indosat mtu refresher ed01 26012012

All Rights Reserved Alcatel-Lucent 2006, #####

Alcatel-Lucent MTU Calculation

A Refresher

1

2All Rights Reserved Alcatel-Lucent 2006, #####

Types of MTU on the 7X50

Port MTU: This MTU governs how large packets can be that fit on a given physical wire.

Service MTU: This MTU is associated with the service and governs how large packets can be that are sent from the customer across the service.

SDP Path MTU: This is the MTU of the SDP between the service endpoints. This MTU governs how large packets can be that are sent over the SDP.

VC-MTU: MTU negotiated by T-LDP and is maximum IP payload size that can be carried inside the tunnel. Derived from Service MTU.

IP-MTU: Can be used in IES and VPRN interfaces for spoke-SDP terminations. Should be equal to the VC-MTU of the spoked Epipe/VPLS.

LSP Path MTU: MTU value that is negotioated by RSVP path/resvmessages. The value is derived from the Port MTU of the network port, special case: ADSPEC. Not covered here.

OSPF MTU: MTU size negotiated by OSPF, derived from Port MTU or administratively set. Not covered here.

First 4 are important to get Services in Operational state UP

There is an MTU associated with all of the data links on the 7X50. This includes both

physical circuits and pseudo wires. Pseudo wires can be built with either MPLS or GRE

on the 7X50. They are used to make the epipe and VPLS services work.

Service MTUs have to match in order for spoke or mesh sdps to come up operationally

even if sdp MTUs are matching. Pay special attention in the case of spoke-sdp to an IES

service from a VPLS service.

To alleviate headaches and give room for future growth, a service provider should select

a large backbone MTU and set it on all links internal to their backbone. The selected

MTU needs to be supported on all platforms in the backbone. The POS MTU of 4470 is a

suggested minimum. A better value would be a nice large round number such as 9000.

9000 is supported by most vendors, and would allow the offering of a premium jumbo-

frame service to those customers who want it of 8000 or more bytes.

If your network supports it 9100 or 9200 would also be good values, since they would

probably allow you to offer a jumbo frame service up to 9000 bytes.

LSP MTU is derived from the network port MTU: in case of ethernet null encapsulation

the LSP MTU = port MTU 14. Cannot be configured. ADSPEC can be set in which case

LSP MTU is adapted to weakest link (lowest MTU) in the path.

OSPF MTU can be set under the OSPF interface configuration


L2 Services Visualizing MTUs (Ethernet example)

Network

Interface

-

Port

MTU

SDP

Path MTU Service MTU

Access

Interface

-

Port

MTU

payload FCS802.1q

Optional (Null/dot1Q/QinQ)(will be stripped)

payloadDA SA FCS

VC-MTU

Service MTU

Ether

Type

DA SAEtherType

Access Port MTU

payloadDA SA FCSEtherType

SAEtherType

Path Label

VCLabel

SDP Path MTU

Network Port MTU

No fragmentation possible

(*) DA

The Physical MTU on an Ethernet access interface needs to be set to at least:

1514 with mode-access and encap-type null

1500 + 14 DLC header

1518 with mode-access and encap-type dot1q

1500 + 14 DLC header + 4 dot1q tag

1522 with mode-access and encap-type qinq

1500 + 14 DLC + 4 (first q-tag) + 4 (second q-tag)

(*) REMARK: If the network interface is dot1q encapsulated, an extra 4 bytes are needed

for the q-tag


Access Port MTU/Service MTU on the 7X50

Access Port MTU:

should be equal or larger than Service MTU + port encapsulation overhead (0 for Null, 4 Dot1q,

8 QinQ Ethernet example)

configure port x/x/x ethernet mtu [512..9212] bytes

default 1514 for Null, 1518 for Dot1Q, 1522 for QinQ

show port

Service MTU:

This is the value to start from: it is the max payload (IP + Ethernet) the service should be able to

offer to the client (FCS not calcualetd)

configure service [vpls|[e|a|i|f]pipe] service-mtu [1..9194] bytes

default 1514

show service id x base

Remark: not used in L3 services

Remarks:

Access Port MTU:

For the Service to get to Operational state UP, at least 1 SAP is needed that is in

Operational State UP

If the Access port MTU < (Service MTU Encapsulation overhead), the access

port will be in Operational DOWN status

Service MTU:

if Service MTU might not be configured, default value is 1514 (see info detail)


SDP Path/VC-MTU on the 7X50

VC-MTU:

derived from the Service MTU (Service MTU 14 Ethernet)

This is the MTU value that will be negotiated by T-LDP and should match the other side

show router ldp bindings (service-id )

SDP Path MTU: MTU the SDP can carry

by default not configured and is derived from Network Port MTU

should not necessarily match on both sides of the SDP (*)

should be equal or larger than service MTU

should be equal or smaller than (Network Port MTU 2 labels ethernet header)(*)

configure service sdp x path-mtu [576..9194] bytes

show service id x base

(*)REMARK: Its possible to configure the SDP Path MTU to a higher value than the one derived

from the Network Port MTU and service can get to Operational UP status

Remark: VC-MTUs should match. SDP Path-MTU not necessarily: you can configure a

service with an SDP path MTU of 2000 on one end and an SDP path MTU of 3000 at the

other end: as long as the VC-MTUs match its OK.

(*) Remark: this is the case when no SDP keep-alives (default).


Layer 2 - Minimum Physical MTU on Network Interfaces (MPLS)

If a router needs to support services offering a 1514 byte service payload over

MPLS:

POS

1514

4

4

(4)

2

n/a

1524 (1528)

Ethernet

1514

4

4

(4)

n/a

14

1536 (1540)

Overhead

Service Payload

MPLS tag used as service ID

MPLS tag used for egress LSP

MPLS tag (if FRR bypass is used)

PPP MPLSCP Header

DLC Header

Total

The Physical MTU on the network interfaces must be greater than the maximum service

MTU to be supported + largest encapsulation type used.

Similarly to the Network MTU, the Path MTU must be sufficient to support all services

which that path has to carry. The Path MTU is determined by the lowest of the MTU

along the path the service needs to transit.

For GRE tunnels, this value is set by the administrator, and it is assumed that reality

matches the config. To determine the minimum value supported the 7X50 supports the

oam sdp-mtu command.

For Signaled MPLS tunnels, the Path MTU is determined by the signaling exchange

(RSVP).

Remark: a 4th label can be used in case OAM VCCV is running over a bypass: a router-

alert table is inserted between the Service and the Transport Label


Minimum Physical MTU on Network Interfaces (GRE)

If that router also had to support a similar service over GRE:

POS

1514

4

8

20

2

n/a

1548

Ethernet

1514

4

8

20

n/a

14

1560

Overhead

Service Payload

MPLS tag used as service ID

GRE Header

IP Header

PPP IPCP Header

DLC Header

Total

Service MTU should be sufficient to support the service in question. For a service

running over a typical Ethernet access interface, this will be 1514 bytes. This comprises

the 1500 byte payload and the 14 byte DLC header. This is the same even if the access

interface is dot1q. This is because the dot1q header is stripped at ingress, and replaced

with the appropriate vlan tag at egress.

config>service>[epipe|vpls]> service-mtu [0..9194]


Show Service id X all

This command can be used to verify MTU

SR12# show service id 5500 all=============================================================================Service Detailed Information=============================================================================Service Id : 5500 Vpn Id : 5500Service Type : EpipeDescription : Distributed epipe service to east coastCustomer Id : 5 Last Mgmt Change : 07/14/2003 03:26:46Adm : Up Oper : DownMTU : 1514SAP Count : 1 SDP Bind Count : 1-----------------------------------------------------------------------------Service Destination Points(SDPs)-----------------------------------------------------------------------------Sdp Id 2:123 -(10.10.10.104)-----------------------------------------------------------------------------Description : GRE-10.10.10.104SDP Id : 2:123 Type : SpokeAdmin Path MTU : 4462 Oper Path MTU : 4462Far End : 10.10.10.104 Delivery : GREAdmin State : Up Oper State : TLDP DownIngress Label : 6600 Egress Label : 5500Ingress Filter : n/a Egress Filter : n/aLast Changed : 07/14/2003 03:29:21 Signaling : TLDP

The show service command allows the administrator to check the MTU that is configured and the

actual MTU that will be supported across the entire network connection. The above output is not

complete. Additional information is available including segment by segment MTU capabilities.

Syntax all

Context show>service>id service-id

Description Displays detailed information for all aspects of the service.

Admin Path MTU

The desired largest service frame size (in octets) that can be transmitted through this SDP to the

far-end router, without requiring the packet to be fragmented.

Oper Path MTU

The actual largest service frame size (in octets) that can be transmitted through this SDP to the

far-end router, without requiring the packet to be fragmented.


MTU Configuration Example

SAP 1/1/1Encap: 100

SAP 4/1/1Encap: Null

Network2/1/1

Network3/1/1

MTU Configuration Example ValuesALA-A ALA-BAccess (SAP) Network Network Access (SAP)

Port (slot/MDA/port) 1/1/1 2/1/1 3/1/1 4/1/1Mode type dot1q network network nullMTU 1518 1536 1536 1514

Configuration Example

In order for the maximum length service frame to successfully travel from a local ingress SAP to a

remote egress SAP, the MTU values configured on the local ingress SAP, the SDP (MPLS), and

the egress SAP must be coordinated to accept the maximum frame size the service can forward.

For example, the targeted MTU values to configure for a distributed Epipe service (ALA-A and

ALA-B) are displayed in the picture above.

Since ALA-A uses dot1q encapsulation, the SAP MTU must be set to 1518 to be able to accept a

1514 byte service frame (done by default/see Table 11 for MTU default values). Each SDP MTU

must be at least 1514 as well. If ALA-As network port (2/1/1) is configured as an Ethernet port

with a MPLS SDP running over it, the MTU value of network ports 2/1/1 and 3/1/1 must each be at

least 1536 bytes (1514 MTU + 8 bytes labels (2) + 14 Ethernet). In case FRR bypass is

configured, 1540 should be used. Finally, the MTU of ALAs SAP (access port 4/1/1) must be at

least 1514, as it uses null encap.

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L3 Services Visualizing MTUs

Network

Interface

-

Port

MTU

SDP


IES/VPRN

Access

Interface

-

Port

MTU

payload FCS802.1q


payload

VC-MTU

DA SAEtherType

Access Port MTU

payload FCSDA SAEther

Type

Path

Label

VC

Label

Network Port MTU

Fragmentation possible (see next slides)

IP-MTU

Note:

-Theres no Service MTU concept IES/VPRN services

-Path MTU is not taken into consideration (but should be set to an appropriate value if

sdp-keepalives, sdp keepalive message-length is not configured and intermediate

transmission gear has MTU restrictions)

(*) Note: fragmentation on IP-MTU takes place in the direction from the

VPRN/IES interface towards the CE.

If the IP interface has a SAP defined, the operational MTU is the smaller of the

SAP MTU (port MTU normalized for the SAP encap) and the administrative IP-

MTU (normalized to the SAP encap) to set the IP interface operational IP-MTU. If

the IP interface has a spoke-SDP defined,the operational MTU is the smaller of

the SDP Path-MTU and the administrative IP-MTU (normalized to the spoke

encap) to set the operational IP-MTU for the IP interface.

11


VPRN Fragmentation toward network side

CE1 10.10.10.110.10.10.3

192.1.1.2

CE2PE1

PE3

1/1/1 1/1/11/1/1 1/1/11/1/31/1/4

192.1.2.2192.1.1.1 192.1.2.1

VPRN service

auto-bind ldp or gre

Port MTU 9212 Port MTU 9212



ce1# ping 192.1.2.2 source 192.1.1.2 do-not-fragment size x

VPRN fragmenting toward network side based on network interface MTU

-> x can be max 938 (938 ICMP data + 8 ICMP header + 20 IP header

+ 20 (5 labels) + 14 Ether = 1000)

-> x can be max 930 (930 ICMP data + 8 ICMP header + 20 IP header + 8 GRE

+ 20 IP header + 14 Ether = 1000)

MPLS:

GRE:

Note: 1) SDP path MTU is NOT taken into account(SPD path MTU is exchanged via LDP whereas VPRNs use BGP to exchange service labels)

2) Pre 5.0 only 3 labels are counted in case of MPLS

In pre-5.0 SW VPRN fragmenting toward network side is based on the lesser of (network

interface MTU 12) and sdp path MTU

The 12 takes into account 3 labels: Transport label, (possible:) Bypass label and Service

Label.

As off 5.0 5 labels are taken into account (Transport, Bypass, Service, Router Alert,

LDPoRSVP), hence network port MTU 20

This is hard coded in the SW and cannot be changed.

Note: when pinging a destination with a given size (ping x.x.x.x size y) the size only

indicates the ICMP data size. Hence, the IP packet size is the size + 8 bytes ICMP

header + 20 bytes IP header. This is important to note when testing IP fragmentation with

ping traffic.

12


VPRN Fragmentation toward access side

CE1 10.10.10.110.10.10.3

192.1.1.2

CE2

PE1PE3

1/1/1 1/1/11/1/1 1/1/11/1/31/1/4

192.1.2.2192.1.1.1 192.1.2.1

VPRN service Change: ip-mtu

port-MTU

ce1# ping 192.1.2.2 source 192.1.1.2 do-not-fragment size x

VPRN fragmenting toward access side based on lesser of:

VPRN interface ip-mtu

acces port MTU ether encapsulation overhead

VPRN interface ip-mtu = 600

access port MTU ether encap = 1514 - 14 = 1500

-> x can be max 572 (572 ICMP data + 8 ICMP header + 20 IP header = 600)

ip-mtu = 600

no port MTU (1514)

no ip-mtu

port MTU 600

VPRN interface ip-mtu not configured = 600 (port MTU) 14 ether header = 586

access port MTU ether encap = 600 - 14 = 586

-> x can be max 558 (558 ICMP data + 8 ICMP header + 20 IP header = 586)

NOTE: even if you configure ip-mtu to a higher value, fragmentation will take place at 586 in this case

Note that in this example ethernet encapsulation is NULL.

Note: when pinging a destination with a given size (ping x.x.x.x size y) the size only

indicates the ICMP data size. Hence, the IP packet size is the size + 8 bytes ICMP

header + 20 bytes IP header. This is important to note when testing IP fragmentation with

ping traffic.

13


Epipe spoke-SDP termination into IES service

Network

Interface

-

Port

MTU

SDP


IES

Access

Interface

-

Port

MTU

payload FCS802.1q


payloadDA SA FCS

VC-MTU

Service MTU

Ether

Type

DA SAEtherType

Access Port MTU

payloadDA SA FCSEther

TypeDA SA

Ether

Type

Path

Label

VC

Label

SDP Path MTU

Network Port MTU

Fragmentation possible on IES/VPRN IP-MTU(*)

IP-MTU

Epipe terminates in IES Service

The Physical MTU on an Ethernet access interface needs to be set to at least:

1514 with mode-access and encap-type null

1500 + 14 DLC header

1518 with mode-access and encap-type dot1q

1500 + 14 DLC header + 4 dot1q tag

1522 with mode-access and encap-type qinq

1500 + 14 DLC + 4 (first q-tag) + 4 (second q-tag)

(*) Note: fragmentation on IP-MTU takes place in the direction from the

VPRN/IES interface towards the CE.

If the IP interface has a SAP defined, the operational MTU is the smaller of the

SAP MTU (port MTU normalized for the SAP encap) and the administrative IP-

MTU (normalized to the SAP encap) to set the IP interface operational IP-MTU. If

the IP interface has a spoke-SDP defined,the operational MTU is the smaller of

the SDP Path-MTU and the administrative IP-MTU (normalized to the spoke

encap) to set the operational IP-MTU for the IP interface.

(*) REMARK: If the network interface is dot1q encapsulated, an extra 4 bytes are needed

for the q-tag

14


The Spoke SDP binding becomes only operationally up if the VC-MTU (signaled via TLDP ) of both ends match

On the Epipe/VPLS side of the spoke the VC-MTU is derived from the configured service MTU (Service MTU is 1514 by default)

On the IES/VPRN side of the spoke-SDP there is no Service MTU concept and the VC-MTU will be equal with the configured operation ip-mtu value of the IES or VPRN interface. If the ip-mtu is not explicitly configured, its value will be set to the SDP operational MTU minus ethernet header size (14)

The best way to fix this is to match the IES/VPRN interface IP-MTU to the VC-MTU from the other side (show router ldp bindings)

Packets can be fragmented on the IP-MTU size

Spoke-SDP termination: MTU Considerations


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