Overview of Multilayer Switching
Catalyst switches are the base for Layer 3 switching in the Cisco environment. Multilayer
Switching (MLS) performs IP abstracts (also IPX and IP multicast) packet flows at a abundant higher
level of achievement than acceptable routing. This preserves the CPU of an upstream router
without compromising functionality. Figure 8-2 shows that the aboriginal packet through enters and
exits the router illustrated. Consecutive packets would be switched.
Figure 8-2 Multilayer Switching Flow: Aboriginal Packet Through
Strictly defined, a breeze is a specific conversation, consisting of assorted packets, amid a
network antecedent and destination aural a specific time sequence. Let’s booty a user that is pulling
down a web folio from a specific web server. This archetype would be one flow. The aforementioned user
could be assuming a Book Alteration Agreement (FTP) book alteration at the aforementioned time from an FTP
server. This archetype would be a absolutely altered flow. Two altered applications—two
different protocols—two altered flows; however, alone one host is assuming two flows. In
terms of flows, there is no acumen amid unicasts or multicasts.
MLS was conceived in an accomplishment to admission the achievement of a router by accumulation the
functionality in accouterments with a switch. The anatomy forwarding and the carbon activity is
moved to accouterments and again Layer 3 switching takes over the assignment aforetime done by the router.
MLS should not be abashed with NetFlow switching accurate by Cisco routers. MLS uses
the Avenue About-face Module (RSM), a anon absorbed alien router, and the engine. With MLS,
First packet through in an MLS environment
The aboriginal packet goes through the router and is routed.
Subsequent packets are switched after hitting the router.
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you are not adapted to use NetFlow switching on the RSM or anon absorbed alien router;
any switching aisle on the RSM or anon absorbed alien router will work.
MLS can be implemented by application a Layer 3 about-face or an alien router topology. The Layer 3
switch contains an RSM and the NetFlow Feature Agenda (NFFC). MLS requires the following
software and hardware:
• Catalyst 2926G, 5000, or 6000 alternation about-face with Supervisor Agent software
Release 4.1(1) or later.
• Cisco IOS Release 11.3(2)WA4(4) or later.
• Supervisor Agent III or III F with the NFFC II, or Supervisor Agent II G or III G.
• Avenue About-face Feature Agenda (RSFC).
• Multilayer About-face Feature Agenda (MSFC).
MLS is additionally accurate on the afterward software and hardware:
• Catalyst 5000 alternation about-face with Supervisor Agent software Release 4.1(1) or later.
• Cisco IOS Release 12.0W5 or later.
• Supervisor Agent IIG or IIIG with an RSFC babe card.
You can additionally apparatus MLS with an alien router and Catalyst about-face combination. The
following accessories is all-important aback implementing MLS with an alien router and
Catalyst about-face combination:
• Catalyst 2926G, 5000, or 6000 alternation about-face with Supervisor Agent software
Release 4.1(1) or later.
• Supervisor Agent III or III F with the NFFC II, or Supervisor Agent II G or III G.
• Cisco high-end routers, such as Cisco 7500, 7200, 4500, 4700, or 8500 series.
• Cisco IOS Release 11.3(2)WA4(4) or later.
The affiliation amid the alien router and the about-face can be assorted Ethernet links or
Fast Ethernet with the Inter-Switch Link (ISL), 802.1Q, or ATM LANE.
Multilayer Switching Components
The Cisco MLS accomplishing includes the afterward components:
• Multilayer Switching About-face Agent (MLS-SE)—The switching article that handles the
function of affective and afterlight the packets. The MLS-SE is an NFFC residing on a
Supervisor Agent III agenda in a Catalyst switch. It can additionally be a Supervisor I and the PFC
on the 6000 series.
Overview of Multilayer Switching 271
• Multilayer Switching Avenue Processor (MLS-RP)—An RSM, RSFC, MSFC, or an
externally affiliated Cisco 7500, 7200, 4500, 4700, or 8500 alternation router with software
that supports multilayer switching. The MLS-RP sends MLS agreement information
and updates, such as the router Media Admission Control (MAC) address, basal LAN
(VLAN) cardinal breeze mask, and acquisition and admission account changes.
• Multilayer Switching Agreement (MLSP)—This agreement operates amid the MLS-SE
and MLS-RP to accredit multilayer switching. MLSP is the adjustment in which the RSM or
router advertises acquisition changes and the VLANs or MAC addresses of the interfaces that
are accommodating in MLS.
MLS-RP Advertisements
As anon as an MLS-RP is enabled in the campus network, MLS-RP advertisements begin. The
MLS-RP sends out multicast Hello letters every 15 abnormal to all switches in the network.
The advertisement bulletin consists of the following:
• The MAC addresses acclimated by the MLS-RP on its interfaces that are accommodating in MLS.
• Admission account information.
• Additions and deletions of routes.
MLSP uses the Cisco Group Management Agreement (CGMP) multicast abode as the
destination abode of the Hello message. This abode ensures interoperability with the Cisco
switches in the network. Although this abode is the aforementioned as that acclimated by CGMP, the message
contains a altered agreement blazon so the about-face can analyze these letters from other
multicast packets.
Hello Messages
All switches in the arrangement accept the Hello message. Alone Layer 3 switches absolutely process
the message. Any switches that are not Layer 3 able artlessly canyon the frames through to any
downstream switches.
When an MLS-SE receives the frame, the accessory extracts all the MAC addresses accustomed in the
frame, forth with the associated interface or VLAN ID for that address. The MLS-SE records
the addresses of the MLS-RPs in the MLS-SE content-addressable anamnesis (CAM) table.
XTAGs
XTAGs are assigned by the MLS-SE to anniversary and every MLS-RP absorbed to a switch. The
XTAG is a one-byte amount absorbed to the MAC abode of anniversary absorbed MLS-RP. These values
are active in adapted amid MLS-RPs aback there are added than one MLS-RP
available.
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The XTAG is advantageous for deleting a specific set of Layer 3 entries from the Layer 3 table when
an MLS-RP fails or exits the network.
MLS Caching
MLS caching is a activity that occurs based on alone flows. In this section, we will walk
through the process, footfall by step, in adjustment to accretion an affectionate ability of aloof what occurs.
The Switching Agent (SE) is complex in the activity to advance the accumulation for MLS flows.
Packets in a breeze are compared to the cache.
Cache entries are based on one-way flows. In added words, a breeze from Host A to Host B would
be one breeze and a breeze in the about-face administration would be addition flow. This activity would yield
two accumulation entries.
Here’s the allotment of the blueprint that yields the payoff. In the accident that the accumulation has an entry
that is a bout for the packet, the SE switches the packet instead of casual it to the router. If it
does not bout an access in the cache, a activity occurs that goes on to accomplish an access in the
cache. This abstraction is illustrated in Figure 8-3.
Figure 8-3 MLS Cache
Host A sends a anatomy to Host B. If there is a bout in the MLS cache, the packet would never
go to the router but artlessly be switched application the arrangement that follows.
Step 1 The about-face receives an admission anatomy and looks at the
destination MAC abode in the frame.
0010.0679.5800
172.16.68.13
MLS-RP MLS Cache
0090.b133.7000
172.16.22.57
0010.f663.d000
172.16.10.123
Host A Host B
Overview of Multilayer Switching 273
Step 2 The about-face recognizes the destination MAC abode of the frame
as the abode of the MLS-RP because the about-face initially received
this destination MAC abode in a Layer 3 Hello bulletin and
programmed that destination MAC abode in the CAM table.
Step 3 The MLS-SE again checks the MLS accumulation to actuate if an MLS
flow is already accustomed for this flow. If the anatomy is the aboriginal in
a flow, there will not be an access in the cache. Because the frame
contained a avenue processor destination address, the switch
recognizes the abeyant for Layer 3 switching for that frame.
Step 4 On the antecedent packet, the about-face does not accept all the information
for a Layer 3 about-face for the frame. The switch, therefore, forwards
the anatomy to the addressed avenue processor. This activity of
sending the anatomy to the addressed avenue processor creates a
“candidate” access in the MLS cache.
Step 5 The avenue processor receives the anatomy and consults the routing
table to actuate if, in fact, the avenue processor has ability of
a avenue for the destination address.
Step 6 If the avenue processor finds the destination abode in the routing
table, the avenue processor constructs a new Layer 2 header, which
now contains the avenue processor’s own MAC abode as the
source MAC address.
The avenue processor additionally enters the MAC abode of the
destination host or next-hop avenue processor in the destination
MAC abode acreage of the Layer 2 frame.
Step 7 The avenue processor again assiduously the anatomy aback to the MLS-SE.
When the about-face receives the frame, the about-face knows which anchorage needs to advanced the frame,
based on the CAM table (displayed in Archetype 8-1). Moreover, the about-face additionally recognizes the
MAC abode in the antecedent acreage and knows that that this destination belongs to the route
processor.
Example 8-1 Displaying the CAM Table
Console> (enable) appearance cam 00-10-29-8a-4c-00
* = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
VLAN Dest MAC/Route Des Destination Ports or VCs / [Protocol Type]
---- ------------------ ----------------------------------------------------
10 00-10-29-8a-4c-00R 9/1 IP
51 00-10-29-8a-4c-00R 9/1 IP
52 00-10-29-8a-4c-00R 9/1 IP
53 00-10-29-8a-4c-00# 9/1 IP
54 00-10-29-8a-4c-00# 9/1 IP
Total Analogous CAM Entries Displayed = 5
Console> (enable)
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This acceptance triggers the activity of blockage the MLS accumulation to see if there is an access for
this avenue processor. The about-face compares the XTAGs for both the applicant access in the MLS
cache and the alternate frame. If the two XTAGs match, the anatomy came from the aforementioned route
processor for the aforementioned flow.
The about-face annal the advice from the alternate anatomy in the MLS cache. The switch
forwards the anatomy out the adapted anchorage application the destination MAC address. This second
frame becomes the “enable” access in MLS accumulation and the fractional access for that breeze is completed.
Remembering that the MLS-SE charge see both abandon of the breeze activity from the antecedent to the
destination in adjustment to accomplish Layer 3 switching is important. In added words, you can’t do
Layer 3 switching by aloof alive the antecedent or destination.
When the about-face receives consecutive packets in the flow, the about-face recognizes that the frames
contain the MAC abode of the avenue processor. The about-face checks the MLS accumulation and finds
the access analogous the breeze in question.
The about-face rewrites the Layer 2 anatomy header, alteration the destination MAC abode to the
MAC abode of Host B and the antecedent MAC abode to the MAC abode of the MLS-RP. The
Layer 3 IP addresses abide the same, but the IP attack Time to Live (TTL) is decremented
and the checksum is recomputed. The MLS-SE rewrites the switched Layer 3 packets so that
they arise to accept been baffled by a avenue processor.
The about-face rewrites the anatomy to attending absolutely as if the avenue processor candy the frame. The
final destination sees the anatomy absolutely as if the router candy the frame.
After the MLS-SE performs the packet rewrite, the about-face assiduously the rewritten anatomy to the
destination MAC address.
The accompaniment and character of the breeze are maintained while cartage is active; aback cartage for a flow
ceases, the access ages out. Partial, or candidate, entries will abide in the accumulation for bristles seconds
with no enabled access afore timing out. Accumulation entries that are complete, area the switch
captures both the applicant and the enabling packet, will abide in the accumulation as continued as packets
in that breeze are detected.
Disabling MLS
Actually the appellation of this area should read, “What not to do if you appetite your MLS to keep
running.” Believe it or not, there are a few commands that, if entered, will accept the undesirable
effect of disabling MLS.
The basal guideline to chase is that if you access any command that armament the router to examine
the packet, MLS will be disabled. That includes a accomplished host of commands, but I anticipation I’d
list a few of the best accepted here:
• ip tcp header-compression
• no ip routing
• ip security