Cisco 7604 Configuration Manual page 891
Ios software configuration guide
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Chapter 52
Configuring Local SPAN, RSPAN, and ERSPAN
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For ERSPAN packets, the "protocol type" field value in the GRE header is 0x88BE.
The payload of a Layer 3 ERSPAN packet is a copied Layer 2 Ethernet frame, excluding any ISL or
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802.1Q tags.
ERSPAN adds a 50-byte header to each copied Layer 2 Ethernet frame and replaces the 4-byte cyclic
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redundancy check (CRC) trailer.
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ERSPAN supports jumbo frames that contain Layer 3 packets of up to 9,202 bytes. If the length of
the copied Layer 2 Ethernet frame is greater than 9,170 (9,152-byte Layer 3 packet), ERSPAN
truncates the copied Layer 2 Ethernet frame to create a 9,202-byte ERSPAN Layer 3 packet.
Regardless of any configured MTU size, ERSPAN creates Layer 3 packets that can be as long as
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9,202 bytes. ERSPAN traffic might be dropped by any interface in the network that enforces an MTU
size smaller than 9,202 bytes.
With the default MTU size (1,500 bytes), if the length of the copied Layer 2 Ethernet frame is
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greater than 1,468 bytes (1,450-byte Layer 3 packet), the ERSPAN traffic is dropped by any
interface in the network that enforces the 1,500-byte MTU size.
The mtu interface command and the system jumbomtu command (see the
Note
Support" section on page
is 9,216 bytes).
All participating routers must be connected at Layer 3 and the network path must support the size
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of the ERSPAN traffic.
ERSPAN does not support packet fragmentation. The "do not fragment" bit is set in the IP header
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of ERSPAN packets. ERSPAN destination sessions cannot reassemble fragmented ERSPAN
packets.
ERSPAN traffic is subject to the traffic load conditions of the network. You can set the ERSPAN
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packet IP precedence or DSCP value to prioritize ERSPAN traffic for QoS.
The only supported destination for ERSPAN traffic is an ERSPAN destination session on a PFC3.
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All ERSPAN source sessions on a router must use the same origin IP address, configured with the
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origin ip address command (see the
page
All ERSPAN destination sessions on a switch must use the same IP address on the same destination
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interface. You enter the destination interface IP address with the ip address command (see the
"Configuring ERSPAN Destination Sessions" section on page
The ERSPAN source session's destination IP address, which must be configured on an interface on
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the destination router, is the source of traffic that an ERSPAN destination session sends to the
destination ports. You configure the same address in both the source and destination sessions with
the ip address command.
The ERSPAN ID differentiates the ERSPAN traffic arriving at the same destination IP address from
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various different ERSPAN source sessions.
OL-4266-08
9-10) set the maximum Layer 3 packet size (default is 1,500 bytes, maximum
52-19).
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Local SPAN, RSPAN, and ERSPAN Configuration Guidelines and Restrictions
"Configuring ERSPAN Source Sessions" section on
"Configuring Jumbo Frame
52-22).
52-13
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