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Juniper Networks/Cisco Systems Switch Interoperability Cookbook

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Jumbo&frame&routing&

Objective!

To validate the ability of Juniper and Cisco switches to correctly route bidirectional

traffic with packet lengths greater than 1,500 bytes.

Background!

Some routing protocols such as open shortest path first (OSPF) require that both routers

agree on the same maximum transmission unit (MTU) when exchanging routing

information. For Ethernet interfaces, the requirement for matched MTUs applies equally

to jumbo frames (those larger than 1,518 bytes) as to standard-length frames.

In part because of the lack of a standard length for jumbo frames, there is confusion in the

marketplace about the maximum frame length possible. Older Linux drivers for Ethernet

interfaces in servers support a maximum length of around 7,000 bytes, though most

Linux drivers now allow frame lengths of 9,000 bytes or more. Ethernet interfaces of

switches and routers typically support a larger protocol data unit (PDU) but there is some

confusion as to whether that PDU should be a maximum of 9,000 bytes or 9,216 bytes.

Adding to the confusion, implementations differ as to whether the 4-byte cyclic

redundancy check (CRC) should or should not be included when stating the maximum

frame length.

Juniper and Cisco switches typically support 9,216-byte jumbo frames, including CRC

This section explains how to configure both vendors’ devices to set up an OSPF routing

session using jumbo frames.

Topology!

In this example, a Juniper Virtual Chassis Fabric (comprising two Juniper QFX5100s and

one Juniper EX4300) configured as an OSPF router exchanges jumbo frames with a

Cisco Nexus 7010 switch. This example uses OSPF because it requires both sides to use

the same MTU when exchanging database description messages

Figure 3 illustrates the configuration used to validate jumbo frame routing. In this

example, an OSPF routing session will be established between the Juniper and Cisco

devices. Both interfaces have IP addresses in the 192.18.64.0/24 subnet.

The Juniper EX9200 supports a maximum Ethernet frame length of 9,192 bytes, or 9,152 bytes when

configured in Virtual Chassis mode, to account for a 40-byte internal header. Other Juniper devices,

including those given in the examples here, support a maximum Ethernet frame length of 9,216 bytes.

This requirement is specified in RFC 2328, section 10.6.

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Cisco Ex SERIES Manuel d'utilisateur Page 13