--------------------------------------------------
ERX NOTES on Padding
--------------------------------------------------
Padding IS-IS Hello Packets
You can use the isis hello padding command to configure IS-IS hello packet
padding. Padding the hello packets promotes early error detection due to
transmission problems with large frames or due to mismatched MTUs on adjacent
interfaces.
When disabled (default), IS-IS hello packets are padded to the full MTU size until an
adjacency is formed with the adjacent interface. After the adjacency is formed, the
hello packets are no longer padded. When enabled, IS-IS hello packets are always
padded.
isis hello padding
Use to pad IS-IS hello packets to their full maximum transmission unit (MTU)
size.
Example
host1(config-if)#isis hello padding
Use the no version to restore the hello padding to its default, no padding.
--------------------------------------------------
Cisco NOTES on Padding
--------------------------------------------------
isis hello padding
To reenable IS-IS hello padding at the interface level, enter the isis hello
padding command in interface configuration mode. To disable IS-IS hello padding,
use the no form of this command.
isis hello padding
no isis hello padding
Syntax Description
This command has no arguments or keywords.
Defaults
IS-IS hello padding is enabled.
Command Modes
Interface configuration
Command History
Release Modification
12.0(5)T
This command was introduced.
12.0(5)S
This command was integrated into Cisco IOS Release 12.0(5)S.
Usage Guidelines
Intermediate System-to-Intermediate System (IS-IS) hellos are padded to the full
maximum transmission unit (MTU) size. The benefit of padding IS-IS hellos to the
full MTU is that it allows for early detection of errors that result from
transmission problems with large frames or errors that result from mismatched
MTUs on adjacent interfaces.
You can disable hello padding in order to avoid wasting network bandwidth in case
the MTU of both interfaces is the same or, in case of translational bridging. While
hello padding is disabled, Cisco routers still send the first five IS-IS hellos
padded to the full MTU size, in order to maintain the benefits of discovering MTU
mismatches.
To selectively disable hello padding for a specific interface, enter the no isis
hello padding command in interface configuration mode. To disable hello padding for
all interfaces on a router for the IS-IS routing process, enter the no hello padding
command in router configuration mode.
--------------------------------------------------
General Notes:
--------------------------------------------------
Starting Point
1500
GRE Overhead = 24 bytes.
JunOSe CALEA Overhead = 36 bytes.
Cisco CLNS auto-calc overhead = 3 bytes.
--------------------------------------------------
ERX:
Auto calc 12 for RSVP/C2C/FRR when RSVP on.
Ethernet overhead is full packet w/CRC
Cisco:
L2 auto calc'd.
Per Juniper:
ERX Protocol Cisco
=================================
12 - RSVP 4
auto | C2C -
auto - FRR 4
4 LDP 4
4 VPN 4
18 Ethernet -
4 VLAN -
=================================
42 Total Overhead 16
Still not verified, but what I think:
ERX Protocol Cisco
=================================
12 - RSVP 4
auto | C2C -
auto - FRR 4
-4 NoC2C -
4 LDP 4
4 VPN 4
18 Ethernet -
4 VLAN -
4 Dyn. MPLS -
Interface
=================================
42 Total Overhead 16
--------------------------------------------------
42-16=26bytes
JunOSe-IP-MTU = Cisco-IP-MTU +26 bytes
JunOSe-CLNS-MTU = Cisco-CLNS-MTU +22 bytes (26 bytes - 4 bytes Dyn MPLS Int).
--------------------------------------------------
So, for 1500 IP MTU DF traffic:
Cisco 1520 (1516 + 4 bytes to make CLNS MTU Equal)
JunOSe 1542
This will allow a IP downstream MTU of 1504 and and upstream of 1500,
but it will keep the CLNS MTU equal.
The CLNS will show as 1517 on Cisco and 1520 on JunOSe, using the
"show clns inter " command.
Cisco MPLS MTU : 1516 = 1520 - 4
JunOSe MPLS MTU: 1508 = 1500 + LDP (4) + VPN (4)
--------------------------------------------------
For minimal needed configuration we need an IP MTU to current traffic
is 1500+36(calea)+24(gre)+16(MPLS)= 1576
Cisco GE w/MPLS MTU = 1576 (IP MTU needed) + 4 JunOSe(DynMPLSInt) = 1580 bytes
JunOSe GE w/MPLS MTU = Cisco GE w/MPLS MTU +22 bytes = 1580+22 = 1602 bytes
This will allow a IP downstream of MTU 1576 and a upstream of 1572,
but it will keep the CLNS MTU equal.
The CLNS will show as 1573 on Cisco and 1576 on JunOSe, using the
"show clns inter " command.
Cisco MPLS MTU : 1576 = 1580 -4
JunOSe MPLS MTU: 1584 = 1576 + LDP (4) + VPN (4)
--------------------------------------------------
Reccomendations would be to set IP MTU to SONET max. of 4470.
Cisco GE w/MPLS MTU = 4470-16 bytes = 4454
JunOSe GE w/MPLS MTU = Cisco GE w/MPLS MTU +22 bytes= 4470+22 = 4492 bytes
This will allow a IP downstream of MTU 4454 and a upstream of 4450,
but it will keep the CLNS MTU equal.
The CLNS will show as 4467 on Cisco and 4470 on JunOSe, using the
"show clns inter " command.
Cisco MPLS MTU : 4466 = 4470 -4
JunOSe MPLS MTU: 4458 = 4450 + LDP (4) + VPN (4)
--------------------------------------------------
From Juniper Test Results:
==========================
1542 ERX
1512/1516 Cisco (with/without FRR)
