Cisco Ex SERIES Uživatelský manuál stáhnout pdf (Strana 16)

Juniper Networks/Cisco Systems Switch Interoperability Cookbook

! ! !

The fact that both routers are in OSPF “Full” state indicates they have agreed to exchange

IP packets up to 9,198 bytes long (9,216 bytes, including Ethernet header and CRC).

OSPF routing sessions will not be fully established unless both sides agree on MTU size.

Jumbo&frame&switching&

Objective!

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

traffic consisting of jumbo frames.

Background!

For many years the IEEE Ethernet specification has defined the maximum length of an

Ethernet frame to be 1,518 bytes (or 1,522 bytes with an 802.1Q VLAN tag). The use of

jumbo frames – those larger than 1,518 bytes – remains nonstandard

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’ switches to exchange jumbo

frames.

Topology!

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

QFX5100s and one Juniper EX4300) exchanges jumbo frames with a Cisco Nexus 7010.

As commonly used in many organizations, VLAN trunk ports connect the switches and

VLAN access ports at the edge accept untagged jumbo frames. However, the ability to

switch jumbo frames does not depend on VLAN tagging. This example would also work

with all interfaces passing untagged traffic.

Recent versions of the 802.3 Ethernet specification have extended the maximum “envelope” frame length

to 2,000 bytes to allow for multiple VLAN headers and various encapsulation methods. However, the

specification’s maximum “basic” frame length remains at 1,518 bytes.

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.

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