Routing protocols – recap and quick notes

By | January 18, 2019

Types of dynamic routing protocols:


RIP : Routing Information Protocol - AD 120
IGRP : Interior Gateway Routing Protocol - AD-100
EIGRP : Enhanced IGRP AD-90
OSPF : Open Shortest Path First AD-110
IS-IS : Intermediate system to intermediate system AD-115
BGP: Border Gateway Protocol AD-20


Property   EIGRP OSPF  BGP
Administrative Distances Internal – 90
External 170
110 EBGP – 20
IBGP – 200
Method   Advanced distance vector  Link state Path vector
Summarization   Auto and manual Manual Auto and Manual
VLSM  Yes Yes Yes
Convergence Speed    Very fast convergence Fast Slow
Timers: Update
Triggered (LAN 5/15, WAN 60/180)  Triggered when network change occurs, send periodic update LSA refreshes every 30 minutes (NBMA 30/120, LAN 10/40) Triggered (60/180)
Network Size  Large Large Very large
Mixed-Vendor Devices No Yes Yes
Use multicast  
Feature  – Partial updates conserve network bandwidth
– Support for IP, AppleTalk, and IPX
– Runs directly over IP, using protocol number 88
– Support for all Layer2 (data link layer) protocols and topologies
– Load balancing across equal-and unequal-cost pathways
– Multicast and unicast instead of broadcast address
– Support for authentication
– Manual summarization at any interface
– 100% loop-free classless routing
 – Minimizes the number of routing table entries
– Contains LSA flooding to a reasonable area
– Each routing device takes a copy of the LSA updates its LSDB and forward the LSA to all neighbor devices within area
– Minimizes the impact of a topology change
– Enforces the concept of a hierarchical network design
 – BGP provides the routing betw these autonomouse systems.
– BGP uses the concept of autonomous systems (AS). An autonomous system is a group of networks under a common administration. The Internet Assigned Numbers Authority (IANA) assigns AS numbers: 1 to 64511 are public AS
numbers and 64512 to 65535 are private AS numbers.
– IGP: A routing protocol that exchanges routing infor within AS. RIP, IGRP, OSPF, IS-IS and EIGRP are examples of IFPs.
– EGP: A routing protocol that exchanges routing infor betw different AS. BGP is an example of an EGP.
– The administrative distance for EBGP routes is 20. The administrative distance for IBGP routes is 200.
– BGP neighbors are called peers and must be statically configured.
– BGP uses TCP port 179. BGP peers exchange incremental, triggered route updates and periodic keepalives.
Operation – IP EIGRP Neighbor Table
– IP EIGRP Topology Table AD+FD
– The IP Routing Table
Neighbor Table
Topology Table LSDB
Routing Table
(LSA-> LSDB-> SPF algorithm-> SPF Tree-> Routing Table)
Function is controlled by EIGRP’s function is controlled by 4 key technologies:
– Neighbor discovery and maintenance: Periodic hello messages
– The Reliable Transport Protocol (RTP): Controls sending, tracking, and acknowledging EIGRP messages
– Diffusing Update Algorithm (DUAL): Determines the best loop-free route
– Protocol-independent modules (PDM): Modules are “plug-ins” for IP, IPX, Novel Netware and AppleTalk versions of EIGRP
Following are several types of areas:
– Backbone area: Area 0, which is attached to every other area.
– Regular area: Nonbackbone area; its database contains both internal and external routes.
– Stub area: It’s database contains only internal routes and a default route.
– Totally Stubby Area: Cisco proprietary area designation. Its database contains routes only for its own area and a
default route.
– Not-so-stubby area (NSSA): Its database contains internal routes, routes redistributed from a connected routing
process, and optionally a default route.
– Totally NSSA: Cisco proprietary area designation. Its database contains only routes for its own area, routes redistributed
from a connected routing process, and a default route.
BGP uses 3 databases. The first two listed are BGP-specific; the third is shared by all routing processes on the router:
– Neighbor database: A list of all configured BGP neighbors. To view it, use the show ip bgp summary
– BGP database, or RIB (Routing Information Base): A list of networks known by BGP, along with their
paths and attributes. To view it, use the show ip bgp command.
– Routing table: A list of the paths to each network used by the router, and the next hop for each network. To view
it, use the show ip route command.
Packet Types/BGP Message Types EIGRP uses 5 packet types:
Hello: Identifies neighbors and serves as a keepalive mechanism sent multicast
Update: Reliably sends route information unicast to a specific router
Query: Reliably requests specific route information query packet multicast to its neighbors
Reply: Reliably responds to a query replies are unicast
ACK: Acknowledgment
The 5 OSPF packet types follow:
Hello: Identifies neighbors and serves as a keepalive.
Link State Request (LSR): Request for a Link State Update (LSU). Contains the type of LSU requested and the
ID of the router requesting it.
Database Description (DBD): A summary of the LSDB, including the RID and sequence number of each LSA
in the LSDB.
Link State Update (LSU): Contains a full LSA entry. An LSA includes topology information; for example, the
RID of this router and the RID and cost to each neighbor. One LSU can contain multiple LSAs.
Link State Acknowledgment (LSAck): Acknowledges all other OSPF packets (except Hellos).
BGP has 4 types of messages:
Open: After a neighbor is configured, BGP sends an open message to try to establish peering with that neighbor.
Includes information such as autonomous system number, router ID, and hold time.
Update: Message used to transfer routing information between peers. Includes new routes, withdrawn routes, and
path attributes.
Keepalive: BGP peers exchange keepalive messages every 60 seconds by default. These keep the peering session
Notification: When a problem occurs that causes a router to end the BGP peering session, a notification message
is sent to the BGP neighbor and the connection is closed.
Neighbor Discovery and Route Exchange Neighbor Discovery and Route Exchange
Step 1. Router A sends out a hello.
Step 2. Router B sends back a hello and an update. The update contains routing information.
Step 3. Router A acknowledges the update.
Step 4. Router A sends its update.
Step 5. Router B acknowledges.
Establishing Neighbors and Exchanging Routes
Step 1. Down state: OSPF process not yet started, so no Hellos sent.
Step 2. Init state: Router sends Hello packets out all OSPF interfaces.
Step 3. Two-way state: Router receives a Hello from another router that contains its own router ID in the neighbor
list. All other required elements match, so routers can become neighbors.
Step 4. Exstart state: If routers become adjacent (exchange routes), they determine which one starts the
exchange process.
Step 5. Exchange state: Routers exchange DBDs listing the LSAs in their LSD by RID and sequence number.
Step 6. Loading state: Each router compares the DBD received to the contents of its LS database. It then sends a
LSR for missing or outdated LSAs. Each router responds to its neighbor’s LSR with a Link State Update.
Each LSU is acknowledged.
Step 7. Full state: The LSDB has been synchronized with the adjacent neighbor.
BGP Peering States
The command show ip bgp neighbors shows a list of peers and the status of their peering session. This status can
include the following states:
Idle: No peering; router is looking for neighbor. Idle (admin) means that the neighbor relationship has been
administratively shut down.
Connect: TCP handshake completed.
OpenSent, or Active: An open message was sent to try to establish the peering.
OpenConfirm: Router has received a reply to the open message.
Established: Routers have a BGP peering session. This is the desired state.
Metric (Calculation) Bandwidth+Delay Cost= 100 Mbps/Bandwidth IBGP – 0
Redistributed routes metric = IGP metric