define?
The source of the route
What are the four types of routing table entries?
- Direct Network Routes
- Remote Network Routes
- Host Routes
- Default Routes
Define a direct network route
A routing table entry for an active router interface or subnet that is automatically added to the routing table
Define a static route
Manually added to the routing table
Define a default route
Entry in the routing table to represent the forwarding path that will be used if no other entries are matched, also known as default gateway
How does a router forward packets?
After receiving a packet, it reads the destination address and looks up a matching destination network address and prefix, if there is a match, the router will forward the packet out of one of its interfaces by encapsulating the packet in a new frame
How does a router forward packets in a LAN/subnet?
The router uses arp (IPV4) or neighbor discover (IPV6) to determine the interface address of the destination host
How does a router forward packets to an external network?
It inserts the next hop router’s MAC address into the new frame during encapsulation
Define hop count
The amount of hops it takes a packet to transverse the internet; each time a packet passes through a router, the hop count increases by one
Define TTL
Counter field in the IP header that records number of hops a packet can make before being dropped; the number of seconds a packet can stay on the network before being discarded; when TTL hits 0, the packet is discarded
Define Dynamic Routing
Routing entries learned from another router via a dynamic routing protocol
How does dynamic routing determine routing information?
By using one of two protocols, distance vector or link state to select the best route
Define the distance vector protocol
Distance vector protocols send their entire routing table to directly connected neighbors to determine the path with the least amount of hops
Define a link state protocol
Dynamic Routing algorigthm/protocol allows a router to store the complete network topology and assess the least-cost paths in the topology database.
Define Convergence
Process where routers agree on routes through the network to establish the same network topology in their routing tables
What is steady state?
A network where all the routers have the same topology
What is an autonomous system (AS)?
A network under the administrative control of a single owner
What is an Interior Gateway Protocol (IGP)?
A dynamic routing protocol used to share forwarding paths between routers in the same autonomous system (AS)
What is an Exterior Gateway Protocol (EGP)?
A dynamic routing protocol that can advertise routes between autonomous systems (AS) in the internetwork
What type of routing protocol is RIP (Routing Information Protocol)?
A distance vector routing protocol that only considers one metric to select the optimal path which is the route with the least amount of hops
How does RIP (Routing Information Protocol) work?
RIP sends its routing database to neighboring routers every 30 seconds, and will send triggered updates whenever changes occur; When a router receives an update from a neighbor it adds the unknown route and increases the hop count by 1
What are the downsides to RIP (Routing Information Protocol)?
- Suited for smaller networks because distance vector algorithms require that routers periodically update their routing table and is unsuitable for larger networks
- Distance vector algorithms provide slower convergence than link state algorithms
Define EIGRP (Enhanced Interior Gateway Routing Protocol)
An advanced distance vector/hybrid routing protocol that uses two metrics to determine the best route
What metrics are used in EIGRP (Enhanced Interior Gateway Routing Protocol) to determine the best route?
- Bandwidth - applies a cost based on the lowest bandwidth link in the path
- Delay/Latency - Applies a cost based on the time it takes for a packet to traverse the link; most important if route is used to carry time sensitive data
How do routers using EIGRP (Enhanced Interior Gateway Routing Protocol) communicate with their neighbors?
- EIGRP sends a full update of its routing database when it first establishes contact with a neighbor, and thereafter only sends updates when there is a topology change
- Sends hello messages to confirm connectivity with neighbors
How do EIGRP (Enhanced Interior Gateway Routing Protocol) routers leverage topology tables
Topology tables are used to prevent loops while support a greater number of max hops (up to 255)
What protocol number does EIGRP (Enhanced Interior Gateway Routing Protocol) use?
Port 88
Define Open Shortest Path First (OSPF) routing protocol
A link-state protocol that uses a hierarchical topology of its connected networks
What is a Link State Advertisement (LSA)?
How OSPF routers communicate link state within the AS
How do routers using Open Shortest Path First (OSPF) communicate?
Routers distribute information about its local state (usable interfaces, reachable neighbors, and cost of each interface) to other routers using LSA advertisements and stores the information in the link state database (LSDB)
What is area routing?
An AS can be divided into areas, which are connected to each other by routers. An area consists of groups of contiguous networks and attached hosts. The topology of an area is invisible to entities outside the area.
What is a border router?
A router that is located on the border of OSPF areas and connects these areas to the backbone and maintains separate link-state databases for each area
Define Area 0 in an OSPF topology
Also known as a backbone, is created by the collection of border routers and is only visible to the border routers
How are messages in OSPF sent?
They’re sent as multicasts and tagged with the protocol number 89 that supports cryptographic authentication
How is BGP (Border Gateway Protocol) used?
Designated routing protocol used between ISPs
Does BGP (Border Gateway Protocol) use classless or classful IP addressing?
Classes network addressing called Network Layer Reachability Information (NLRI)
How does BGP (Border Gateway Protocol) select the best path/route?
Based on hop count, weight, local preference, origin, and community
What port does BGP (Border Gateway Protocol) use?
TCP port 179
Define the “longest prefix match”
Presented with a packet to route, a router will route the packet to the the network with the largest prefix
What is Administrative Distance (AD)?
The first criteria a router uses to determine which routing protocol to use if two protocols have the same destination in their routing table
List routing protocols by Administrative Distance (AD) from lowest to highest
- Local interface/directly connected
- Static route
- BGP
- EIGRP
- OSPF
5.RIP
What is an Anatonoumous System Number (ASN)?
Identifies a group of network prefixes under the control of a single entity
Define an edge router
A perimeter router that physically links remote networks and subdivides autonomous IP networks into multiple subnets
Define an internal router
Positioned to implement internal routing topologies and to create VLANS
What command is used to list the routing table in Linux and windows?
Route
If you can ping a host’s default gateway but not the host, what is most likely the issue?
A routing issue
What command is used to check for a route to a specific network?
show up route x.x.x.x
What is a routing loop?
Occurs when two routers use each other as the path to a network and causes a packet to be forwarded in a loop until its TTL expires
What mechanisms are implemented to stop routing loops?
Distance vector protocols use max hop count, holddown timer, and split horizon
What is the holddown timer metric?
Routers in an AS start a holddown timer for a route if a node declares a network is unreachable to allow all routers in the AS to converge routing tables
Define asymmetric routing
a packet traverses from a source to a destination in one path and takes a different path when it returns to the source