The Bulk Block
A bulk block is appropriate to affix two or added about-face blocks in a campus network. Because
all cartage casual to and from all about-face blocks, server blocks, the WAN block, and the Internet
must cantankerous the bulk block, the bulk charge be as able and airy as possible. The bulk is the
basic foundation of the campus arrangement and carries abundant added cartage than any added block.
A arrangement bulk can use any technology (frame, cell, or packet) to carriage campus data. Some
network designs advance ATM in the core, while others are based on Ethernet. Because most
campus networks are now appliance Gigabit Ethernet as a bulk technology, Ethernet bulk blocks
will be advised at length. (Refer to Affiliate 6, “Trunking with ATM LANE,” for a discussion
of ATM LANE features.)
Recall that the administration band provides Band 3 functionality, and that the bulk is usually
designed with Band 2 devices. Therefore, alone IP subnets are acclimated to affix all
distribution and bulk switches. At atomic two subnets should be acclimated to accommodate resiliency and
load acclimation into the core, although a audible VLAN could be used. As VLANs end at the
distribution layer, they are baffled into the core.
The bulk block could abide of a audible Band 2 switch, demography in the two bombastic links from
the administration band switches. Due to the accent of the bulk block in a campus network,
two or added identical switches should be implemented in the bulk to accommodate redundancy. As
well, either Band 2 or Band 3 accessories can be acclimated in the core, depending on design
requirements. This affair will be discussed added in the afterward sections.
HSRP HSRP
To bulk layer
Access
layer
Distribution
layer
X
X
X
X
44 Affiliate 2: Campus Arrangement Architecture Models
The links amid layers should additionally be advised to backpack at atomic the bulk of cartage load
handled by the administration switches. The links amid bulk switches in the aforementioned bulk subnet
should be of acceptable admeasurement to backpack the accumulated bulk of cartage advancing into the bulk switch.
Consider the boilerplate articulation utilization, but acquiesce for approaching growth. An Ethernet bulk allows for
simple and scalable upgrades of consequence (Ethernet, Fast Ethernet, Fast EtherChannel,
Gigabit Ethernet, Gigabit EtherChannel, and so on).
Two basal bulk block designs are presented in the afterward sections, anniversary advised for the size
of a campus network:
• Burst core
• Bifold core
Collapsed Core
A burst bulk block is one area the bulk band of the bureaucracy is burst into the
distribution layer. Here, both administration and bulk functions are provided aural the same
switch devices. This bearings is usually begin in abate campus networks, area a separate
core band (and added amount or performance) is not warranted.
Figure 2-9 shows the basal burst bulk design. Although the administration and bulk layer
functions are performed in the aforementioned device, befitting these functions audible and properly
designed is important. Note additionally that the burst bulk is not an absolute architecture block,
but is chip into the administration band of the alone standalone about-face blocks.
In the burst bulk design, anniversary admission band about-face has a bombastic articulation to anniversary distribution/
core band switch. All Band 3 subnets present in the admission band abolish at the Band 3 ports
of the administration switches, as in the basal about-face block design. The distribution/core switches
are affiliated to anniversary added by one or added links, commutual a aisle to be acclimated during a
redundancy failover. Spanning Tree will accumulate one of the bombastic links to the admission layer
blocked to anticipate Band 2 bridging loops.
Figure 2-9 Burst Bulk Block Design
Switch block 2
Access
layer
Distribution/Core
layer
Switch block 1
Core links
Modular Arrangement Architecture 45
However, at Band 3, back-up is provided through HSRP for IP. The two distribution
switches will allotment a accepted absence aperture address, but alone one will be alive at any time.
In the accident of a distribution/core about-face failure, connectivity to the bulk will be maintained as
the bombastic Band 3 about-face takes over.
Why be anxious about the differences amid Band 2 and Band 3 redundancy? Although
the distribution/core switches accept Band 3 functionality, as in the case of MLS, understanding
how MLS works in switches like the Catalyst 5000 and 6000 families is useful. A Band 3
routing accommodation is aboriginal fabricated on a cartage breeze (either by an chip or alien router). The
switches accumulation this information, and Band 3 switching begins afterwards a aisle is bent by the
routing decision. Therefore, both Band 2 and Band 3 operations are still occurring on the
distribution/core switches, anniversary with altered back-up requirements. Affiliate 8 covers MLS
in added detail.
Dual Core
A bifold bulk connects two or added about-face blocks in a bombastic fashion. Although the collapsed
core can affix two about-face blocks with some redundancy, the bulk is not scalable back more
switch blocks are added. Figure 2-10 illustrates the bifold core. Apprehension that this bulk appears as
an absolute bore and is not alloyed into any added block or layer.
Normally, the bifold bulk is congenital with Band 2 switches to accommodate the simplest and best efficient
throughput. Architecture a bifold bulk with Band 3 is possible, as discussed in the area “Layer 3
Core” after in the chapter. The bifold bulk uses two identical switches to accommodate redundancy.
Redundant links affix the administration band allocation of anniversary about-face block to anniversary of the dual
core switches. Note the absence of any links amid the two bulk switches. In a Band 2 core,
the switches are not affiliated to abstain any bridging loops.
Figure 2-10 Bifold Arrangement Bulk Design
Switch block 1 About-face block 2
Core
block
Subnet A Subnet B
Access
Distribution
46 Affiliate 2: Campus Arrangement Architecture Models
In the bifold core, anniversary administration about-face has two equal-cost paths to the core, accouterment twice
the accessible bandwidth. Both paths abide alive because the administration band uses Band 3
devices that can administer equal-cost paths in acquisition tables. In fact, the Band 3 path
determination beyond the bulk occurs after any assurance on Spanning Tree at all. Designing
the bulk after links amid the bulk switches removes any achievability of loops and
eliminates the charge for Spanning Tree in the core. The acquisition agreement in use determines the
availability or accident of a adjoining Band 3 device. Therefore, if one bulk about-face fails, the
routing agreement will reroute cartage appliance an alternating aisle through the absolute bulk switch.
Core Admeasurement in a Campus Network
The bifold bulk is fabricated up of bombastic switches, and is belted and abandoned by Band 3
devices. Acquisition protocols actuate paths and advance the operation of the core. As with any
network, you charge pay some absorption to the all-embracing architecture of the routers and acquisition protocols
in the network. As acquisition protocols bear updates throughout the network, network
topologies ability be ability change. The admeasurement of the arrangement (the cardinal of routers) then
affects acquisition agreement performance, as updates are exchanged and arrangement aggregation takes
place.
While the arrangement apparent in Figure 2-10 ability attending baby with alone two about-face blocks of two
Layer 3 switches (route processors aural the administration band switches) each, ample campus
networks can accept abounding about-face blocks affiliated into the bulk block. Band 2 accessories are used
in the bulk with usually alone a audible VLAN or subnet beyond the core. Therefore, all route
processors affix into a audible advertisement area at the core. Anniversary avenue processor must
communicate with and accumulate advice about anniversary of its anon affiliated peers. Thus, most
routing protocols accept applied banned on the cardinal of associate routers that can be connected.
Because two equal-cost paths from anniversary administration about-face into the core, anniversary router forms two
peer relationships with every added router. Therefore, the absolute best cardinal of switch
blocks that can be accurate is bisected the cardinal of administration band routers. For example, if
routing protocols such as Open Shortest Aisle Aboriginal (OSPF) and Enhanced Interior Gateway
Routing Agreement (EIGRP) could abutment a best of 50 peers, alone 25 about-face blocks could
be affiliated into the core. Additionally accumulate in apperception that added types of campus modules (server
blocks, WAN blocks, mainframe blocks, and so forth) affix into the bulk and create
additional associate relationships.
One final bulk admeasurement appliance is accompanying to the acquisition protocols and their abutment of equalcost
paths. In the case of bifold bulk design, these paths charge advance to abandoned VLANs or subnets
if a acquisition agreement supports two equal-cost paths. In added words, anniversary aisle charge be connected
to a abstracted concrete bulk switch. Two equal-cost paths are acclimated in a bifold bulk architecture with two
Layer 2 switches. Likewise, a acquisition agreement that supports six equal-cost paths requires that
the six administration about-face links be affiliated to absolutely six Band 2 accessories in the core.
Although this bureaucracy sounds complicated, it gives six times the back-up and six times the
available bandwidth into the core.
Modular Arrangement Architecture 47
This leads to a final architecture point for the absolute bulk switch—scale the bulk about-face to bout the
incoming load. At a minimum, the bulk about-face charge be able to handle switching anniversary of its
incoming administration links at 100% capacity.
Core Scalability
As the cardinal of about-face blocks increases, the bulk block charge additionally be able of scaling
without redesign. Traditionally, hierarchical arrangement designs accept acclimated Band 2 switches at the
access layer, Band 3 accessories at the administration layer, and Band 2 switches at the core. This
design has been actual amount able and has provided high-performance connectivity between
switch blocks in the campus.
Network advance dictates added about-face blocks, which in about-face requires added administration switches
with bombastic paths into the core. The bulk charge again be scaled to abutment the back-up and
the added campus cartage load.
Providing bombastic paths from the administration switches into the bulk block allows the Layer
3 administration switches to analyze several equal-cost paths beyond the core. If the cardinal of core
switches charge be added for scalability, the cardinal of equal-cost paths can become
excessive—more than the acquisition protocols can handle.
Because the bulk block is formed with Band 2 switches, the Spanning-Tree Agreement is acclimated to
prevent bridging loops. Two architecture decisions can be fabricated in the core:
• Interconnect the bulk switches with bombastic links.
• Interconnect the bulk switches alone with administration accessories (not added bulk switches).
The aboriginal architecture accommodation creates the achievability of bridging loops by basic a triangle between
a administration about-face and two bulk switches. Therefore, Spanning Tree charge be enabled to
dismantle the loop. Blocking one of the bombastic distribution/core links finer removes
both back-up and amount balancing. Also, the Spanning Tree can be actual apathetic to converge
(more than 50 seconds) if the arrangement cartography changes. Ample portions of the bulk block can
become abandoned while the arrangement reconverges.
The added architecture accommodation allows Spanning Tree to abide to run aural the core, but loops
will never anatomy because the bulk switches are not affiliated together. The Band 3 accessories in the
distribution band will use the equal-cost paths into the bulk to the best advantage.
However, as about-face blocks are added, you charge booty affliction not to beat the cardinal of equalcost
paths that can be accurate by the acquisition protocols. As well, the about-face block routers will
appear to all sit on a audible subnet because they are affiliated to a Band 2 about-face area in
the core. As discussed earlier, the cardinal of routers that can be anon affiliated to aeon is
limited. One band-aid to this botheration is to breach up the bulk into assorted VLANs so that router
peering is bargain in anniversary VLAN.
48 Affiliate 2: Campus Arrangement Architecture Models
Layer 3 Core
Layer 3 switching can additionally be acclimated in the bulk to absolutely calibration the bulk block for ample campus
networks. This admission additionally overcomes the problems of apathetic convergence, amount balancing
limitations, and router analytical limitations. Figure 2-11 shows a arrangement architecture appliance a Layer
3 core. Apprehension that the arrangement anatomy is identical to one appliance a Band 2 bifold core. The main
difference is that the bulk accessories accomplish at Band 3. Additionally apprehension that the bulk switches can have
direct links to anniversary other. Because of Band 3 functionality, the absolute links do not appoint any
bridging loops.
Figure 2-11 Band 3 Bulk Design
With a Band 3 core, the aisle assurance intelligence occurs in both the administration and core
layers, acceptance the cardinal of bulk accessories to be added for scalability. Bombastic paths also
can be acclimated to interconnect the bulk switches after affair for Band 2 bridging loops.
Router analytical problems are additionally affected as the cardinal of routers affiliated to individual
subnets is reduced. Administration accessories are no best advised aeon with all other
distribution devices. Instead, a administration accessory aeon alone with a bulk about-face on anniversary link
into the core. This advantage becomes abnormally important in actual ample campus networks
involving added than 100 about-face blocks.
The capital apropos with implementing a Band 3 bulk block are amount and performance. The
Layer 3 accessories appropriate are added big-ticket than Band 2 devices. The Band 3 accessories also
need to accept switching latencies commensurable to their Band 2 counterparts. Appliance a Band 3 core
also adds added acquisition hops to cross-campus traffic. However, the added scalability
and adaptability of a Band 3 bulk far outweigh any slight achievement risks. In fact, with MLS
and CEF performed in hardware, the added aerial is negligible.
Switch block 1 About-face block 2
Layer 3
core block
Access
Distribution
Foundation Arbitrary 49
Foundation Summary
The Foundation Arbitrary is a accumulating of tables and abstracts that provides a acceptable review
of abounding key concepts in this chapter. For those of you already adequate with the capacity in
this chapter, this arbitrary could advice you anamnesis a few details. For those of you who aloof read
this chapter, this analysis should advice coalesce some key facts. For any of you accomplishing your final
preparation afore the exam, the afterward tables and abstracts will hopefully be a convenient
way to analysis the day afore the exam.
Table 2-6 Layers of Abstracts Communications
OSI Band Agreement Abstracts Unit Mechanism to Process PDU
7 (application)
6 (presentation)
5 (session)
4 (transport) TCP articulation TCP port
3 (network) Packet Router
2 (data link) Anatomy Switch/bridge
1 (physical)
Table 2-7 Types of Arrangement Services
Service Blazon Location of Service Extent of Cartage Flow
Local Aforementioned segment/VLAN as user Admission band only
Remote Altered segment/VLAN as user Admission to administration layers
Enterprise Central to all campus users Admission to administration to bulk layers
50 Affiliate 2: Campus Arrangement Architecture Models
Table 2-8 Comparison of Hierarchical Layers
Layer Attributes
Access High anchorage body to affix to end users
Low cost
Uplinks to college layers of the campus network
Layer 2 casework (traffic clarification and VLAN membership)
Distribution Aggregation of admission band devices
High Band 3 throughput—InterVLAN routing
Robust Band 3 functionality
Security
Media translation
Core Fast abstracts transport
No “expensive” Band 3 processing
Table 2-9 Catalyst Switches for the Admission Layer
Catalyst
Model
Max Port
Density Uplinks Max Backplane Added features
1900 24 10BaseT 2 100BaseX 1 Gbps Fast EtherChannel
2820 24 10BaseT 100BaseX/FDDI/ATM 1 Gbps Fast EtherChannel
2900 XL 48 10/100 2 100 or 1000BaseX
or ATM
24 Gbps QoS
3500 XL 48 10/100 2 1000BaseX 10 Gbps Stackable Gigabit
4000 96 10/100
36 1000BaseX
100 or 1000BaseX 24 Gbps Security, QoS
5000 396 10/100 Any 3.6 Gbps Modular, “any-to-any”
switching
Foundation Arbitrary 51
Table 2-10 Arbitrary of Catalyst About-face Products and Typical Hierarchical Band Applications
Campus Admeasurement Band Catalyst About-face Key Features
Any Admission 1900 <>
100FX uplinks
2820 <>
100FX/FDDI/ATM uplinks
2900XL <>
100 or GE uplinks
3500XL <>
GE uplinks
Stackable
4000 <>
GE uplinks
5000 > 100 users any media
Any blazon uplinks
Low price/port
Small Campus Administration 2926G <>
GE uplinks
MLS
2948G-L3 <>
GE uplinks
Layer 3 switching (CEF)
4908G-L3 8 GE admission devices
Layer 3 switching (CEF)
5000 High 100 and GE densities
Any media supported
MLS
6000 High 100 and GE densities
High performance
MLS
Scalable for approaching growth
Core Usually combined
with distribution
continues
52 Affiliate 2: Campus Arrangement Architecture Models
Medium Campus Administration 5000 High 100 and GE densities
Any media supported
MLS
6000 High 100 and GE densities
High performance
MLS
Scalable for approaching growth
Core 5000 High anchorage densities
Any media
6000 High GE densities
High performance
Security and QoS
Scalability
8500 High performance
100/GE/ATM/SONET
Security and QoS
Scalability
Large Campus Administration 6000 High 100 and GE densities
MLS
Security and QoS
High performance
Core 6000 High 100 and GE densities
Security and QoS
High performance
8500 100/GE/ATM/SONET
Non-blocking MLS (CEF)
High performance