Routing Protocol Selection Guide - IGRP, EIGRP, OSPF, IS-IS, BGP
Overview
The
purpose of routing protocols is to learn of available routes that exist
on the enterprise network, build routing tables and make routing
decisions. Some of the most common routing protocols include IGRP,
EIGRP, OSPF, IS-IS and BGP. There are two primary routing protocol types
although many different routing protocols defined with those two types.
Link state and distance vector protocols comprise the primary types.
Distance vector protocols advertise their routing table to all directly
connected neighbors at regular frequent intervals using a lot of
bandwidth and are slow to converge. When a route becomes unavailable,
all router tables must be updated with that new information. The problem
is with each router having to advertise that new information to its
neighbors, it takes a long time for all routers to have a current
accurate view of the network. Distance vector protocols use fixed length
subnet masks which aren't scalable. Link state protocols advertise
routing updates only when they occur which uses bandwidth more
effectively. Routers don't advertise the routing table which makes
convergence faster. The routing protocol will flood the network with
link state advertisements to all neighbor routers per area in an attempt
to converge the network with new route information. The incremental
change is all that is advertised to all routers as a multicast LSA
update. They use variable length subnet masks, which are scalable and
use addressing more efficiently.
Interior Gateway Routing Protocol (IGRP)
Interior
Gateway Routing Protocol is a distance vector routing protocol
developed by Cisco systems for routing multiple protocols across small
and medium sized Cisco networks. It is proprietary which requires that
you use Cisco routers. This contrasts with IP RIP and IPX RIP, which are
designed for multi-vendor networks. IGRP will route IP, IPX, Decnet and
AppleTalk which makes it very versatile for clients running many
different protocols. It is somewhat more scalable than RIP since it
supports a hop count of 100, only advertises every 90 seconds and uses a
composite of five different metrics to select a best path destination.
Note that since IGRP advertises less frequently, it uses less bandwidth
than RIP but converges much slower since it is 90 seconds before IGRP
routers are aware of network topology changes. IGRP does recognize
assignment of different autonomous systems and automatically summarizes
at network class boundaries. As well there is the option to load balance
traffic across equal or unequal metric cost paths.
Characteristics
- Distance Vector
- Routes IP, IPX, Decnet, Appletalk
- Routing Table Advertisements Every 90 Seconds
- Metric: Bandwidth, Delay, Reliability, Load, MTU Size
- Hop Count: 100
- Fixed Length Subnet Masks
- Summarization on Network Class Address
- Load Balancing Across 6 Equal or Unequal Cost Paths ( IOS 11.0 )
- Update Timer: 90 seconds
- Invalid Timer: 270 seconds
- Holddown Timer: 280 seconds
- Metric Calculation = destination path minimum BW * delay (usec)
- Split Horizon
Enhanced Interior Gateway Routing Protocol (EIGRP)
Enhanced
Interior Gateway Routing Protocol is a hybrid routing protocol
developed by Cisco systems for routing many protocols across an
enterprise Cisco network. It has characteristics of both distance vector
routing protocols and link state routing protocols. It is proprietary
which requires that you use Cisco routers. EIGRP will route the same
protocols that IGRP routes (IP, IPX, Decnet and Appletalk) and use the
same composite metrics as IGRP to select a best path destination. As
well there is the option to load balance traffic across equal or unequal
metric cost paths. Summarization is automatic at a network class
address however it can be configured to summarize at subnet boundaries
as well. Redistribution between IGRP and EIGRP is automatic as well.
There is support for a hop count of 255 and variable length subnet
masks.
Convergence
Convergence
with EIGRP is faster since it uses an algorithm called dual update
algorithm or DUAL, which is run when a router detects that a particular
route is unavailable. The router queries its neighbors looking for a
feasible successor. That is defined as a neighbor with a least cost
route to a particular destination that doesn't cause any routing loops.
EIGRP will update its routing table with the new route and the
associated metric. Route changes are advertised only to affected routers
when changes occur. That utilizes bandwidth more efficiently than
distance vector routing protocols.
Autonomous Systems
EIGRP
does recognize assignment of different autonomous systems which are
processes running under the same administrative routing domain.
Assigning different autonomous system numbers isn't for defining a
backbone such as with OSPF. With IGRP and EIGRP it is used to change
route redistribution, filtering and summarization points.
Characteristics
- Advanced Distance Vector
- Routes IP, IPX, Decnet, Appletalk
- Routing Advertisements: Partial When Route Changes Occur
- Metrics: Bandwidth, Delay, Reliability, Load, MTU Size
- Hop Count: 255
- Variable Length Subnet Masks
- Summarization on Network Class Address or Subnet Boundary
- Load Balancing Across 6 Equal or Unequal Cost Paths (IOS 11.0)
- Hello Timer: 5 seconds on Ethernet / 60 seconds on Non-Broadcast
- Holddown Timer: 15 seconds on Ethernet / 180 seconds on Non-Broadcast
- Metric Calculation = destination path minimum BW * delay (msec) * 256
- Split Horizon
- LSA Multicast Address: 224.0.0.10
Open Shortest Path First (OSPF)
Open
Shortest Path First is a true link state protocol developed as an open
standard for routing IP across large multi-vendor networks. A link state
protocol will send link state advertisements to all connected neighbors
of the same area to communicate route information. Each OSPF enabled
router, when started, will send hello packets to all directly connected
OSPF routers. The hello packets contain information such as router
timers, router ID and subnet mask. If the routers agree on the
information they become OSPF neighbors. Once routers become neighbors
they establish adjacencies by exchanging link state databases. Routers
on point-to-point and point-to-multipoint links (as specified with the
OSPF interface type setting) automatically establish adjacencies.
Routers with OSPF interfaces configured as broadcast (Ethernet) and NBMA
(Frame Relay) will use a designated router that establishes those
adjacencies.
Areas
OSPF
uses a hierarchy with assigned areas that connect to a core backbone of
routers. Each area is defined by one or more routers that have
established adjacencies. OSPF has defined backbone area 0, stub areas,
not-so-stubby areas and totally stubby areas. Area 0 is built with a
group of routers connected at a designated office or by WAN links across
several offices. It is preferable to have all area 0 routers connected
with a full mesh using an Ethernet segment at a core office. This
provides for high performance and prevents partitioning of the area
should a router connection fail. Area 0 is a transit area for all
traffic from attached areas. Any inter-area traffic must route through
area 0 first. Stub areas use a default route injected from the ABR to
forward traffic destined for any external routes (LSA 5,7) to the area
border router. Inter-area (LSA 3,4) and intra-area (LSA 1,2) routing is
as usual. Totally Stubby areas are a Cisco specification that uses a
default route injected from the ABR for all Inter-area and external
routes. The Totally Stubby area doesn't advertise or receive external or
Inter-area LSA's. The Not-So-Stubby area ABR is a transit area that
will import external routes with type 7 LSA and flood them to other
areas as type 5 LSA. External routes aren't received at that area type.
Inter-area and intra-area routing is as usual. OSPF defines internal
routers, backbone routers, area border routers (ABR) and autonomous
system boundary routers (ASBR). Internal routers are specific to one
area. Area border routers have interfaces that are assigned to more than
one area such as area 0 and area 10. An autonomous system boundary
router has interfaces assigned to OSPF and a different routing protocol
such as EIGRP or BGP. A virtual link is utilized when an area doesn't
have a direct connection to area 0. A virtual link is established
between an area border router for an area that isn't connected to area
0, and an area border router for an area that is connected to area 0.
Area design involves considering geographical location of offices and
traffic flows across the enterprise. It is important to be able to
summarize addresses for many offices per area and minimize broadcast
traffic.
Convergence
Fast
convergence is accomplished with the SPF (Dijkstra) algorithm which
determines a shortest path from source to destination. The routing table
is built from running SPF which determines all routes from neighbor
routers. Since each OSPF router has a copy of the topology database and
routing table for its particular area, any route changes are detected
faster than with distance vector protocols and alternate routes are
determined.
Designated Router
Broadcast
networks such as Ethernet and Non-Broadcast Multi Access networks such
as Frame Relay have a designated router (DR) and a backup designated
router (BDR) that are elected. Designated routers establish adjacencies
with all routers on that network segment. This is to reduce broadcasts
from all routers sending regular hello packets to its neighbors. The DR
sends multicast packets to all routers that it has established
adjacencies with. If the DR fails, it is the BDR that sends multicasts
to specific routers. Each router is assigned a router ID, which is the
highest assigned IP address on a working interface. OSPF uses the router
ID (RID) for all routing processes.
Characteristics
- Link State
- Routes IP
- Routing Advertisements: Partial When Route Changes Occur
- Metric: Composite Cost of each router to Destination (100,000,000/interface speed)
- Hop Count: None (Limited by Network)
- Variable Length Subnet Masks
- Summarization on Network Class Address or Subnet Boundary
- Load Balancing Across 4 Equal Cost Paths
- Router Types: Internal, Backbone, ABR, ASBR
- Area Types: Backbone, Stubby, Not-So-Stubby, Totally Stubby
- LSA Types: Intra-Area (1,2) Inter-Area (3,4), External (5,7)
- Hello Timer Interval: (10 seconds for Ethernet / 30 seconds for Non-Broadcast)
- Dead Timer Interval: 40 seconds for Ethernet / 120 seconds for Non-Broadcast)
- LSA Multicast Address: 224.0.0.5 and 224.0.0.6 (DR/BDR) Don't Filter!
- Interface Types: Point to Point, Broadcast, Non-Broadcast, Point to Multipoint, Loopback
Integrated IS-IS
Integrated
Intermediate System - Intermediate System routing protocol is a link
state protocol similar to OSPF that is used with large enterprise and
ISP customers. An intermediate system is a router and IS-IS is the
routing protocol that routes packets between intermediate systems. IS-IS
utilizes a link state database and runs the SPF Dijkstra algorithm to
select shortest paths routes. Neighbor routers on point to point and
point to multipoint links establish adjacencies by sending hello packets
and exchanging link state databases. IS-IS routers on broadcast and
NBMA networks select a designated router that establishes adjacencies
with all neighbor routers on that network. The designated router and
each neighbor router will establish an adjacency with all neighbor
routers by multicasting link state advertisements to the network itself.
That is different from OSPF, which establishes adjacencies between the
DR and each neighbor router only. IS-IS uses a hierarchical area
structure with level 1 and level 2 router types. Level 1 routers are
similar to OSPF intra-area routers, which have no direct connections
outside of its area. Level 2 routers comprise the backbone area which
connects different areas similar to OSPF area 0. With IS-IS a router can
be an L1/L2 router which is like an OSPF area border router (ABR) which
has connections with its area and the backbone area. The difference
with IS-IS is that the links between routers comprise the area borders
and not the router. Each IS-IS router must have an assigned address that
is unique for that routing domain. An address format is used which is
comprised of an area ID and a system ID. The area ID is the assigned
area number and the system ID is a MAC address from one of the router
interfaces. There is support for variable length subnet masks, which is
standard with all link state protocols. Note that IS-IS assigns the
routing process to an interface instead of a network.
Characteristics
- Link State
- Routes IP, CLNS
- Routing Advertisements: Partial When Routing Changes Occur
- Metric: Variable Cost (default cost 10 assigned to each interface)
- Hop Count: None (limited by network)
- Variable Length Subnet Masks
- Summarization on Network Class Address or Subnet Boundary
- Load Balancing Across 6 Equal Cost Paths
- Hello Timer Interval: 10 seconds
- Dead Timer Interval: 30 seconds
- Area Types: Hierarchical Topology similar to OSPF
- Router Types: Level 1 and Level 2
- LSP Types: Internal L1 and L2, External L2
- Designated Router Election, No BDR
Border Gateway Protocol (BGP)
Border
Gateway Protocol is an exterior gateway protocol, which is different
from the interior gateway protocols discussed so far. The distinction is
important since the term autonomous system is used somewhat differently
with protocols such as EIGRP than it is with BGP. Exterior gateway
protocols such as BGP route between autonomous systems, which are
assigned a particular AS number. AS numbers can be assigned to an office
with one or several BGP routers. The BGP routing table is comprised of
destination IP addresses, an associated AS-Path to reach that
destination and a next hop router address. The AS-Path is a collection
of AS numbers that represent each office involved with routing packets.
Contrast that with EIGRP, which uses autonomous systems as well. The
difference is their autonomous systems refer to a logical grouping of
routers within the same administrative system. An EIGRP network can
configure many autonomous systems. They are all managed by the company
for defining route summarization, redistribution and filtering. BGP is
utilized a lot by Internet Service Providers (ISP) and large enterprise
companies that have dual homed internet connections with single or dual
routers homed to the same or different Internet Service Providers. BGP
will route packets across an ISP network, which is a separate routing
domain that is managed by them. The ISP has its own assigned AS number,
which is assigned by InterNIC. New customers can either request an AS
assignment for their office from the ISP or InterNIC. A unique AS number
assignment is required for customers when they connect using BGP. There
are 10 defined attributes that have a particular order or sequence,
which BGP utilizes as metrics to determine the best path to a
destination. Companies with only one circuit connection to an ISP will
implement a default route at their router, which forwards any packets
that are destined for an external network. BGP routers will redistribute
routing information (peering) with all IGP routers on the network
(EIGRP, RIP, OSPF etc) which involve exchange of full routing tables.
Once that is finished, incremental updates are sent with topology
changes. The BGP default keepalive timer is 60 seconds while the
holddown timer is 180 seconds. Each BGP router can be configured to
filter routing broadcasts with route maps instead of sending/receiving
the entire internet routing table.
Characteristics
- Path Vector
- Routes IP
- Routing Advertisements: Partial When Route Changes Occur
- Metrics: Weight, Local Preference, Local Originated, As Path, Origin Type, MED
- Hop Count: 255
- Variable Length Subnet Masks
- Summarization on Network Class Address or Subnet Boundary
- Load Balancing Across 6 Equal Cost Paths
- Keepalive Timer: 60 seconds
- Holddown Timer: 180 seconds
- Designated Router: Route Reflector
BGP Routing Table Components
- Destination IP Address / Subnet Mask
- AS-Path
- Next Hop IP Address
Copyright
© 2013 Shaun L. Hummel All Rights Reserved
https://supportforums.cisco.com/docs/DOC-30205
Red Hat, Inc.
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