Routing Flashcards
configure router subinterface
- i.e. interface f-/0.10
- encapsulation dot1q 10
- ip and mask
configure static route
ip route network-address mask {ip | exit interface}
default route
static routed used w/ bits matching
configured as 0.0.0.0 0.0.0.0
configure RIP
#router rip #network *ip address*
RIP troubleshooting commands
show ip route
show ip protocols
show ip rip
switch router to RIPv2
#router rip #version 2
enable VLSM for RIPv2
no auto-summary
EIGRP
- distance vector
- classless
- cisco proprietary
- RTP
- bounded updates
- diffusing update algorithm (DUAL)
- establishing adjacencies
- neighbor and topology table
EIGRP TLV types (type/length/value)
0x0001 EIGRP parameters
0x0102 IP internal routes
0x0103 IP external routes
Opcode in EIGRP header
specifies EIGRP packet type
AS # in EIGRP packet header
specifies antonymous system # since multiple EIGRP instances can be run
5 EIGRP packet-types
- hello
- update
- acknowledgement
- query
- reply
configure EIGRP summary route
enter interface config mode
ip summary-address eigrp as-# network# mask
command to include static route in routing updates
redistribute static
command to modify bandwidth metric on EIGRP interface
bandwidth speed in kilobits
3 tables useful for troubleshooting EIGRP
- routing table
- neighbor table
- topology table
sucessor
neighboring router used for packet forwarding and is the least-cost route to the destination network
feasible distance
lowest calculated metric to destination network
feasibility
met when neighbors reported distance to a network is less than the local routers FD to the same network
OSPF
Open Shortest Path First
- link state protocol
- open standard
5 OSPF packet types
-hello
-DBD (database description)
-LSR (link-state request)
-LSU (link-state update)
LSAck (link state acknowledgement)
OSPF hello packets
- discover OSPF neighbors & establish adjacencies
- advertise parameters on which 2 routers must agree to become neighbors
- elect the designated router & backup designated router on multiaccess networks
matching interface values required for OSPF adjacency
- hello interval
- dead interval
- network type
designated router
- responsible for updating all other OSPF routers on a multi-access network
- router w/ highest OSPF interface priority is chosen
- 2nd highest is backup designated router
link state routing process
- each router learns about its own links
- each router uses hello packets to establish adjacencies
- each router builds a link-state packet
- each router floods the LSP to all neighbors
- each router uses the database to construct a complete map of the topology, SPF determines the best paths
command to enable OSPF
#router ospf process-id #network address wildcard mask area area-id
OSPF area
group of routers that share link-state information
OSPF metric
cost
10 to the 8th/bandwidth in bps
modify OSPF bandwidth
use bandwidth command to modify bandwidth value
use ip ospf cost command
change router OSPF priority to control DR & BDR selection
ip ospf priority {0-255}
basic routing troubleshooting
- check routing tables for convergence using show ip route
- ip routes are missing investigate protocol operation using show ip protocols
- check for missing neighbors using show ip (ospf or eigrp) neighbors
- show run config
troubleshooting VLSM
- pay attention to see if design really uses VLSM, check for classless routing protocol
- check for overlapping subnets
- outward symptoms include not sending packets out of local subent
- use traceroute
- do the math on subnets to be sure they don’t overlap
troubleshoot RIP
- incorrect network statements
- dis-contiguous networks
EIGRP and OSPF troubleshooting commands
show ip eigrp interfaces
show ip ospf interface brief
show ip protocols
configure DHCP server on cisco router
1) use ip dhcp excluded-address [low-address (high-address)] to identify excluded addresses
2) create DHCP pool using ip dhcp pool pool-name command
3) configure ip addressing parameters
distance vector routing protocols
RIP(1&2) IGRP, EIGRP
link state routing protocols
OSPF, IS-IS
path vector routing protocols
BGP, EGP
IGP & EGP
interior gateway protocols used for intra AS routing
exterior gateway protocols route between ASs
what is a distance vector protocol?
- use distance & direction
- don’t know topology
Works best with
- flat, simple networks
- administrators don’t understand link state protocols
- specific types of networks (hub & spoke) are being implemented
- worst-case convergence times are not a concern
what are link-state routing protocols
- have complete topology
- only send updates when topology changes
works best
- hierarchal networks
- large networks
- knowledgeable administrators
- fast convergence is crucial
how does routing decide between multiple paths to same destination?
metric
-metric can viewed with show ip route command
administrative distance
- defines preference of a routing source
- all routes are prioritized by AD regardless of protocol, static routes, etc.
- lower value = higher priority
- can be 0 to 255
verify w/ show ip protocols command
default AD of directly connected & static routes
0 and 1 respectively
default EIGRP AD
90
default OSPF AD
110
default RIP AD
120
RIPv2 metric
hop count
OSPF metric
bandwidth
EIGRP metric
function of bandwidth and delay(bandwidth+delay)*256
preventing routing loops w/distance vector protocols
- define maximum metirc
- hold down timers
- split horizon
- route poisoning or poison reverse
- triggered updates
- TTL field in ip header
hold-down timers
instructs routers to hold any changes that might affect routes for a specific period of time. If a route is identified as down or possibly down any other information for that route containing the same status or worse, is ignored for the hold down period so the network can converge
split horizon
doesn’t allow advertisement to be sent out the same interface from which they originated
route poisoning or poison reverse
used to mark route as unreachable in a routing update sent to other routers
LSDB & LSA
link state database
link state advertisements
SPF
shortest path first
- run on LSDF to create SPF tree
- adds cost associated w/each link between itself
- lowest cost path is added to routing table
how to accomplish inter VLAN routing
- one router interface per vlan
- router on a stick using trunk link and sub interfaces
- layer 3 switch
feasible successor
a neighbor who has a loop free backup path to the same network as the successor by satisfying the feasibility condition
standard ACLs
filter based on source address
extended ACL
filters based on source & destination address, protocol, or ports
ACL guidelines
- 3 Ps 1 ACL per protocol, per direction, per interface allowed
- organize from top down, specific to general
- there is an implicit deny any at end of ACL
- extended ACLs close to source
- standard ACL close to destination
standard numbered ACLs
1-99
1300-1999
extended numbered ACLs
access-list access-list # {permit | deny} protocol source source-wildcard [operator port] destination destination-wildcard [operator port] [established] [log]
100-199
2000-2699
configure named ACLs
- name the ACL ip-access list standard name
- create ACL
- apply ACL
extended named ACL
- name the ACL ip access-list extended name
- create ACL
- apply ACL
add comments to ACLs
use remark in place of permit or deny in syntax
ACL show commands
show access-lists
show ip access-list
show ip interface
show running-config
standard numbered ACLs
1-99
1300-1999
extended numbered ACLs
access-list access-list # {permit | deny} protocol source source-wildcard [operator port] destination destination-wildcard [operator port] [established] [log]
100-199
2000-2699
configure named ACLs
- name the ACL ip-access list standard name
- create ACL
- apply ACL
extended named ACL
- name the ACL ip access-list extended name
- create ACL
- apply ACL
add comments to ACLs
use remark in place of permit or deny in syntax
ACL show commands
show access-lists
show ip access-list
show ip interface
show running-config
standard numbered ACLs
1-99
1300-1999
extended numbered ACLs
access-list access-list # {permit | deny} protocol source source-wildcard [operator port] destination destination-wildcard [operator port] [established] [log]
100-199
2000-2699
configure named ACLs
- name the ACL ip-access list standard name
- create ACL
- apply ACL
extended named ACL
- name the ACL ip access-list extended name
- create ACL
- apply ACL
add comments to ACLs
use remark in place of permit or deny in syntax
ACL show commands
show access-lists
show ip access-list
show ip interface
show running-config
What is an OSPF router that has interfaces in multiple areas called?
Area border router
OSPF area 0
There must be an area 0
It should be used or the backbone
EIGRP IPv4 multicast address
224.0.0.10
AD in context of EIGRP?
Advertised distance.
The total metric along a path to a destination network as advertised by an upstream neighbor
How is feasible distance calculated in EIGRP?
advertised distance + metric to neighbor
What can cause adjacency issues in EIGRP?
The interface between the devices is down.
The routers have mismatching EIGRP autonomous systems.
The EIGRP process is not enabled on one of the interfaces that connects the devices.
One of the interfaces that connects the devices is configured as a passive interface.
mismatched K values
Issues that prevent routing table from learning appropriate routes
Networks are not being advertised on remote routers.
An access list is blocking advertisements of remote networks.
Automatic route summarization is causing confusion in your discontiguous network.
EIGRP IPv6 multicast address
FF02::A
differences in configuring EIGRP for IPv6
after configuring the routing process you must issue a no shutdown command in routing configuration mode
there a now network statements. instead you add an interface to routing by issuing a ipv6 eigrp AS# command in interface configuration mode
What are OSPF dbd packets?
Database Description packets contain LSA headers that help routers build the link state database.
What are OSPF LSR packets?
Once DBD packets are exchanged, each router checks the LSA Headers against its own database. If it it does not have current information for any LSA, it generates an LSR packets and sends it to its neighbor to request updated LSAs.
What are OSPF LSU packets?
LSU packets contains a list of LSAs that are to be updated. This packet is often used in flooding.
What are OSPF LSAck packets
LSAck packets help to ensure reliable transmission of LSAs. Each LSA is explicitly acknowledged.
The default OSPF costs are based on a reference of 100mbps. What are the 3 options to adjust this for faster links?
Set the reference bandwidth globally
manually set the interface cost on specific interfaces
manually configure the reference bandwidth per interface
What process is used for OSPF routers to exchange information
The routers exchange one or more DBD packets. A DBD includes information about the LSA entry header that appears in the LSDB of the router. Each LSA entry header includes information about the link-state type, the address of the advertising router, the cost of the link, and the sequence number. The router uses the sequence number to determine the “newness” of the received link-state information.
When the router receives the DBD, it acknowledges the receipt of the DBD that is using the LSAck packet.
Routers compare the information that they receive with the information that they have. If the received DBD has a more up-to-date link-state entry, the router sends an LSR to the other router to request the updated link-state entry.
The other router responds with complete information about the requested entry in an LSU packet. The other router adds the new link-state entries to its LSDB.
When the router receives an LSU, it sends an LSAck.
OSPF backbone area, or area 0
The primary function of this OSPF area is to quickly and efficiently move IP packets. Backbone areas interconnect with other OSPF area types. The OSPF hierarchical area structure requires that all areas connect directly to the backbone area. In the figure, links between Area 1 and Area 2 routers are not allowed. Generally, end users are not found within a backbone area, which is also known as OSPF Area 0.
Normal or non backbone OSPF area
The primary function of this OSPF area is to connect users and resources. Normal areas are usually set up according to functional or geographical groupings. By default, a normal area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas must cross a transit area such as Area 0.
What does an OSPF autonomous system consist of ?
all of the OSPF areas and routers that are running the OSPF routing protocol.
What are OSPF internal routers?
non backbone routers that have all interfaces in a single area
What are OSPF area border routers?
routers that connect nonbackbone areas to the backbone area
What functions does an OSPF area border router perform?
It separates LSA flooding zones.
It becomes the primary point for area address summarization.
It functions regularly as the source for default routes.
It maintains the LSDB for each area with which it is connected.
What is the ideal design for how many areas an ABR connects to?
The ideal design is to have each ABR connected to two areas only, the backbone and another area, with three areas being the upper limit.
What is an autonomous system boundary router ASBR?
An ABR located between an OSPF autonomous system and a non-OSPF network. ASBRs run both OSPF and another routing protocol, such as RIP. ASBRs must reside in a nonstub OSPF area.
Type 1 LSA
router LSA
Every router generates router link advertisements for each area to which it belongs. Router link advertisements describe the state of the router links to the area and are flooded only within this particular area.
Type 2 LSA
network LSA
Network LSAs are generated only by designated routers and describe the set of routers attached to a particular nonbroadcast multiaccess (NBMA) or broadcast network. The purpose of the network LSA is to ensure that only one LSA is generated for the NBMA or broadcast network (as opposed to one from each attached router). This is a form of internal OSPF summarization.
Type 3 LSA
summary LSA
Summary LSAs are generated by area border routers (ABRs) and describe inter-area routes to various networks. Specifically, Type 3 LSAs describe networks that are within the OSPF autonomous system but outside of the particular OSPF area that is receiving the LSA.
Type 4 LSA
ASBR summary LSA
Each summary LSA describes a route to a destination outside the OSPF area yet still inside the autonomous system (that is, an inter-area route). the Type 4 summary LSAs describe routes to autonomous system boundary routers (ASBRs) and are also generated by ABRs.
Type 5 LSA
Autonomous system LSA
Type 5 LSAs are generated by the ASBRs. These LSAs describe routes to destinations that are external to the AS. Type 5 LSAs are flooded everywhere, with the exception of stub areas.
max number of routers that should participate in an OSPF area
50
What are the major components of OSPF troubleshooting?
OSPF neighbor adjacencies
OSPF routing table
OSPF path selection
Key OSPFv3 Characteristics
As in OSPFv2, each OSPF router is uniquely identified by its RID, a 32-bit number that is based on an IPv4 address of the router. If no IPv4 address is configured, you will be prompted to configure it using the router-id command.
Adjacencies and next-hop attributes use IPv6 link-local addresses.
IPv6 is used for transport of LSAs.
OSPFv3 is enabled per link instead per network as in OSPFv2.
OSPFv3 communicates with its peers using IPv6 multicast addresses.
OSPFv3 multicast addresses
FF02::5 for all OSPF routers
FF02::6 for the OSPF DR and OSPF BDR.
command to enable OSPFv3 on an interface
ipv6 ospf process-id area area-id
show command to get general OSPFv3 info
show ipv6 ospf
