---
description: MTU
---

# Mpls

The maximum frame size (in bytes) that is permitted over a certain link or service\
Oversized frames arriving at a layer 2 interface are not fragmented, but simply discarded\
For an Ip/Mpls network, the following MTU entities must be considered:\
1\) Service MTU\
2\) Access port, or SAP MTU\
3\) SDP path MTU\
4\) Network port MTU

### Service MTU

Service MTU defines the maximum customer payload that can be carried end to end in a service.\
The default MTU for an **ethernet VPN** service is **1514 bytes.**\
1514 = 1500 bytes (payload) + 14 bytes (DLC header)\
FCS is not carried but recalculated at the far end.&#x20;

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### SAP MTU&#x20;

Derived from the physical access port MTU\
İncludes VLAN tags arriving at the SAP\
Vlan tags are service-delimiting by default, thus stripped on arrival \
The SAP MTU must be >= service MTU + Vlan encapsulation for the SAP to  be operationally up.\


<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FyHgzk4Kyn6PLIMB1ffaJ%2Fimage.png?alt=media&#x26;token=b672d774-4cdb-407e-87c8-a0d1df1ecca0" alt=""><figcaption></figcaption></figure>



### SAP MTU Calculation

Null encapsulation default MTU is 1514\
Dot1q encapsulation default MTU is 1514+4 = 1518 \
Q-in-Q encapsulation default MTU is 1514 + 4 + = 1522

![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2F2XpyCs4QwbRiYueeJ4sY%2Fimage.png?alt=media\&token=dfcdb6a7-4d61-4c43-a18b-9c2d8e415202)\


### SDP Path and Network Port MTU

The SDP path MTU defines the maximum payload size that can be carried in the SDP transport tunnel.&#x20;

* By default, it is derived from the network port.
* SDP path MTU =  egress network port MTU -SDP encapsulation
* SPD path MTU >= service MTU

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FAznIVnwl4NjbOtv2fNN1%2Fimage.png?alt=media&#x26;token=fb391af1-dc8e-48c8-b031-f128f3a764b4" alt=""><figcaption></figcaption></figure>

### SDP  Path MTU Calculation

For a gigabit ethernet network port with a default MTU of 9212

#### If SDP uses MPLS encapsulation

\


<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2Fo1qzwFbLl7IulW5ZEiX2%2Fimage.png?alt=media&#x26;token=d620e7fc-ce9d-43b9-ad9f-3bf8b78af319" alt=""><figcaption></figcaption></figure>

#### If SDP uses Gre encapsulation

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### Epipe MTU Case Study

Epipe services ID 50 is configured between PE1 and PE2 with a service MTU of 1514 bytes. \
The customer sites connect to the PE routers using dot1q ethernet encapsulation\
The SDP between the PE routers uses RSVP-signaled LSPs for transport.

New requirement for epipe 50 to support a maximum customer packet size of 5000 bytes.

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FcfjlzNf0mQOelJlQiqw8%2Fimage.png?alt=media&#x26;token=bcb196c7-8f92-40d3-8c6d-91f4f71f4ea9" alt=""><figcaption></figcaption></figure>

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Service Mtu değeri 1514 byte. Sap Mtu değeri vlan tag'i de dahil olduğu içij 4 byte daha eklenmiş hali olan 1518 byte.

SDP, Mpls encapsulation kullandığı için MTU değeri 9190.&#x20;

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FrqbuEEA1Uev0SOOrixlI%2Fimage.png?alt=media&#x26;token=fdc07aaa-93df-4ce0-99fc-3e4a9362ef60" alt=""><figcaption></figcaption></figure>

1/1/1 portu SAP portu 1518 byte, 1/1/2 portu Network portu.&#x20;

### SDP (Service Distribution Point)&#x20;

A service distribution (SDP) defines a transport tunnel between the PE routers.\
A SDP is bound to a VPLS service using either a spoke or a mesh SDP.\
The SDP binding type determines how flooded traffic is transmitted.

### Spoke SDP Flooding Behavior

Flooded traffic received on a spoke SDP is replicated on all other spoke SDPs,mesh SDPs and local SAPs.

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FLP7ZhhWjn9nyFZjHWg3I%2Fimage.png?alt=media&#x26;token=307e8139-e2a0-4d0a-930e-7d969af93104" alt=""><figcaption></figcaption></figure>

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### Mesh SDP Flooding Behavior

Flooded traffic received on a mesh SDP is replicated on all other spoke SDPs and local SAPs.

* Not replicated on other mesh SDPs.

![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2F2QKl2gClRsCVX0lFF53a%2Fimage.png?alt=media\&token=b7f6d115-8984-4416-8491-d59970070379)![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FOyrdsXLhk16M54qugrk0%2Fimage.png?alt=media\&token=b1c614dd-3db2-4c8f-8324-4ec3a740810d)



<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FgnEKoSPhiVF72BWPKk7K%2Fimage.png?alt=media&#x26;token=537fd77b-619c-46d4-94a2-061beb40a028" alt=""><figcaption></figcaption></figure>

![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FJPPBvUpwZgl5oOEzB4Af%2Fimage.png?alt=media\&token=c7e896d9-5958-49ee-9f8a-4c3b5de888a2)![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FHMe9jYnfdFSD11eURTXV%2Fimage.png?alt=media\&token=9115eeb4-015a-4810-a46f-a12b60e1868d)



### VPLS Flooding Behavior

Vpls provides a layer 2 multipoint connectivity\
Traffic is forwarded based on destination Mac address.\
Each PE dynamically builds its forwarding database (FDB) based on the source MAC address in traffic offered to the VPLS\
VPLS forwarding behavior depends on the type of traffic sent.

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FUjMuU3liVcrbnem2I0nT%2Fimage.png?alt=media&#x26;token=f8b6a912-eaf7-4939-bbfb-3e3fc8feaeed" alt=""><figcaption></figcaption></figure>

### Known Unicast Traffic

Traffic destined to a known unicast address is sent only to the destination.

* A known destination is one that is present in the FDB

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FD0JP5KeZ9vyCRyrySFEI%2Fimage.png?alt=media&#x26;token=92d73ad3-7e17-4d93-bbfe-244d85c75e4f" alt=""><figcaption></figcaption></figure>

### Broadcast, Unknown Unicast and Multicast (BUM) Traffic

Traffic destined to a BUM address is flooded to all local SAPs and remote PEs in the service.

* In a basic VPLS, the SDP is bound to  the service as a mesh SDP

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FtnvnZfcFaKxE4nBBlRv4%2Fimage.png?alt=media&#x26;token=dadd566d-4719-4d6d-9ab2-4e0e482afb39" alt=""><figcaption></figcaption></figure>

### Mac Learning in a VPLS&#x20;

* Vpls is fully meshed

![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FqjOPTjb7JPC4LiIPyCx9%2Fimage.png?alt=media\&token=ada2904d-1587-4e55-b431-3fdbc164e454)![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2Fwc01G7QUuXVX7gNb4a7Y%2Fimage.png?alt=media\&token=faa80ca8-08b7-471c-9669-9bb9ec1c9672)



### Mac Aging

FDB entry is added with an age of 0\
Age resets when receiving a packet with a matching source mac address.\
Age increase during inactive flow\
FDB entry is removed if age is greater than the age timer

![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2F9w48okMki9x5LJwDYTI1%2Fimage.png?alt=media\&token=2cce7490-37ff-4bfd-b394-ab722e2c52d2)![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FqLpzBzkmcqJHkmVAiH9r%2Fimage.png?alt=media\&token=7de4f431-5ce3-478b-bcd8-dd0e3c82fecc)



![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FXHzIHdkWJoYiPJeqI8p0%2Fimage.png?alt=media\&token=c7a83781-bfa5-450b-a383-f557bab2f609)



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![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FGl8rjgGsUVQUTsJPVz2l%2Fimage.png?alt=media\&token=67d0249c-04cd-47de-b864-e69793f290ec)![](https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FdnuZYxoki1R1Bykgxofh%2Fimage.png?alt=media\&token=6a3c4565-6dcd-4bf4-b1ba-5171de976295)



### VPRN (Virtual Private Routed Network)



* Connects multiple sites in a single routed domain over the provider network.
* The provider network is invisible to customer
  * The vprn appears as a virtual IP router.

#### Virtual Routing and Forwarding (VRF) Table

* Contains customer routes for the VPRN
* A PE maintains a VRF for each provisioned VPRN service

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FdGzZVRHxNWwMObKd93uJ%2Fimage.png?alt=media&#x26;token=6253e357-29da-4ac2-baae-8f06b56a144f" alt=""><figcaption></figcaption></figure>

### Distribution of Customer Routes in VPRN

* CE-to-PE Routing&#x20;
  * The CE router distributes routes to the local PE router, the PE router stores them in the VRF for the VPRN service.

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FeN5xuYUEgGiqJHyfbU2L%2Fimage.png?alt=media&#x26;token=158efe12-7e02-4ef5-9503-333bc0e19cc9" alt=""><figcaption></figcaption></figure>

PE-to-PE Routing

* MP-BGP distributes customer routes between PE routers

PE-to-CE Routing&#x20;

* The PE router distributes the VRF routes learned from remote PE routers to the local CE routers.

#### CE-to-PE Routing

* The PE router peers with the CE router to exchange customer routes&#x20;
*   Customer routes are kept in the VRF for the VPRN service

    * CE is effectively peering with the VRF on the PE routes.



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The CE router can use static routes, RIP,ISIS,OSPF or eBGP to exchange routes with the VRF

#### PE-to-PE Routing: MP-BGP

A single instance of MP-BGP handles route exchange for all VPRN services.\
The RD (Route Distinguisher) is used to distinguish between customer routes from different VPRN services.\
Each VPRN is configured with a unique RD\
The RD is a added to the IPv4 prefix to create a VPN-İpv4 prefix.

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FVCc4jA8QBOU0QgaUHc8k%2Fimage.png?alt=media&#x26;token=34325174-e89e-458c-8424-f7bce33bf318" alt=""><figcaption></figcaption></figure>

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#### PE-to-PE Routing: Route Target (RT)

Question: How does receiving PE determine which routes belong to which RF ?

The route Target (RT) is a bgp extended community added by the advertising PE when the route is exported from the VRF into MP-BGP

* The receiving PE uses the RT to select the routes that will be imported into a VRF

<figure><img src="https://2292156749-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDfDBccroFkvJ25uVFqYX%2Fuploads%2FhY8eC2Obs4B8dPul9Zre%2Fimage.png?alt=media&#x26;token=c58549f6-11e0-4cb0-b37c-0ff7c1182bc2" alt=""><figcaption></figcaption></figure>

#### PE-to-CE Routing

VPN routes received from remote PEs are added to the VRFs and need to be advertised to local CE routes.

* A routing protocol or a static route is used for PE-CE routing

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{% embed url="https://www.youtube.com/watch?v=seVVs_u8Mec&list=PLgKNvl454BxeIe_LVIbM-dlgLKaGxwPl-&index=5&ab_channel=Nokia" %}





























